CA1295372C - Wireless communication system using current formed underground vertical plane polarized antennas - Google Patents
Wireless communication system using current formed underground vertical plane polarized antennasInfo
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- CA1295372C CA1295372C CA000597466A CA597466A CA1295372C CA 1295372 C CA1295372 C CA 1295372C CA 000597466 A CA000597466 A CA 000597466A CA 597466 A CA597466 A CA 597466A CA 1295372 C CA1295372 C CA 1295372C
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- current driver
- conductors
- earth
- current
- transmitter
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Abstract
ABSTRACT OF THE DISCLOSURE (U) A wireless communication system and method for employing underground or low profile surface deployed current drivers for inducing ground currents in the earth which define loop antennas for sending and receiving electromagnetic signals propagated through the atmosphere over a wide bandwidth. The system uses a current driver whose impedance is adjusted so that it matches the impedance of a transmitter/receiver connected thereto.
The current driver is capacitively coupled to the sur-rounding ground so that the current driver and the earth effectively function together as a vertical plane polarized antenna which propagates a vertically polarized electromagnetic signal. The system can be operated using either a single current driver or, where increased signal gain is desired, using an array of current drivers that are spaced in parallel. A unidirectional vertically polarized signal may be produced by shortening the length of one current driver conductor with respect to the other arm. Unwanted cancellation currents may be substantially eliminated from the region surrounding the current driver by positioning low conductivity materials so as to extend into the flow paths of the cancellation currents.
The current driver is capacitively coupled to the sur-rounding ground so that the current driver and the earth effectively function together as a vertical plane polarized antenna which propagates a vertically polarized electromagnetic signal. The system can be operated using either a single current driver or, where increased signal gain is desired, using an array of current drivers that are spaced in parallel. A unidirectional vertically polarized signal may be produced by shortening the length of one current driver conductor with respect to the other arm. Unwanted cancellation currents may be substantially eliminated from the region surrounding the current driver by positioning low conductivity materials so as to extend into the flow paths of the cancellation currents.
Description
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1 WIRELESS CO.~MU~IC.~TIO~ SYSTE.Y A~DI~ETHOD USI~G
1 WIRELESS CO.~MU~IC.~TIO~ SYSTE.Y A~DI~ETHOD USI~G
2 CURRE~'T FOR~ED'_'`;DERGP~OU~'D VERTICAL PLA~TE
_ _ _ POL~ IZ~D ~TTE~AS(U) (U) 7 Field of the Invention (U) (S) 11 This invention relates to an improved wireless 12 communication system and method, and more particularly to 13 a system and method for employing underground or low 14 profile, surface deployed current drivers for inducing ground currents in the earth in such a way that the 16 current drivers and earth function together as vertical 17 plane polarized antennas which perform as loop or 18 long-wire guided wave antennas and which send and receive 19 vertically polarized electromagnetic signals propagated through the atmosphere over a wide bandwidth.
21 The Prior Art (U) .
(U) 22 Aboveground wireless communication systems have been 23 known and used for many years. Generally, such systems 24 employ aboveground antennas which extend high into the air for transmitting and receiving low, medium and high 26 frequency electromagnetic signals which travel through ~' ... . .. _._~___ . _.. _. _.. -'--' `' ' . ~ '''--'' --. ' --' . --'~'--`I' ,~ _.. _ . . . __ _ _.. ...... _.. _ _.. _ _.. _ ._ .
(U) l the atmosphere. In a military sense, such aboveground 2 communication systems are considered soft" for securit~
_ _ _ POL~ IZ~D ~TTE~AS(U) (U) 7 Field of the Invention (U) (S) 11 This invention relates to an improved wireless 12 communication system and method, and more particularly to 13 a system and method for employing underground or low 14 profile, surface deployed current drivers for inducing ground currents in the earth in such a way that the 16 current drivers and earth function together as vertical 17 plane polarized antennas which perform as loop or 18 long-wire guided wave antennas and which send and receive 19 vertically polarized electromagnetic signals propagated through the atmosphere over a wide bandwidth.
21 The Prior Art (U) .
(U) 22 Aboveground wireless communication systems have been 23 known and used for many years. Generally, such systems 24 employ aboveground antennas which extend high into the air for transmitting and receiving low, medium and high 26 frequency electromagnetic signals which travel through ~' ... . .. _._~___ . _.. _. _.. -'--' `' ' . ~ '''--'' --. ' --' . --'~'--`I' ,~ _.. _ . . . __ _ _.. ...... _.. _ _.. _ _.. _ ._ .
(U) l the atmosphere. In a military sense, such aboveground 2 communication systems are considered soft" for securit~
3 purposes because they are relatively easy to destroy.
4 "Hardness" (or "softness") is a military term used to denote the system's vulnerability to destruction under 6 attack. The harder a system is, the less vulnerable to 7 destruction it is.
(U) 8 The hardness of a communication system is generally 9 measured by such criteria as its ability to withstand substantial shock, as in the case of a powerful e~plosion 11 occurring very near to the system and its ability to 12 survive high energy electromagnetic pulse radiation which 13 may be produced by a nuclear blast.
(U) 14 Even though a powerful explosion may be centered some distance from an aboveground communication system, 16 the resulting shock waves will likely damage or destroy 17 the system antennas. Furthermore, aboveground antennas 18 which transmit or receive high frequency signals are very 19 susceptible to the adverse effects of electromagnetic pulse radiation. Even though attempts have been made to 21 increase the hardness of aboveground communication 22 systems by constructing backup systems, factors such as 23 cost and environmental considerations make it very 24 difficult to justify and obtain the redundancy required to make such systems secure in the event of attack.
~6 1~5372 (U) 1 In order to increase system hardness, it is desir-2 able to deploy communication system antennas under the ~ ground or flat upon the ground, and in some cases it is 4 desirable to deploy at least par~ of the antennas above 5 but in close proximity to the surface of the ground.
(U) 8 The hardness of a communication system is generally 9 measured by such criteria as its ability to withstand substantial shock, as in the case of a powerful e~plosion 11 occurring very near to the system and its ability to 12 survive high energy electromagnetic pulse radiation which 13 may be produced by a nuclear blast.
(U) 14 Even though a powerful explosion may be centered some distance from an aboveground communication system, 16 the resulting shock waves will likely damage or destroy 17 the system antennas. Furthermore, aboveground antennas 18 which transmit or receive high frequency signals are very 19 susceptible to the adverse effects of electromagnetic pulse radiation. Even though attempts have been made to 21 increase the hardness of aboveground communication 22 systems by constructing backup systems, factors such as 23 cost and environmental considerations make it very 24 difficult to justify and obtain the redundancy required to make such systems secure in the event of attack.
~6 1~5372 (U) 1 In order to increase system hardness, it is desir-2 able to deploy communication system antennas under the ~ ground or flat upon the ground, and in some cases it is 4 desirable to deploy at least par~ of the antennas above 5 but in close proximity to the surface of the ground.
6 Underground and near surface above ground deployed 7 antennas ("low profile antennas") are able to withstand 8 the effects of nearby explosions to a much greater degree ~ than conventional aboveground antennas. Further, such underground and surface proximity antennas are exposed to 11 less electromagnetic pulse radiation. Because of these 12 advantages, a communication system utilizing underground 13 or low profile antennas requires less redundancy to 14 achieve system security than a comparable communication system using typical aboveground antennas. However, 16 although system hardness is improved, prior art 17 undergroundlow profile antenna systems have been 18 substantially less efficient in their operation than the l9 high, aboveground antennas. Because of their poor performance characteristics, such antenna sys~ems have 21 had limited and very specific applications and have been 22 wholly unable to adequately function as a replacement for 23 the high, aboveground antenna systems.
(U) 24 Such inadequate performance characteristics are embodied in the various wireless subterranean signaling 26 systems which have been proposed in the past, wherein 129S3 ~Z
(U) 1 electromagnetic signals are transmitted through the earth 2 between underground antennas. For example, electro-~ magnetic wave~ of relatively low frequencies ranging from 4 lOO ~z to lOO KHz have been propagated through the earth between horizontally polarized electric dipole antennas 6 buried in the earth. Such underground transmission of 7 signals is inherently susceptible to significant signal 8 attenuation due to the large dielectric coefficient and 9 high conductivity of the earth. This is due to the fact that in a conductive (i.e., lossy) medium such as the 1~ earth or water, energy is dissipated through currents 12 that are generated by the electric and magnetic field 13 components of the wave. This energy loss results in an 14 appreciable exponential attenuation of field strength with distance. In contrast, electromagnetic waves 16 propagated through the atmosphere lose little energy to 17 the medium. Thus, excess attenuation beyond inverse R2 18 loss is negligible in the atmospheric case except at 19 microwave and higher frequencies.
(U) 20 In order to achieve system hardness while utilizing 21 the atmosphere for signal transmission, several past 22 proposals have involved the positioning of a dipole 23 antenna upon or beneath the surface of the earth. Such 24 systems have experienced significant reduction in signal strength as compared with aboveground, vertically ,.
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(U) 1 oriented dipole antennas as a result of signal 2 attenuation and losses in the earth.
(U) 3 Comparisons of the performance of subsurface dipole 4 antennas to conventional above surface antennas are presented in Fenwick and Weeks, Submerged Antenna 6 Characteristics, I.E.E.E. TR~NSACTIONS ON ANTENNAS AND
7 PROPAGATION, p. 296 (May, 1963), where it is seen that in 8 many common situations the strength of the underground 9 produced signal is more than 40 dB weaker than the signal produced by the reference antenna, which is a perfect 11 quarter-wave vertical monopole antenna. Such reduced 12 signal power is simply not acceptable for many 13 communications systems applications, especially when such 14 applications may involve long-range signal tranmission.
In addition to the above problem, dipole antennas produce 16 electromagneti.c signals which propagate in directions 17 generally normal to the longitudinal axis of the dipole 18 antenna. As a result, much of the signal strength is l9 directed substantially straight upwards or into the ground where it is lost, resulting in significant amounts 21 of power loss and reduced efficiency in the communication 22 system.
(U) 23 In order to provide an underground antenna system 24 while transmitting usable signals through the atmosphere, it has been proposed in the prior art to employ a buried 26 loop antenna for generating a horizontally polarized lZ~!53 ,z ~
(U) 1 magnetic wave which in turn generates a surface wave 2 having a vertically polarized electric component to be ~ received by a vertical whip receiving antenna. ~lthough 4 a substantial portion of the resulting signal propagates along the earth's surface, this antenna system still is 6 very low in efficiency which is mainly a result of the 7 use of horizontally polarized waves and the losses 8 associated therewith. Another disadvantage of this type 9 of prior art antenna is the very large physical antenna size needed at low and even medium range frequencies.
(S) 11 It becomes clear that the most efficient means for 12 obtaining system hardness while providing for trans-13 mission and reception of elec~romagnetic signals through 14 the atmosphere would be to utilize a buried wire loop or traveling wave antenna which could produce vertical plane 16 polarized electromagnetic signals. However, it is well 17 known that the size of such an antenna is directly 18 related to the wavelength of the signal in earth at the l9 frequency of operation. In fact, for optimal operation the perimeter length oP the underground loop antenna 21 should be approximately 1.4 wavelengths in earth at the 22 operating frequency. For an operating frequency in the 23 MF range of 400 KHz, the necessary loop antenna perimeter 24 length would be approximately lOO meters. It becomes immediately apparent that, even if physically possible, 26 the cost of trenching, supporting and burying such an , . . . ~ .. , . ..... . , .. , . .. ... . . .. . . , ,~.. ... .. ... . . . . ... .. .
c `- l;~gS3 ~2 (S) 1 antenna in the vertical position would mal;e use of the 2 antenna unrealistic if n~t impossible.
(U) ~A further problem that is common to all underground 4 antennas is the lower power gain which is experienced as a result of signal attenuation prior to signal entry into 6 the atmosphere. Although this attenuation can be mini-7 mized by positioning the antenna close to the surface, it 8 still exists in significant amounts. No adequate method 9 has heretofore been found for substantially increasing the gain of atmospheric transmission signals emanating 11 from an underground source and thus, this reduced perfor-12 mance capability has continued to be a long-standing, 13 unresolved problem in the art.
(U) 14In light of the above considerations, it is apparent that the great need that has heretofore gone unsatis~ied 16 is to provide a two-way, wireless underground or near 17 surface aboveground deployed communication system capable 18 of effectively receiving and transmitting signals over a 19 wide band of frequencies, with the system being suffi-ciently "hard" to withstand a near miss of a nuclear 21 weapon. The system should have reduced vulnerability to 22 jamming and, even immediately following a nuclear explo-23 sion, should permit long distance transmission of 24 electromagnetic signals with reasonable data rates. The system should be capable of transmitting communication 26 signals in either broad or narrow beam configurations, ~29S~ ~2 ~U) 1 and in conjunction with enhanced signal processing, 2 should be capable of performance comparable to e~isting 3 aboveground antenna systems. Furthermore, the system 4 should feasibly permit redundancy sufficient to satisfy the need for system security without excessive costs.
6 The underground communication systems heretofore employed 7 have not been able to satisfy these important needs.
8 BRIEF SUMMARY ~D OBJECTS OF THE I~E~TION (U) (S) 9 The present invention advantageously provides a unique two-way, wireless communication system and method 11 that employs an underground or surface or low profile 12 current driver arrangement for sending and receiving 13 electromagnetic signals propagated through the atmo-14 sphere. The system is operable on a wide frequency band, and produces steerable signals having a power gain and 16 data rates comparable to signals transmitted from more 17 conventional aboveground systems.
(5) 18 The present invention involves use of at least one 19 current driver comprising a pair of conductors and a balun positioned flat upon the earth's surface, or 21 positioned above but in proximity to the earth's surface 22 or buried beneath the surface of the ground in proximity 23 to the earth's surface. The elements of the current 24 driver are constructed so that it is capacitively coupled with the surrounding earth in a manner such that induced 26 ground currents cause the current driver and the earth to lZ9~i3, ~ ,~
(S) l effectively function together as a vertical plane 2 polarized loop antenna. A vertically polari ed signal is 3 transmitted from this antenna into the atmosphere where 4 it continues to propagate as a vertically polarized wave.
(S) 5 In one preferred embodiment, the two current driver 6 conducting arms are positioned end-to-end and joined at 7 the middle to a balance-to-unbalance ("balun") impedance 8 matching transformer. When the current driver is thus 9 configured, with the impedance of the current driver and earth substantially matched or correlated to that of a 11 transmitter/receiver connected thereto, the capa.itively 12 coupled current causes the current driver and the earth 13 to effectively function as a vertical plane loop antenna 14 which produces a bidirectional vertically polarized electromagnetic signal.
(S) 16 In another preferred embodiment, the length of one 17 of the two current driver conducting arms is substantial-18 ly shorter than the other conducting arm. A length ratio 19 for the arms of approximately 2 to l has been found to be adequate for many applications while larger ratios 21 provide further improvement in antenna gain. In this 22 configuration, the capacitively coupled current causes 23 the current driver and the earth to effectively function 24 as a long-wire traveling wave antenna which produces a substantially unidirectional vertically polarized ~6 electromagnetic si ~ 1. For a length ratio of 2 to 1 the Cl lZ953~2 (S) power gain is approximately 4 dB greater than the above-described bidirectional signal.
(5) ~ Power gain in each of the embodiments is further improved by positioning several current drivers parallel to one another and at spacings which may be as close as half a skin depth in ground. These are fed in parallel using a power splitter and appropriate lengths of line.
(S) Conducting tree elements may be affixed to the ends of the current driver conducting arms to ~ove the capacitive coupling further out toward the ends of the 11 arms, thus reducing by as much as one-third the current driver length required to receive or transmit at a given 13 frequency. The communication system described herein may 14 be used in a surface deployed mode (i.e., flat on the ground) in an aboveground, low profile mode, or it may be buried under the earth near the earth air interface.
(S) Further operating efficiency is achieved in the 18 underground deployed system by placing porous, low conductivity material adjacent the entire upper portion of the current driver, outward from the central portion 21 of its sides and below its central portion. By this 22 means, the flow of unwanted cancellation currents above 23 and along side the current driver are partially prevented, as is the induction of undesirable ground currents close to the current driver's center. Addition ~6 of a drainage means below the low conductivity material 1 C~
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(S) 1 and optional placement of water resistant material above 2 the current driver prevent pooling of water around the 3 current driver and permit all weather operation.
(U) 4 Accordingly, it is a primary object of the present invention to provide a communication system and method 6 which permits high quality, two-way communication while 7 having the capability of surviving all but a direct hit 8 by a nuclear weapon.
(U) g It is another object of the present invention to provide for high quality, long distance atmospheric 11 transmission of communication signals between underground 12 or aboveground, low profile sending and receiving ter-13 minals, (U) 14 It is still another object of the present invention to provide a communication system having underground or 16 aboveground, low profile sending and receiving terminals 17 capable of sending and receiving signals on a wide 18 frequency band at data rates which are comparable to 19 aboveground communication systems.
(U) 20 It is a further obiect of the present invention to 21 provide a communication system having underground or 22 aboveground, low profile sending and receiving terminals 23 which are configured so as to significantly reduce the 24 current driver length required to transmit and receive at specified frequencies.
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53 o2 (U) 1 Another object of the present invention is to 2 provide an undergrcund or aboveground, low profile 3 communication system which may be configured to produce 4 either bidirectional or unidirectional vertically polarized electromagnetic signals and which may be 6 operated in substantially all weather conditions without 7 significant degradation of signal quality.
(U) 8 It is still another object of the present invention 9 to provide a communication system which has a low suscep-tibility to jamming or to interfering electromagnetic 11 pulse radiation.
(U) 12 Still a further object of the present invention is 13 to provide a long distance communication system using an 14 underground or aboveground, surface proximity deployed current driver arrangement which makes redundancy for 16 purposes of system security economically feasible.
(U) 17 It is still another object of the present invention 18 to provide a communication system which permits initial 19 construction at low cost and which has a very low mainte-nance cost, thus making the system highly desirable in 21 both military and commercial broadcasting and receiving 22 systems.
(U) 23 Another valuable object of the present invention is 24 to provide a high power, long distance, underground or aboveground, surface proximity deployed communication ~2~53 ~;~
(U) 1 system which does not degrade the surrounding environ-2 ment, and which is easily camouflage~.
(U) ~ These and other objects and features of the present 4 invention will become more îully apparent from the following description and appended claims taken in 6 conjunction with the accompanying drawings.
7 BRIEF DESCRIPTION OF THE DR~WINGS (U) (U) 8 Figure l is a schematic illustration of a ground 9 surface deployed current driver ~hich embodies the principles of the present invention.
(U) 11 Figure 2 is a graph which represents the input 12 resistance of a wire loop antenna as a function of the 13 length of the loop antenna perimeter in wavelengths when 14 the loop antenna is located in air.
(U) 15 Figure 3 is a graph which represents the input 16 reactance of a wire loop antenna as a function of the 17 length of the loop antenna perimeter in wavelengths when 18 the loop antenna is located in air, (U) 19 Figure 4 is a schematic illustration of the current driver of Figure l with current and voltage magnitudes 21 represented next to the current paths for the case when 22 the loop is approximately 1.4 wavelengths.
(U) 23 Figure 5 is a schematic illustration of a low 24 profile deployed current driver which embodies the principles of the present invention.
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(U) 1 Figure 6 is a schematic illustration of an end-fed 2 current driver which embodies the principles of the 8 present invention.
(U) 4 Figure 7 is a top plan view of the end-fed current driver of Figure 6.
(U) 6 Figure 8 is a partial cross-sectional view of the 7 middle portion of the end-fed current driver of Figure 6.
(U) 8 Figure 9 is a top plan view of the end-fed current 9 driver of Figure 7, with the resulting electromagnetic signal wave pattern superimposed thereon.
(U) 11 Figure 10 is a schematic representation of a cen-12 ter-fed current driver embodying the principles of the 13 present invention.
(U) 14 Figure 11 is a top plan view of the center-fed current driver of Figure 10.
(U) 16 Figure 12 is a top plan view of the center-fed 17 current driver of Figure 10, with the resulting 18 electromagnetic signal wave pattern superimposed thereon.
(S) 19 Figure 13 is a schematic representation of another preferred embodiment of the present invention comprising 21 a current driver array having several center-fed current 22 drivers positioned in parallel.
(U) 23 Figure 14 is a top plan view of the current driver 2~ array of Figure 13.
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(S) 1 Figure 15 is a top plan view of the current\driver 2 array of Figure 13 with the resulting electromagnetic 1 signal wave pattern superimposed thereon.
(S) 4 Figure 16 is a schematic representation of another preferred embodiment of the present invention comprising 6 a center-fed current driver array, with extensions of the 7 parallel current drivers configured as conductive tree 8 terminations.
(U) 9 Figure 17 is a top plan view of the center-fed current driver array of Figure 16.
(U) 11 Figure 18 is a top plan view of a center-fed current 12 driver array such as that of Figure 16~ wit,h the 13 resulting electromagnetic signal wave pattern 14 superimposed thereon.
(S) 15 Figure 19 is a top plan view of another embodiment 16 of the present invention, comprising a switched current 17 driver array capable of transmitting signals over a 18 relatively wide band of frequencies.
(S) 19 Figure 20 is a schematic representation of another preferred embodiment of the present invention comprising 21 a unidirectional current driver embodying the principles 22 of the present invention.
(U) 23 Figure 21 is a top plan view of the unidirectional 24 current driver of Figure 20, with the resulting electromagnetic signal wave pattern superimposed thereon.
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3 ~ 2 (S) 1 Figure 22 is a top plan view of a unidirectional 2 current driver array, with the resulting electromagnetic 3 signal wave pattern superimposed thereon.
(S) 4 Figure 23 is a schematic illustration of one pre-ferred embodiment of a center-fed current driver in 6 combination with a cancellation current barrier embodying 7 the principles of the present invention.
(U) 8 Figure 24 is a top plan view of the center-fed 9 current driver and cancellation barrier of Figure 23.
(S)10 Figure 25 is a schematic illustration of another 11 preferred embodiment of a center-fed current driver in 12 combination with a cancellation current barrier embodying 13 the principles of the present invention.
(s)14 Figure 26 is a schematic illustration of still another preferred embodiment of a center-fed current 16 driver in combination with a cancellation current barrier 17 embodying the principles of the present invention.
(s)lB Figure 27 is a top plan view of a center-fed current 19 driver array in combination with a cancellation current barrier embodying the principles of the present in-21 vention.
(u)22 Figure 28 is a graph which represents the signal 23 wavelength in the earth as a function of frequency.
(u)24 Figure 29 is a graph which represents the signal penetration or skin depth in the earth as a function of 26 frequency.
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~2~ 3~2 1 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (U) (U) 2 Reference is now made to the drawings wherein like ~ parts are designated with like numerals throughout.
4 1. General Discussion (U) (S) 5 The communication system of the present invention 6 may be generally described by reference to Figures 1 7 through 5. With particular reference to Figure 1, it is 8 seen that a current driver (generally designated at 100) 9 is positioned in proximity to the earth's surface 11.
Current driver 100 is configured similarly to a dipole 11 antenna, in that it has first and second conductors 12 referred to hereinafter as conducting arms 10~ and 104 13 respectively, positioned in end-to-end relationship but 14 separated from electrical contact. Conducting arms 102 and 104 are protected about substantially their entire lfi exterior surface by a sheath of electrical insulating 17 material such as teflon or one of many other commercially 18 available electrical insulators. By this means direct 19 electrical contact with the earth 11 is prevented, while capacitive coupling of the current driver 100 to the 21 earth 11 is encouraged.
(S) 22 The adjacent ends of conducting arms 102 and 104 are 23 connected to a balun 106 that provides impedance matching 24 as well as balance-to-unbalance transformer action in a manner to be described more fully hereinafter. The balun 26 106 preferably should be ca~able of ~proximately lZ~
(S) l matching all impedance values of the curren ~ lver lO0 2 and the earth 11 which may exist beyond one loop 3 wavelength at the operating frequency to the impedance of 4 a signal source (not shown) which is connected to balun 106.
(S) 6When the impedance of the current driver 100 and 7 earth ll are appro~imately matched by use of balun 106 to 8 the impedance of a signal source (not shown), ground 9 currents 108 are induced into the surrounding earth 11 through current driver 100. If the length of conducting 11arms 102 and 104 is sufficiently long, ground currents 12 108 will define a loop whose perimeter is of a size 13sufficient to cause the earth 11 and current driver 100 14 to essentially function together as a vertical plane polarized antenna. The physical means by which this is 16 accomplished may best be described by reference to 17 Figures 2 through 4.
(S) 18Figure 2 illustrates the input resistance of a 19 square wire loop antenna in air as a function of the loop perimeter, while Figure 3 illustrates the input reactance 21 of such an antenna. The peak values of ~esistance and 22 reactance are reduced when the antenna is near or below 23 the earth's surface. Peak resistance in this case would 24 be about 500 ohms. Dipole radiators are generally half wave in length at a given operating frequency, and are 26 operated at a frequency close to the point where the ., .. , .. , .. . , .. .. .. , . . . . .. . . . .. , ~ ............................ . .. .. .. . .. .. .. .
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12~53 ~ 2 (S) 1 reactance is near zero ohms. This point is i~lustrated 2at location 112 in Figure 2 and at 113 in Figure 3. In ~ this situation, a center-fed dipole antenna will have 4 maximum currents and minimum voltage levels at the center feed point, and minimum current with maximum voltage 6 levels àt the ends of the conducting arms. Ma.ximum 7 signal radiation will thus occur outwardly in a direction 8 substantially normal to the central axis of the dipole antenna.
(S) 10In contrast, the current driver 100 of the present 11system is operated in the region above one wavelength 12loop perimeter where the resistance and reactance are 13high. This is is essentially opposite to the 14above-described operating conditions for a typical dipole 15antenna. In this case, the impedance of the current 16driver 100 and the earth is approximately matched to that 17of the signal source (normally a transmitter/receiver), 18by means of the balun and impedance transformer 106. The l9current driver 100 is configured so as to have an overall 20length which is sufficient to induce ground currents into 21the earth 11 by means of capacitive coupling so as to 22form a current loop having a perimeter which is in excess 23of one wavelength in the ground at the system operating 24frequency. The current driver operating range in which 25this result c ~ be accomplished includes substantially 26all conditions w ~ the wavelen~th is ~reater than one, I ~l ~ 53.~
(S) 1 as is partially illustrated at 114 in Figure 2 and at 115 2 in Figure 3. The point at which peak operation of the ~ current driver lOO as a loop is approached is illustrated 4 at 116 in Figure 2 and at 117 in Figure 3. This position defines a current loop perimeter which is about 1.4 6 wavelengths of the operating frequency in the earth. The 7 system also approaches peak operating conditions at just 8 less than the half wavelength point between each 9 additional full wavelength (i.e. 2.5, 3.5, 4.5 et. seq.).
When the length of the current driver 100 exceeds about ]1 one wavelength, the antenna formed by the current driver 12 100 and the earth begins to operate as a long-wire guided 13 wave antenna with greatly increased antenna gain.
(S) 14 The particular voltage and current waveforms which are present when current driver 100 is operated at 16 operating point 116 can be described by reference to 17 Figure 4. In this operating condition, the primary 18 current path induced by current driver 100 is illustrated ~9 at 127. The current standing wave is indicated at 118.
Likewise, the voltage field produced when current driver 21 100 is fed at point 106 is illustrated at 122. In this 22 operating configuration it can be seen that voltage 23 minimums occur near the positions identified at 125. The 24 voltage on antenna element 100 causes a resulting ground current to be induced in the earth by means of capacitive 26 coupling; the resulting current forming a loop 127 which .
(S) l extends into the earth 11 with its ends coupled to the 2 current driver lOO in the vicinity of 128. The current ~ in loop 127 additionally travels along the body of 4 current driver lOO in the direction illustra~ed by the arrows in Figure 4, so as to form a vertical plane loop.
(S) 6 It should be noted that the position along current 7 driver lO0 of the current maximum and voltage minimum 125 8 varies in response to variation of frequency, being at 9 the center for one wavelength loop perimeter and moving outward toward the ends as the frequency increases.
11 Thus, for proper operation the current driver 100 should 12 be of sufficient length to permit capacitive coupling in 13 the preferential coupled region of the driver lOO.
(S) 14 When the current driver lOO is long compared to a lS wavelength there is a substantial increase in antenna l6 gain due to the current driver and earth acting as a 17 long-wire guided wave antenna. Thus, there is a low peak 18 response when the loop perimeter is about 1.4 19 wavelengths, followed by a partial null when the loop perimeter is about two wavelengths. This is then 21 followed by a substantial increase in antenna gain as the 22 antenna begins to operate as a long-wire guided wave 23 antenna. The performance typically continues to increase 24 with increasing frequency until a maximum is reached in the 15 to 20 MHz range. The gain then slowly drops off r'~ ~
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(S) 1 with increasing frequency because antenna losses ibcrease 2 faster than directional gain.
(U) 3Examples of conventional aboveground long-wire 4 guided wave antennas are set forth in chapter 5 of 5.~ntenna Theory and Design, (pages 239-244) by Warren L.
6 Stutzman and Gary A. Thiele (John Wiley and Sons 1981).
(S) 7The current and voltage relationships described 8 above remain substantially the same for given earth 9 conditions, and for current drivers of a given length, no matter whether the current driver is positioned upon the 11 surface of the earth as illustrated in Figure 4, or 12 whether it is positioned so as to have a portion of its 13 length in proximity to but above the surface of the earth 14 as illustrated in Figure 5, or whether it is buried beneath the surface of the earth close to the interface 16 as will be described more fully hereinafter.
(S) 17By reference to Figure 5 one low profile embodiment 18 of the current driver 100 is illustrated. In this 19 embodiment balun 106 is positioned on or near the surface of the earth 11, with the inner portions of insulated 21conducting arms 102 and 104 being supported above the 22 surface of the earth by means of nonconducting or low 23 conducting posts or stakes 124. The outer portions of 24conducting arms 102 and 104 are positioned upon the surface of the earth 11. In this manner, capacitive 26coupling between current driver 100 and earth 11 only (S) 1 occurs along those portions of conduc~ing arms 102 and 2 104 which are positioned upon the earth itself. Thus, in 3 the configuration of Figure 5, ground currents are 4 induced into the earth so as to form current loops 108 which e~tend only between the outer portions of 6 conducting arms 102 and 104. By this means, interior 7 currents which normally travel between the inner portions 8 Of arms 102 and 104, and which can cause losses are 9 eliminated. Thus, a more clearly defined vertical plane polarized antenna is defined by the currents 108 and 11 current driver 100.
(U) 12 Particular examples of several preferred embodiments 13 of the underground configuration of the current driver 14 100 are more fully described hereinafter. Examples of 15 low profile configurations of current drivers which 16 embody the principles of the present invention are 17 outlined in the applicant~s U.S. Patent No. 4,809,010 18 which issued on 28 February, 1989.
21 2. The End-Fed Current Driver (Figures 6 throu~h 9) (U) (S) 22 One preferred embodiment of a communication system 23 configuration of the present invention is illustrated in 24 Figures 6 through 9. With reference to Figures 6 and 7, 25 a current driver (generally designated at 10) is illus-~6 trated as being general a dipole. The ~3 12~53, 2 (S) 1 current driver 10 may be constructed using insulated 2 cables or insulated pipes.
(S) 3As shown in Figure 6, the current driver 10 is 4 buried below the earth's surface 11. The dèpth of b~rial is typically between zero and ten meters. For example, 6 the required hardness for withstanding a near miss in 7 many applications would require a burial depth of about 8 one meter. The method of burial may be by conventional 9 trenching and refilling where the ground is soft (i.e., sand, gravel or the like) or by conventional rotary 11 drilling where the ground is rocky or hard. When using 12 trenching as the method of burial, it is typical to use 13 wires or cables for the current driver elements. When 14 using rotary drilling, it is typical to use pipe for the current driver elements. In either case, the current 16 driver 10 is buried substantially parallel to the surface 17 of the earth 11.
(S) 18Approximately at its mid point 12 current driver 10 l9 is provided with a dielectric partition 21 (see Figure 8). As shown best in Figure 8, the dielectric partition 2121 is threadingly connected between the pipes 16 and 18 22 which define the upper and lower conducting arms of 23 current driver 10, respectively. With further reference 24 to Figure 8 a coaxial cable 14 extends through the center of current driver 10. The inner conductor 20 of coaxial 26 cable 14 extends through the dielectric partition 21 and 3-~ 2 (S) 1 is secured to the lower conducting arm defined by yipe 2 18. The outer conductor ~2 of coaxial cable 14 is ~ secured to the upper conducting arm defined by pipe 16.
(S) 6Coaxial cable 14 is connected at its upper end to an impedance matching circuit 17 (see Figures 6 and 7) that 6 is used to adjust impedances so as to approximately match 7 or correlate the impedance of the current driver 10 and 8 the earth to the impedance of transmitter/receiver 13.
9 The impedance matching circuit 25 may be, for example, a transmission line transformer balun that provides 11 impedance matching as well as balance-to-unbalance 12 transformer action, as described in the text Solid State 13Radio Engineering, (pages 371 to 379) by Herbert L.
14 Krauss, Charles W. Bostian and Frederick H. Raab (John Wiley and Sons 1980). As hereinafter more fully 16 explained, this impedance matching technique greatly 17 improves the performance of the communication system.
(S) 18Impedance matching circuit Z5 is connected through l9 cable 14 to the transmitter/receiver 13, which comprises a conventional transmitter for sending electromagnetic 21 signals and a conventional receiver for processing 22 received electromagnetic signals. Transmitter/receiver 23 13 may be positioned below the surface of the ground 11 2~ and adjacent to one end of the antenna current driver 10, and it may be interconnected with other communication ~ 5 ~LZ~53*2 (S) 1 equipment (not shown) located on the surface of the 2 ground 11 by means of cables 19.
(S) ~ ~n alternative configuration uses a standard 50 ohm 4 coax line 14 which passes through the upper conductor 16 and is connected to a small transmission line transformer 6 balun 17 located in the dielectric section 21. This 7 balun in turn connects to upper and lower conductors 16 8 and 18 and provides the necessary unbalance-to-balance 9 line transformation and the necessary impedance transformation to match the impedance of the current 11 driver and earth to the coax line. This avoids the need 12 for high impedance coax as would be required in the 13 configuration of Figure 8.
(S) 14 In one presently preferred embodiment the conducting pipes or arms 16 and 18 and the dielectric partition 21 16 of current driver 10 are protected by an external sheath 17 of electrical insulating material 24 (see Figure 8).
18 Sheath 24 prevents short circuiting of the current driver 19 by water that may collect in the bore hole or trench, and it helps to prevent corrosion. In addition, sheath 24 21 encourages capacitive coupling of the current driver 10 22 to the surrounding ground during system operation.
(S) 23 Sheath 24 may consist of preformed tapes or sheets 24 of electrical insulating material wrapped about the surface of current driver 10. Alternatively, sheath 24 26 may be formed by immersing current driver 10 in a liquid ~ . . . . . .
.. . .
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~2~3 ~ 2 (S) l bath of electrical insulating material, removing it and 2 then permitting it to dry. Other methods are also known ~ and used for providing such protection.
(5) 4Although current driver 10 appears to be physically constructed as a dipole antenna, when the impedance of 6 the current driver 10 and earth is approximately matched 7 to the impedance of the transmitter/receiver 13, and when 8 arms 16 and 18 are of a length sufficient to induce a 9 current loop in the earth so that the current loop has a perimeter which is greater in length than one wavelength 11 of the electromagnetic signals in earth, then current 12 driver lO does not act as a dipole antenna. Under these 13 conditions, the interaction between current driver lO and 14 the surrounding earth causes current driver 10 and the earth to effectively function together as a vertical 16 plane polarized antenna which produces a vertically 17 polarized electromagnetic wave. The current loop is 18 formed as a result of capacitive coupling which occurs l9 between the conducting arms 16 and 18 of current driver 10 and the surrounding earth. For good performance the 21 capacitive reactance must be small with respect to the 22 current driver resistance.
(S) 23The current loop action is illustrated by reference 24to the ground current loop paths 15 and 23 of Figures 6 and 7. Current loop paths 15 represent vertical paths, 26 while current loop paths 23 represent current paths to (S) l the sides of current driver lO. In the embodiment of 2 Figure 8 the electrical current from the ~ transmitter/receiver 13 passes by way of coaxial cable 14 4 to one of the conducting arms 16 or 18 of current driver lO, as for example conducting arm 16 which is connected 6 to conductor 22 of the coa~ial cable 14. The current is 7 capacitively coupled to the surrounding earth through 8 which it travels along current loop paths 15 and 23 to 9 the other conducting arm 18. The current is then capacitively coupled to conducting arm 18, from which it ll returns to the generator of transmitter/receiver 13 by 12 way of conductor 20 (see Figure 8).
(S) 13 As shown by the current loop paths 15 and 23 in 14 Figures 6 and 7, electrical currents flow on all sides of current driver lO. However, the electromagnetic waves 16 from current loop paths 23 (see Figure 7) cancel each 17 other so that no resultant signal is produced since the 18 current loop paths on one side of current driver 10 are 19 opposite in direction to the current loop paths on the other side oE current driver 10, as schematically shown 21 by the directional arrows. As is also schematically 22 illustrated by the directional arrows on the ground 23 current loop paths 15 (see Figure 6), the top and bottom 24 loops are also opposite their respective directions.
However, as shown in Figure 6, when current driver lO is 26 positioned near the ground surface 11 the area which 53~2 (S) 1 defines the path of the upper ground currènts becomes 2 smaller. Thus, the upper ground currents do not signifi-3 cantly cancel the electromagnetic waves resulting from 4 the bottom or lower ground currents. Thus, the lower ground currents act together with conductive arms 16 and 6 18 to define a vertical plane loop which produces verti-7 cally polarized electromagnetic signals.
(S) 8 The electromagnetic waves emitted from the commu-9 nication system are subject to slightly reduced gain as a result of the cancellation effects of the upper loop with 11 respect to the larger lower loop of vertical paths 15.
(U) 24 Such inadequate performance characteristics are embodied in the various wireless subterranean signaling 26 systems which have been proposed in the past, wherein 129S3 ~Z
(U) 1 electromagnetic signals are transmitted through the earth 2 between underground antennas. For example, electro-~ magnetic wave~ of relatively low frequencies ranging from 4 lOO ~z to lOO KHz have been propagated through the earth between horizontally polarized electric dipole antennas 6 buried in the earth. Such underground transmission of 7 signals is inherently susceptible to significant signal 8 attenuation due to the large dielectric coefficient and 9 high conductivity of the earth. This is due to the fact that in a conductive (i.e., lossy) medium such as the 1~ earth or water, energy is dissipated through currents 12 that are generated by the electric and magnetic field 13 components of the wave. This energy loss results in an 14 appreciable exponential attenuation of field strength with distance. In contrast, electromagnetic waves 16 propagated through the atmosphere lose little energy to 17 the medium. Thus, excess attenuation beyond inverse R2 18 loss is negligible in the atmospheric case except at 19 microwave and higher frequencies.
(U) 20 In order to achieve system hardness while utilizing 21 the atmosphere for signal transmission, several past 22 proposals have involved the positioning of a dipole 23 antenna upon or beneath the surface of the earth. Such 24 systems have experienced significant reduction in signal strength as compared with aboveground, vertically ,.
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(U) 1 oriented dipole antennas as a result of signal 2 attenuation and losses in the earth.
(U) 3 Comparisons of the performance of subsurface dipole 4 antennas to conventional above surface antennas are presented in Fenwick and Weeks, Submerged Antenna 6 Characteristics, I.E.E.E. TR~NSACTIONS ON ANTENNAS AND
7 PROPAGATION, p. 296 (May, 1963), where it is seen that in 8 many common situations the strength of the underground 9 produced signal is more than 40 dB weaker than the signal produced by the reference antenna, which is a perfect 11 quarter-wave vertical monopole antenna. Such reduced 12 signal power is simply not acceptable for many 13 communications systems applications, especially when such 14 applications may involve long-range signal tranmission.
In addition to the above problem, dipole antennas produce 16 electromagneti.c signals which propagate in directions 17 generally normal to the longitudinal axis of the dipole 18 antenna. As a result, much of the signal strength is l9 directed substantially straight upwards or into the ground where it is lost, resulting in significant amounts 21 of power loss and reduced efficiency in the communication 22 system.
(U) 23 In order to provide an underground antenna system 24 while transmitting usable signals through the atmosphere, it has been proposed in the prior art to employ a buried 26 loop antenna for generating a horizontally polarized lZ~!53 ,z ~
(U) 1 magnetic wave which in turn generates a surface wave 2 having a vertically polarized electric component to be ~ received by a vertical whip receiving antenna. ~lthough 4 a substantial portion of the resulting signal propagates along the earth's surface, this antenna system still is 6 very low in efficiency which is mainly a result of the 7 use of horizontally polarized waves and the losses 8 associated therewith. Another disadvantage of this type 9 of prior art antenna is the very large physical antenna size needed at low and even medium range frequencies.
(S) 11 It becomes clear that the most efficient means for 12 obtaining system hardness while providing for trans-13 mission and reception of elec~romagnetic signals through 14 the atmosphere would be to utilize a buried wire loop or traveling wave antenna which could produce vertical plane 16 polarized electromagnetic signals. However, it is well 17 known that the size of such an antenna is directly 18 related to the wavelength of the signal in earth at the l9 frequency of operation. In fact, for optimal operation the perimeter length oP the underground loop antenna 21 should be approximately 1.4 wavelengths in earth at the 22 operating frequency. For an operating frequency in the 23 MF range of 400 KHz, the necessary loop antenna perimeter 24 length would be approximately lOO meters. It becomes immediately apparent that, even if physically possible, 26 the cost of trenching, supporting and burying such an , . . . ~ .. , . ..... . , .. , . .. ... . . .. . . , ,~.. ... .. ... . . . . ... .. .
c `- l;~gS3 ~2 (S) 1 antenna in the vertical position would mal;e use of the 2 antenna unrealistic if n~t impossible.
(U) ~A further problem that is common to all underground 4 antennas is the lower power gain which is experienced as a result of signal attenuation prior to signal entry into 6 the atmosphere. Although this attenuation can be mini-7 mized by positioning the antenna close to the surface, it 8 still exists in significant amounts. No adequate method 9 has heretofore been found for substantially increasing the gain of atmospheric transmission signals emanating 11 from an underground source and thus, this reduced perfor-12 mance capability has continued to be a long-standing, 13 unresolved problem in the art.
(U) 14In light of the above considerations, it is apparent that the great need that has heretofore gone unsatis~ied 16 is to provide a two-way, wireless underground or near 17 surface aboveground deployed communication system capable 18 of effectively receiving and transmitting signals over a 19 wide band of frequencies, with the system being suffi-ciently "hard" to withstand a near miss of a nuclear 21 weapon. The system should have reduced vulnerability to 22 jamming and, even immediately following a nuclear explo-23 sion, should permit long distance transmission of 24 electromagnetic signals with reasonable data rates. The system should be capable of transmitting communication 26 signals in either broad or narrow beam configurations, ~29S~ ~2 ~U) 1 and in conjunction with enhanced signal processing, 2 should be capable of performance comparable to e~isting 3 aboveground antenna systems. Furthermore, the system 4 should feasibly permit redundancy sufficient to satisfy the need for system security without excessive costs.
6 The underground communication systems heretofore employed 7 have not been able to satisfy these important needs.
8 BRIEF SUMMARY ~D OBJECTS OF THE I~E~TION (U) (S) 9 The present invention advantageously provides a unique two-way, wireless communication system and method 11 that employs an underground or surface or low profile 12 current driver arrangement for sending and receiving 13 electromagnetic signals propagated through the atmo-14 sphere. The system is operable on a wide frequency band, and produces steerable signals having a power gain and 16 data rates comparable to signals transmitted from more 17 conventional aboveground systems.
(5) 18 The present invention involves use of at least one 19 current driver comprising a pair of conductors and a balun positioned flat upon the earth's surface, or 21 positioned above but in proximity to the earth's surface 22 or buried beneath the surface of the ground in proximity 23 to the earth's surface. The elements of the current 24 driver are constructed so that it is capacitively coupled with the surrounding earth in a manner such that induced 26 ground currents cause the current driver and the earth to lZ9~i3, ~ ,~
(S) l effectively function together as a vertical plane 2 polarized loop antenna. A vertically polari ed signal is 3 transmitted from this antenna into the atmosphere where 4 it continues to propagate as a vertically polarized wave.
(S) 5 In one preferred embodiment, the two current driver 6 conducting arms are positioned end-to-end and joined at 7 the middle to a balance-to-unbalance ("balun") impedance 8 matching transformer. When the current driver is thus 9 configured, with the impedance of the current driver and earth substantially matched or correlated to that of a 11 transmitter/receiver connected thereto, the capa.itively 12 coupled current causes the current driver and the earth 13 to effectively function as a vertical plane loop antenna 14 which produces a bidirectional vertically polarized electromagnetic signal.
(S) 16 In another preferred embodiment, the length of one 17 of the two current driver conducting arms is substantial-18 ly shorter than the other conducting arm. A length ratio 19 for the arms of approximately 2 to l has been found to be adequate for many applications while larger ratios 21 provide further improvement in antenna gain. In this 22 configuration, the capacitively coupled current causes 23 the current driver and the earth to effectively function 24 as a long-wire traveling wave antenna which produces a substantially unidirectional vertically polarized ~6 electromagnetic si ~ 1. For a length ratio of 2 to 1 the Cl lZ953~2 (S) power gain is approximately 4 dB greater than the above-described bidirectional signal.
(5) ~ Power gain in each of the embodiments is further improved by positioning several current drivers parallel to one another and at spacings which may be as close as half a skin depth in ground. These are fed in parallel using a power splitter and appropriate lengths of line.
(S) Conducting tree elements may be affixed to the ends of the current driver conducting arms to ~ove the capacitive coupling further out toward the ends of the 11 arms, thus reducing by as much as one-third the current driver length required to receive or transmit at a given 13 frequency. The communication system described herein may 14 be used in a surface deployed mode (i.e., flat on the ground) in an aboveground, low profile mode, or it may be buried under the earth near the earth air interface.
(S) Further operating efficiency is achieved in the 18 underground deployed system by placing porous, low conductivity material adjacent the entire upper portion of the current driver, outward from the central portion 21 of its sides and below its central portion. By this 22 means, the flow of unwanted cancellation currents above 23 and along side the current driver are partially prevented, as is the induction of undesirable ground currents close to the current driver's center. Addition ~6 of a drainage means below the low conductivity material 1 C~
lZ~53~Z
(S) 1 and optional placement of water resistant material above 2 the current driver prevent pooling of water around the 3 current driver and permit all weather operation.
(U) 4 Accordingly, it is a primary object of the present invention to provide a communication system and method 6 which permits high quality, two-way communication while 7 having the capability of surviving all but a direct hit 8 by a nuclear weapon.
(U) g It is another object of the present invention to provide for high quality, long distance atmospheric 11 transmission of communication signals between underground 12 or aboveground, low profile sending and receiving ter-13 minals, (U) 14 It is still another object of the present invention to provide a communication system having underground or 16 aboveground, low profile sending and receiving terminals 17 capable of sending and receiving signals on a wide 18 frequency band at data rates which are comparable to 19 aboveground communication systems.
(U) 20 It is a further obiect of the present invention to 21 provide a communication system having underground or 22 aboveground, low profile sending and receiving terminals 23 which are configured so as to significantly reduce the 24 current driver length required to transmit and receive at specified frequencies.
~ \
53 o2 (U) 1 Another object of the present invention is to 2 provide an undergrcund or aboveground, low profile 3 communication system which may be configured to produce 4 either bidirectional or unidirectional vertically polarized electromagnetic signals and which may be 6 operated in substantially all weather conditions without 7 significant degradation of signal quality.
(U) 8 It is still another object of the present invention 9 to provide a communication system which has a low suscep-tibility to jamming or to interfering electromagnetic 11 pulse radiation.
(U) 12 Still a further object of the present invention is 13 to provide a long distance communication system using an 14 underground or aboveground, surface proximity deployed current driver arrangement which makes redundancy for 16 purposes of system security economically feasible.
(U) 17 It is still another object of the present invention 18 to provide a communication system which permits initial 19 construction at low cost and which has a very low mainte-nance cost, thus making the system highly desirable in 21 both military and commercial broadcasting and receiving 22 systems.
(U) 23 Another valuable object of the present invention is 24 to provide a high power, long distance, underground or aboveground, surface proximity deployed communication ~2~53 ~;~
(U) 1 system which does not degrade the surrounding environ-2 ment, and which is easily camouflage~.
(U) ~ These and other objects and features of the present 4 invention will become more îully apparent from the following description and appended claims taken in 6 conjunction with the accompanying drawings.
7 BRIEF DESCRIPTION OF THE DR~WINGS (U) (U) 8 Figure l is a schematic illustration of a ground 9 surface deployed current driver ~hich embodies the principles of the present invention.
(U) 11 Figure 2 is a graph which represents the input 12 resistance of a wire loop antenna as a function of the 13 length of the loop antenna perimeter in wavelengths when 14 the loop antenna is located in air.
(U) 15 Figure 3 is a graph which represents the input 16 reactance of a wire loop antenna as a function of the 17 length of the loop antenna perimeter in wavelengths when 18 the loop antenna is located in air, (U) 19 Figure 4 is a schematic illustration of the current driver of Figure l with current and voltage magnitudes 21 represented next to the current paths for the case when 22 the loop is approximately 1.4 wavelengths.
(U) 23 Figure 5 is a schematic illustration of a low 24 profile deployed current driver which embodies the principles of the present invention.
lZ9~3 ~ Z
(U) 1 Figure 6 is a schematic illustration of an end-fed 2 current driver which embodies the principles of the 8 present invention.
(U) 4 Figure 7 is a top plan view of the end-fed current driver of Figure 6.
(U) 6 Figure 8 is a partial cross-sectional view of the 7 middle portion of the end-fed current driver of Figure 6.
(U) 8 Figure 9 is a top plan view of the end-fed current 9 driver of Figure 7, with the resulting electromagnetic signal wave pattern superimposed thereon.
(U) 11 Figure 10 is a schematic representation of a cen-12 ter-fed current driver embodying the principles of the 13 present invention.
(U) 14 Figure 11 is a top plan view of the center-fed current driver of Figure 10.
(U) 16 Figure 12 is a top plan view of the center-fed 17 current driver of Figure 10, with the resulting 18 electromagnetic signal wave pattern superimposed thereon.
(S) 19 Figure 13 is a schematic representation of another preferred embodiment of the present invention comprising 21 a current driver array having several center-fed current 22 drivers positioned in parallel.
(U) 23 Figure 14 is a top plan view of the current driver 2~ array of Figure 13.
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(S) 1 Figure 15 is a top plan view of the current\driver 2 array of Figure 13 with the resulting electromagnetic 1 signal wave pattern superimposed thereon.
(S) 4 Figure 16 is a schematic representation of another preferred embodiment of the present invention comprising 6 a center-fed current driver array, with extensions of the 7 parallel current drivers configured as conductive tree 8 terminations.
(U) 9 Figure 17 is a top plan view of the center-fed current driver array of Figure 16.
(U) 11 Figure 18 is a top plan view of a center-fed current 12 driver array such as that of Figure 16~ wit,h the 13 resulting electromagnetic signal wave pattern 14 superimposed thereon.
(S) 15 Figure 19 is a top plan view of another embodiment 16 of the present invention, comprising a switched current 17 driver array capable of transmitting signals over a 18 relatively wide band of frequencies.
(S) 19 Figure 20 is a schematic representation of another preferred embodiment of the present invention comprising 21 a unidirectional current driver embodying the principles 22 of the present invention.
(U) 23 Figure 21 is a top plan view of the unidirectional 24 current driver of Figure 20, with the resulting electromagnetic signal wave pattern superimposed thereon.
I r~
3 ~ 2 (S) 1 Figure 22 is a top plan view of a unidirectional 2 current driver array, with the resulting electromagnetic 3 signal wave pattern superimposed thereon.
(S) 4 Figure 23 is a schematic illustration of one pre-ferred embodiment of a center-fed current driver in 6 combination with a cancellation current barrier embodying 7 the principles of the present invention.
(U) 8 Figure 24 is a top plan view of the center-fed 9 current driver and cancellation barrier of Figure 23.
(S)10 Figure 25 is a schematic illustration of another 11 preferred embodiment of a center-fed current driver in 12 combination with a cancellation current barrier embodying 13 the principles of the present invention.
(s)14 Figure 26 is a schematic illustration of still another preferred embodiment of a center-fed current 16 driver in combination with a cancellation current barrier 17 embodying the principles of the present invention.
(s)lB Figure 27 is a top plan view of a center-fed current 19 driver array in combination with a cancellation current barrier embodying the principles of the present in-21 vention.
(u)22 Figure 28 is a graph which represents the signal 23 wavelength in the earth as a function of frequency.
(u)24 Figure 29 is a graph which represents the signal penetration or skin depth in the earth as a function of 26 frequency.
I ~
~2~ 3~2 1 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (U) (U) 2 Reference is now made to the drawings wherein like ~ parts are designated with like numerals throughout.
4 1. General Discussion (U) (S) 5 The communication system of the present invention 6 may be generally described by reference to Figures 1 7 through 5. With particular reference to Figure 1, it is 8 seen that a current driver (generally designated at 100) 9 is positioned in proximity to the earth's surface 11.
Current driver 100 is configured similarly to a dipole 11 antenna, in that it has first and second conductors 12 referred to hereinafter as conducting arms 10~ and 104 13 respectively, positioned in end-to-end relationship but 14 separated from electrical contact. Conducting arms 102 and 104 are protected about substantially their entire lfi exterior surface by a sheath of electrical insulating 17 material such as teflon or one of many other commercially 18 available electrical insulators. By this means direct 19 electrical contact with the earth 11 is prevented, while capacitive coupling of the current driver 100 to the 21 earth 11 is encouraged.
(S) 22 The adjacent ends of conducting arms 102 and 104 are 23 connected to a balun 106 that provides impedance matching 24 as well as balance-to-unbalance transformer action in a manner to be described more fully hereinafter. The balun 26 106 preferably should be ca~able of ~proximately lZ~
(S) l matching all impedance values of the curren ~ lver lO0 2 and the earth 11 which may exist beyond one loop 3 wavelength at the operating frequency to the impedance of 4 a signal source (not shown) which is connected to balun 106.
(S) 6When the impedance of the current driver 100 and 7 earth ll are appro~imately matched by use of balun 106 to 8 the impedance of a signal source (not shown), ground 9 currents 108 are induced into the surrounding earth 11 through current driver 100. If the length of conducting 11arms 102 and 104 is sufficiently long, ground currents 12 108 will define a loop whose perimeter is of a size 13sufficient to cause the earth 11 and current driver 100 14 to essentially function together as a vertical plane polarized antenna. The physical means by which this is 16 accomplished may best be described by reference to 17 Figures 2 through 4.
(S) 18Figure 2 illustrates the input resistance of a 19 square wire loop antenna in air as a function of the loop perimeter, while Figure 3 illustrates the input reactance 21 of such an antenna. The peak values of ~esistance and 22 reactance are reduced when the antenna is near or below 23 the earth's surface. Peak resistance in this case would 24 be about 500 ohms. Dipole radiators are generally half wave in length at a given operating frequency, and are 26 operated at a frequency close to the point where the ., .. , .. , .. . , .. .. .. , . . . . .. . . . .. , ~ ............................ . .. .. .. . .. .. .. .
....... .
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12~53 ~ 2 (S) 1 reactance is near zero ohms. This point is i~lustrated 2at location 112 in Figure 2 and at 113 in Figure 3. In ~ this situation, a center-fed dipole antenna will have 4 maximum currents and minimum voltage levels at the center feed point, and minimum current with maximum voltage 6 levels àt the ends of the conducting arms. Ma.ximum 7 signal radiation will thus occur outwardly in a direction 8 substantially normal to the central axis of the dipole antenna.
(S) 10In contrast, the current driver 100 of the present 11system is operated in the region above one wavelength 12loop perimeter where the resistance and reactance are 13high. This is is essentially opposite to the 14above-described operating conditions for a typical dipole 15antenna. In this case, the impedance of the current 16driver 100 and the earth is approximately matched to that 17of the signal source (normally a transmitter/receiver), 18by means of the balun and impedance transformer 106. The l9current driver 100 is configured so as to have an overall 20length which is sufficient to induce ground currents into 21the earth 11 by means of capacitive coupling so as to 22form a current loop having a perimeter which is in excess 23of one wavelength in the ground at the system operating 24frequency. The current driver operating range in which 25this result c ~ be accomplished includes substantially 26all conditions w ~ the wavelen~th is ~reater than one, I ~l ~ 53.~
(S) 1 as is partially illustrated at 114 in Figure 2 and at 115 2 in Figure 3. The point at which peak operation of the ~ current driver lOO as a loop is approached is illustrated 4 at 116 in Figure 2 and at 117 in Figure 3. This position defines a current loop perimeter which is about 1.4 6 wavelengths of the operating frequency in the earth. The 7 system also approaches peak operating conditions at just 8 less than the half wavelength point between each 9 additional full wavelength (i.e. 2.5, 3.5, 4.5 et. seq.).
When the length of the current driver 100 exceeds about ]1 one wavelength, the antenna formed by the current driver 12 100 and the earth begins to operate as a long-wire guided 13 wave antenna with greatly increased antenna gain.
(S) 14 The particular voltage and current waveforms which are present when current driver 100 is operated at 16 operating point 116 can be described by reference to 17 Figure 4. In this operating condition, the primary 18 current path induced by current driver 100 is illustrated ~9 at 127. The current standing wave is indicated at 118.
Likewise, the voltage field produced when current driver 21 100 is fed at point 106 is illustrated at 122. In this 22 operating configuration it can be seen that voltage 23 minimums occur near the positions identified at 125. The 24 voltage on antenna element 100 causes a resulting ground current to be induced in the earth by means of capacitive 26 coupling; the resulting current forming a loop 127 which .
(S) l extends into the earth 11 with its ends coupled to the 2 current driver lOO in the vicinity of 128. The current ~ in loop 127 additionally travels along the body of 4 current driver lOO in the direction illustra~ed by the arrows in Figure 4, so as to form a vertical plane loop.
(S) 6 It should be noted that the position along current 7 driver lO0 of the current maximum and voltage minimum 125 8 varies in response to variation of frequency, being at 9 the center for one wavelength loop perimeter and moving outward toward the ends as the frequency increases.
11 Thus, for proper operation the current driver 100 should 12 be of sufficient length to permit capacitive coupling in 13 the preferential coupled region of the driver lOO.
(S) 14 When the current driver lOO is long compared to a lS wavelength there is a substantial increase in antenna l6 gain due to the current driver and earth acting as a 17 long-wire guided wave antenna. Thus, there is a low peak 18 response when the loop perimeter is about 1.4 19 wavelengths, followed by a partial null when the loop perimeter is about two wavelengths. This is then 21 followed by a substantial increase in antenna gain as the 22 antenna begins to operate as a long-wire guided wave 23 antenna. The performance typically continues to increase 24 with increasing frequency until a maximum is reached in the 15 to 20 MHz range. The gain then slowly drops off r'~ ~
~53 ,~ ~
(S) 1 with increasing frequency because antenna losses ibcrease 2 faster than directional gain.
(U) 3Examples of conventional aboveground long-wire 4 guided wave antennas are set forth in chapter 5 of 5.~ntenna Theory and Design, (pages 239-244) by Warren L.
6 Stutzman and Gary A. Thiele (John Wiley and Sons 1981).
(S) 7The current and voltage relationships described 8 above remain substantially the same for given earth 9 conditions, and for current drivers of a given length, no matter whether the current driver is positioned upon the 11 surface of the earth as illustrated in Figure 4, or 12 whether it is positioned so as to have a portion of its 13 length in proximity to but above the surface of the earth 14 as illustrated in Figure 5, or whether it is buried beneath the surface of the earth close to the interface 16 as will be described more fully hereinafter.
(S) 17By reference to Figure 5 one low profile embodiment 18 of the current driver 100 is illustrated. In this 19 embodiment balun 106 is positioned on or near the surface of the earth 11, with the inner portions of insulated 21conducting arms 102 and 104 being supported above the 22 surface of the earth by means of nonconducting or low 23 conducting posts or stakes 124. The outer portions of 24conducting arms 102 and 104 are positioned upon the surface of the earth 11. In this manner, capacitive 26coupling between current driver 100 and earth 11 only (S) 1 occurs along those portions of conduc~ing arms 102 and 2 104 which are positioned upon the earth itself. Thus, in 3 the configuration of Figure 5, ground currents are 4 induced into the earth so as to form current loops 108 which e~tend only between the outer portions of 6 conducting arms 102 and 104. By this means, interior 7 currents which normally travel between the inner portions 8 Of arms 102 and 104, and which can cause losses are 9 eliminated. Thus, a more clearly defined vertical plane polarized antenna is defined by the currents 108 and 11 current driver 100.
(U) 12 Particular examples of several preferred embodiments 13 of the underground configuration of the current driver 14 100 are more fully described hereinafter. Examples of 15 low profile configurations of current drivers which 16 embody the principles of the present invention are 17 outlined in the applicant~s U.S. Patent No. 4,809,010 18 which issued on 28 February, 1989.
21 2. The End-Fed Current Driver (Figures 6 throu~h 9) (U) (S) 22 One preferred embodiment of a communication system 23 configuration of the present invention is illustrated in 24 Figures 6 through 9. With reference to Figures 6 and 7, 25 a current driver (generally designated at 10) is illus-~6 trated as being general a dipole. The ~3 12~53, 2 (S) 1 current driver 10 may be constructed using insulated 2 cables or insulated pipes.
(S) 3As shown in Figure 6, the current driver 10 is 4 buried below the earth's surface 11. The dèpth of b~rial is typically between zero and ten meters. For example, 6 the required hardness for withstanding a near miss in 7 many applications would require a burial depth of about 8 one meter. The method of burial may be by conventional 9 trenching and refilling where the ground is soft (i.e., sand, gravel or the like) or by conventional rotary 11 drilling where the ground is rocky or hard. When using 12 trenching as the method of burial, it is typical to use 13 wires or cables for the current driver elements. When 14 using rotary drilling, it is typical to use pipe for the current driver elements. In either case, the current 16 driver 10 is buried substantially parallel to the surface 17 of the earth 11.
(S) 18Approximately at its mid point 12 current driver 10 l9 is provided with a dielectric partition 21 (see Figure 8). As shown best in Figure 8, the dielectric partition 2121 is threadingly connected between the pipes 16 and 18 22 which define the upper and lower conducting arms of 23 current driver 10, respectively. With further reference 24 to Figure 8 a coaxial cable 14 extends through the center of current driver 10. The inner conductor 20 of coaxial 26 cable 14 extends through the dielectric partition 21 and 3-~ 2 (S) 1 is secured to the lower conducting arm defined by yipe 2 18. The outer conductor ~2 of coaxial cable 14 is ~ secured to the upper conducting arm defined by pipe 16.
(S) 6Coaxial cable 14 is connected at its upper end to an impedance matching circuit 17 (see Figures 6 and 7) that 6 is used to adjust impedances so as to approximately match 7 or correlate the impedance of the current driver 10 and 8 the earth to the impedance of transmitter/receiver 13.
9 The impedance matching circuit 25 may be, for example, a transmission line transformer balun that provides 11 impedance matching as well as balance-to-unbalance 12 transformer action, as described in the text Solid State 13Radio Engineering, (pages 371 to 379) by Herbert L.
14 Krauss, Charles W. Bostian and Frederick H. Raab (John Wiley and Sons 1980). As hereinafter more fully 16 explained, this impedance matching technique greatly 17 improves the performance of the communication system.
(S) 18Impedance matching circuit Z5 is connected through l9 cable 14 to the transmitter/receiver 13, which comprises a conventional transmitter for sending electromagnetic 21 signals and a conventional receiver for processing 22 received electromagnetic signals. Transmitter/receiver 23 13 may be positioned below the surface of the ground 11 2~ and adjacent to one end of the antenna current driver 10, and it may be interconnected with other communication ~ 5 ~LZ~53*2 (S) 1 equipment (not shown) located on the surface of the 2 ground 11 by means of cables 19.
(S) ~ ~n alternative configuration uses a standard 50 ohm 4 coax line 14 which passes through the upper conductor 16 and is connected to a small transmission line transformer 6 balun 17 located in the dielectric section 21. This 7 balun in turn connects to upper and lower conductors 16 8 and 18 and provides the necessary unbalance-to-balance 9 line transformation and the necessary impedance transformation to match the impedance of the current 11 driver and earth to the coax line. This avoids the need 12 for high impedance coax as would be required in the 13 configuration of Figure 8.
(S) 14 In one presently preferred embodiment the conducting pipes or arms 16 and 18 and the dielectric partition 21 16 of current driver 10 are protected by an external sheath 17 of electrical insulating material 24 (see Figure 8).
18 Sheath 24 prevents short circuiting of the current driver 19 by water that may collect in the bore hole or trench, and it helps to prevent corrosion. In addition, sheath 24 21 encourages capacitive coupling of the current driver 10 22 to the surrounding ground during system operation.
(S) 23 Sheath 24 may consist of preformed tapes or sheets 24 of electrical insulating material wrapped about the surface of current driver 10. Alternatively, sheath 24 26 may be formed by immersing current driver 10 in a liquid ~ . . . . . .
.. . .
~ c~
~2~3 ~ 2 (S) l bath of electrical insulating material, removing it and 2 then permitting it to dry. Other methods are also known ~ and used for providing such protection.
(5) 4Although current driver 10 appears to be physically constructed as a dipole antenna, when the impedance of 6 the current driver 10 and earth is approximately matched 7 to the impedance of the transmitter/receiver 13, and when 8 arms 16 and 18 are of a length sufficient to induce a 9 current loop in the earth so that the current loop has a perimeter which is greater in length than one wavelength 11 of the electromagnetic signals in earth, then current 12 driver lO does not act as a dipole antenna. Under these 13 conditions, the interaction between current driver lO and 14 the surrounding earth causes current driver 10 and the earth to effectively function together as a vertical 16 plane polarized antenna which produces a vertically 17 polarized electromagnetic wave. The current loop is 18 formed as a result of capacitive coupling which occurs l9 between the conducting arms 16 and 18 of current driver 10 and the surrounding earth. For good performance the 21 capacitive reactance must be small with respect to the 22 current driver resistance.
(S) 23The current loop action is illustrated by reference 24to the ground current loop paths 15 and 23 of Figures 6 and 7. Current loop paths 15 represent vertical paths, 26 while current loop paths 23 represent current paths to (S) l the sides of current driver lO. In the embodiment of 2 Figure 8 the electrical current from the ~ transmitter/receiver 13 passes by way of coaxial cable 14 4 to one of the conducting arms 16 or 18 of current driver lO, as for example conducting arm 16 which is connected 6 to conductor 22 of the coa~ial cable 14. The current is 7 capacitively coupled to the surrounding earth through 8 which it travels along current loop paths 15 and 23 to 9 the other conducting arm 18. The current is then capacitively coupled to conducting arm 18, from which it ll returns to the generator of transmitter/receiver 13 by 12 way of conductor 20 (see Figure 8).
(S) 13 As shown by the current loop paths 15 and 23 in 14 Figures 6 and 7, electrical currents flow on all sides of current driver lO. However, the electromagnetic waves 16 from current loop paths 23 (see Figure 7) cancel each 17 other so that no resultant signal is produced since the 18 current loop paths on one side of current driver 10 are 19 opposite in direction to the current loop paths on the other side oE current driver 10, as schematically shown 21 by the directional arrows. As is also schematically 22 illustrated by the directional arrows on the ground 23 current loop paths 15 (see Figure 6), the top and bottom 24 loops are also opposite their respective directions.
However, as shown in Figure 6, when current driver lO is 26 positioned near the ground surface 11 the area which 53~2 (S) 1 defines the path of the upper ground currènts becomes 2 smaller. Thus, the upper ground currents do not signifi-3 cantly cancel the electromagnetic waves resulting from 4 the bottom or lower ground currents. Thus, the lower ground currents act together with conductive arms 16 and 6 18 to define a vertical plane loop which produces verti-7 cally polarized electromagnetic signals.
(S) 8 The electromagnetic waves emitted from the commu-9 nication system are subject to slightly reduced gain as a result of the cancellation effects of the upper loop with 11 respect to the larger lower loop of vertical paths 15.
12 However, as the current driver 10 is positioned closer to 13 the surface ll of the ground, the upper loop grows 14 smaller and thus less signal degradation is apparent.
I5 The design criteria for determining optimum current 16 driver depth is explained more fully below.
(S) 17 Figure 9 illustrates a typical signal wave pattern 18 of the end-fed current driver 10. The azimuth wave l9 pattern for this and all bidirectional single current driver embodiments of the present invention is a figure 21 ei~ht pattern with lobes 40 in line with the axis of 22 current drive!r 10 and with a deep null 41 normal to the 23 axis of current driver lO which is just the opposite of a 24 dipole in air. The 3 dB beam width of this pattern is about 90 degrees and thus one such bidirectional current ~6 driver configuration produces a signal wave which covers 3 ~ 2 \\
(S) l two 90 degree sectors for a total coverage of 180 2 degrees. The elevation pattern (not shown) is that of a ~ typical vertical loop antenna. Typically, in a single 4 current driver embodiment, the signal wave radiation along the surface of the earth is less by about 3 dB than 6 at the peak of the pattern, with this value being 7 variable with signal frequency and with the conductivity 8 and dielectric constant of the earth. The radiated 9 polarization near the earth's surface is vertical, as is necessary for efficient ground wave propagation.
ll 3. The Center-Fed Current Driver (Figures 10 through 12 ~ (U) (S) 13Another preferred embodiment of the present in-14vention employs a single, center-fed current driver 50, 15as illustrated in Figures 10 through 12. With reference lfito Figure 10, current driver 50 includes two conducting l7arms 16 and 18 which are the same in their construction 18as the conducting arms 16 and 18 of current driver lO
~9described above. However, unlike current driver 10, the 20conducting arms 16 and 18 of current driver 50 are not 21 joined at the middle by a dielectric partition, and are 22 instead connected at their adjacent~to the balun 17. .~s 23 in the previously described embodiment, balun 17 is 24 connected by the coaxial cable 14 to a transmit-ter/receiver 13.
~6 12~S3 ~;~
(S) 1It has been found that the underground communication 2 system of this invention functions properly at current 3 driver/earth combination impedances which correspond to those current loop perimeters which are greater than one wavelength in size at the operating frequency. System 6 performance approaches a peak when the current loop 7 perimeter is approximately 1.5 wavelengths in size, while 8 other peak performance regions are found at each 9 additional full wavelength increase from this value (i.e.
10at wavelengths of 2.5, 3.5, 4.5, et. seq.). The 11 long-wire guided wave action of the curernt driver 12 conductors 16 and 18 further improve performance when the 13 loop path length exceeds about 2.5 wavelengths. By using 14 balun 17 to match the impedance of the current driver, and earth to the impedance of the transmitter/receiver 13 16 and coaxial cable 14, a maximum power transfer may be 17 realized.
(S) 18The bandwidth operation of the communication system l9 of this invention becomes fairly large when using a transmission line transformer balun. Bandwidths of as 21 much as three octaves have been demonstrated. An example 22 is a 3 to 30 MHz system for HF operation. Other well 23 known, commercially available impedance matching circuits 24 may also be used for accomplishing the desired impedance matching, although some may tend to have reduced 26 bandwith.
(S) 1 As in the case of the previously descri'~ed end-fed 2 current driver, the current driver 50 and the surrounding 8 earth effectively function as a vertical plane polarized 4 antenna which produces a vertically polarized electromagnetic wave. The loop action is illustrated 6 best by reference to the ground currént loop paths 15 and 7 23 (see Figures 10 and 11). The electrical curren~ from 8 the generator of transmitter/receiver 13 is conducted 9 through coaxial cable 14 to balun 17 and from balun 17 through conductor 22 to the conducting arm 16. Conduct-ll ing arm 16 is capacitively coupled ~o the surrounding 12 earth so that ground currents will flow on all sides of 13 the current driver 50 to the opposite conducting arm 18, 14 which is also capacitively coupled to the earth so that 15 it picks up the current. From conducting arm 18 the l6 current is returned through the balun 17 and through 17 cable 14 to the generator of transmitterlreceiver 13.
(S) 18 As in the case of the previously described embodi-19 ment, the electromagnetic waves resulting from the side 20 ground currents 23 (see Figure ll) are cancelled because 21 the ground current loops on each side of the current 22 driver 50 are in opposite directions. The cancellation 23 effects resulting from the vertical ground currents 24 illustrated by paths 15 (see Figure 10) can be minimized by properly positioning the center-fed current driver 50 26 at a proper distance from the surface ll of the ground, 3 ~:~
12953 ~2 (S) 1 as hereinafter more fully described. The resu~t is that 2 the current driver 50 functions with the earth in a 3 vertical plane loop mode and they produce a vertically 4 polarized electromagnetic signal that is essentially identical in its characteristics (see, e.g., Figure 12) 6 to the vertically polarized electromagnetic signal 7 previously described in connection with Figure 9.
(S) 8 The center-fed current driver 50 has been found to 9 have one particular advantage over the end-fed current driver lO. Since the coaxial cable 14 for the end-fed ll current driver 10 is not positioned at the center point l2 of the current driver 10, if the conductors 20 and 22 of 13 cable 14 cross at the end of the current driver lO a 14 shorting effect may result. This is due to the fact that the fields inside coaxial cable 14 become coupled to the 1~ outside of the coaxial cable. By feeding the current 17 driver from its center point, it is much easier to avoid 18 crossing the current driver 50 with the coaxial cable 14.
19 4. he Center-Fed Current Driver Array (Figures 13 through 15) (U) (S) 21 The efficiency of a system using a single buried 22 current driver may be low due to signal losses in the 23 earth. Efficiency can be improved by using multiple 24 current drivers 50 connected in a configuration such as that illustrated in Figure 14. To form a current driver 26 array two or more center-fed current drivers 50 are I
lZ~53 12 (S) 1 configured so that their conducti~g arms 16` and 18 are 2 respectively in parallel alignment, both vertically (see 3 Figure 13) and horizontally (see Figure 14). In the 4 illustrated embodiment four identical current drivers $0 are shown, although of course the number may vary as a 6 matter of design choice and system requirements. The 7 individual current drivers 50 are fed with power of the 8 same amplitude and phase. This is accomplished by using 9 a conventional power splitter 30 connected to an impedance matching balun 17 for each current driver 50 1! connected by coaxial cables 14. It is important that the 12 interconnecting coaxial cables 14 between baluns 17 and 13 power splitter 30 be of equal length so that current 14 driver elements 50 are fed with power of the same phase angle.
(S) 16 Each of the individual current drivers 50 in the 17 array function with the earth as a vertical plane 18 polarized antenna in the same manner as the current l9 driver 50 which was previously described in connection with Figures 10 through 12. With the current drivers 50 21 connected and balanced as illustrated in Figure 14, and 22 with proper parallel spacing between the current drivers 23 50, the power gain and efficiency of the current driver 24 array becomes a direct multiple of the number of current drivers contained in the array. For example, the power ~6 gain and efficiency of an array containing four current ~5;372 (S) 1 drivers is approximately four times the power gain and 2 efficiency of a single center-fed current driver.
(S) ~ The parallel spacing between the current drivers in 6 the array is an important design consideration in achiev-ing improved power gain and efficiency. For wide beam 6 operation the basic requirement is that the entire width 7 of the parallel array be no more than half a wavelength 8 in air at the system's operating frequency. If the 9 current driver array exceeds this dimension, the trans-mitted signal beam pattern becomes proportionately 11 reduced. On the other hand, the parallel current drivers 12 50 may not be positioned so close to each other as to 13 experience significant mutual impedance or other coupling 14 effects between the adjacent current drivers.
(S) 15 The size and spacing of an aboveground antenna array 16 are quite different from that of the present invention.
I5 The design criteria for determining optimum current 16 driver depth is explained more fully below.
(S) 17 Figure 9 illustrates a typical signal wave pattern 18 of the end-fed current driver 10. The azimuth wave l9 pattern for this and all bidirectional single current driver embodiments of the present invention is a figure 21 ei~ht pattern with lobes 40 in line with the axis of 22 current drive!r 10 and with a deep null 41 normal to the 23 axis of current driver lO which is just the opposite of a 24 dipole in air. The 3 dB beam width of this pattern is about 90 degrees and thus one such bidirectional current ~6 driver configuration produces a signal wave which covers 3 ~ 2 \\
(S) l two 90 degree sectors for a total coverage of 180 2 degrees. The elevation pattern (not shown) is that of a ~ typical vertical loop antenna. Typically, in a single 4 current driver embodiment, the signal wave radiation along the surface of the earth is less by about 3 dB than 6 at the peak of the pattern, with this value being 7 variable with signal frequency and with the conductivity 8 and dielectric constant of the earth. The radiated 9 polarization near the earth's surface is vertical, as is necessary for efficient ground wave propagation.
ll 3. The Center-Fed Current Driver (Figures 10 through 12 ~ (U) (S) 13Another preferred embodiment of the present in-14vention employs a single, center-fed current driver 50, 15as illustrated in Figures 10 through 12. With reference lfito Figure 10, current driver 50 includes two conducting l7arms 16 and 18 which are the same in their construction 18as the conducting arms 16 and 18 of current driver lO
~9described above. However, unlike current driver 10, the 20conducting arms 16 and 18 of current driver 50 are not 21 joined at the middle by a dielectric partition, and are 22 instead connected at their adjacent~to the balun 17. .~s 23 in the previously described embodiment, balun 17 is 24 connected by the coaxial cable 14 to a transmit-ter/receiver 13.
~6 12~S3 ~;~
(S) 1It has been found that the underground communication 2 system of this invention functions properly at current 3 driver/earth combination impedances which correspond to those current loop perimeters which are greater than one wavelength in size at the operating frequency. System 6 performance approaches a peak when the current loop 7 perimeter is approximately 1.5 wavelengths in size, while 8 other peak performance regions are found at each 9 additional full wavelength increase from this value (i.e.
10at wavelengths of 2.5, 3.5, 4.5, et. seq.). The 11 long-wire guided wave action of the curernt driver 12 conductors 16 and 18 further improve performance when the 13 loop path length exceeds about 2.5 wavelengths. By using 14 balun 17 to match the impedance of the current driver, and earth to the impedance of the transmitter/receiver 13 16 and coaxial cable 14, a maximum power transfer may be 17 realized.
(S) 18The bandwidth operation of the communication system l9 of this invention becomes fairly large when using a transmission line transformer balun. Bandwidths of as 21 much as three octaves have been demonstrated. An example 22 is a 3 to 30 MHz system for HF operation. Other well 23 known, commercially available impedance matching circuits 24 may also be used for accomplishing the desired impedance matching, although some may tend to have reduced 26 bandwith.
(S) 1 As in the case of the previously descri'~ed end-fed 2 current driver, the current driver 50 and the surrounding 8 earth effectively function as a vertical plane polarized 4 antenna which produces a vertically polarized electromagnetic wave. The loop action is illustrated 6 best by reference to the ground currént loop paths 15 and 7 23 (see Figures 10 and 11). The electrical curren~ from 8 the generator of transmitter/receiver 13 is conducted 9 through coaxial cable 14 to balun 17 and from balun 17 through conductor 22 to the conducting arm 16. Conduct-ll ing arm 16 is capacitively coupled ~o the surrounding 12 earth so that ground currents will flow on all sides of 13 the current driver 50 to the opposite conducting arm 18, 14 which is also capacitively coupled to the earth so that 15 it picks up the current. From conducting arm 18 the l6 current is returned through the balun 17 and through 17 cable 14 to the generator of transmitterlreceiver 13.
(S) 18 As in the case of the previously described embodi-19 ment, the electromagnetic waves resulting from the side 20 ground currents 23 (see Figure ll) are cancelled because 21 the ground current loops on each side of the current 22 driver 50 are in opposite directions. The cancellation 23 effects resulting from the vertical ground currents 24 illustrated by paths 15 (see Figure 10) can be minimized by properly positioning the center-fed current driver 50 26 at a proper distance from the surface ll of the ground, 3 ~:~
12953 ~2 (S) 1 as hereinafter more fully described. The resu~t is that 2 the current driver 50 functions with the earth in a 3 vertical plane loop mode and they produce a vertically 4 polarized electromagnetic signal that is essentially identical in its characteristics (see, e.g., Figure 12) 6 to the vertically polarized electromagnetic signal 7 previously described in connection with Figure 9.
(S) 8 The center-fed current driver 50 has been found to 9 have one particular advantage over the end-fed current driver lO. Since the coaxial cable 14 for the end-fed ll current driver 10 is not positioned at the center point l2 of the current driver 10, if the conductors 20 and 22 of 13 cable 14 cross at the end of the current driver lO a 14 shorting effect may result. This is due to the fact that the fields inside coaxial cable 14 become coupled to the 1~ outside of the coaxial cable. By feeding the current 17 driver from its center point, it is much easier to avoid 18 crossing the current driver 50 with the coaxial cable 14.
19 4. he Center-Fed Current Driver Array (Figures 13 through 15) (U) (S) 21 The efficiency of a system using a single buried 22 current driver may be low due to signal losses in the 23 earth. Efficiency can be improved by using multiple 24 current drivers 50 connected in a configuration such as that illustrated in Figure 14. To form a current driver 26 array two or more center-fed current drivers 50 are I
lZ~53 12 (S) 1 configured so that their conducti~g arms 16` and 18 are 2 respectively in parallel alignment, both vertically (see 3 Figure 13) and horizontally (see Figure 14). In the 4 illustrated embodiment four identical current drivers $0 are shown, although of course the number may vary as a 6 matter of design choice and system requirements. The 7 individual current drivers 50 are fed with power of the 8 same amplitude and phase. This is accomplished by using 9 a conventional power splitter 30 connected to an impedance matching balun 17 for each current driver 50 1! connected by coaxial cables 14. It is important that the 12 interconnecting coaxial cables 14 between baluns 17 and 13 power splitter 30 be of equal length so that current 14 driver elements 50 are fed with power of the same phase angle.
(S) 16 Each of the individual current drivers 50 in the 17 array function with the earth as a vertical plane 18 polarized antenna in the same manner as the current l9 driver 50 which was previously described in connection with Figures 10 through 12. With the current drivers 50 21 connected and balanced as illustrated in Figure 14, and 22 with proper parallel spacing between the current drivers 23 50, the power gain and efficiency of the current driver 24 array becomes a direct multiple of the number of current drivers contained in the array. For example, the power ~6 gain and efficiency of an array containing four current ~5;372 (S) 1 drivers is approximately four times the power gain and 2 efficiency of a single center-fed current driver.
(S) ~ The parallel spacing between the current drivers in 6 the array is an important design consideration in achiev-ing improved power gain and efficiency. For wide beam 6 operation the basic requirement is that the entire width 7 of the parallel array be no more than half a wavelength 8 in air at the system's operating frequency. If the 9 current driver array exceeds this dimension, the trans-mitted signal beam pattern becomes proportionately 11 reduced. On the other hand, the parallel current drivers 12 50 may not be positioned so close to each other as to 13 experience significant mutual impedance or other coupling 14 effects between the adjacent current drivers.
(S) 15 The size and spacing of an aboveground antenna array 16 are quite different from that of the present invention.
17 For example, at 400 KHz the wavelength in air is 750 18 meters. Typical required spacing of parallel antennas 19 aboveground would be half a wavelength or 375 meters.
Mutual impedance effects would begin occurring at 21 spacings which are only a little less than this.
(S) 22 In contrast, at 400 KHz the wavelength propagated 23 through the ground is in the range of lOO meters or less 24 and thus the half wavelength spacing would be about 50 25 meters. However, current drivers of the type described 26 herein experience essentially no significant mutual ~3 ~
~z~53 ~ 2 (S) 1 coupling even at parallel spacings as close as approxi-2 mately a skin depth. Operation with spacings as close as ~ .5 skin depths have been tested with satisfactory 4 results. A skin depth is the distance at which an electromagnetic wave would attenuate by e 1 , where e is 6 the base of the natural logarithm (2.718). For typical 7 low conductivity soils~ one skin depth is on the order of 8 lO meters at high frequencies and 40 meters or more at 9 low frequencies (see, e.g., Figure 29 for typical values of skin depth). Thus, with the present invention it is ll possible to produce a communication system with a very l2 high power gain and high signal quality using an 13 underground current driver array which occupies a 14 relatively small area. This is one of the principal advantages of the present invention.
(S) 16 With reference to Figure 15, it is seen that the 17 signal pattern 40 of the current driver array is l8 virtually the same as that of the single current driver l9 10 or 50 (see Figures 9 and 12, respectively) so long as the overall current driver a-rray width does not exceed 21 half a wavelength in air at the operating ~requency of 22 the system. If the array width exceeds this value, then 23 the azimuth beam angle will decrease as in conventional 24 antenna arrays located aboveground.
(S) 25 The current driver array has the additional advan-26 tage that its si ~ pattern 40 (see Figure 15) may be 3 ~
`` ~2~3S37;~
(S) 1 steered with the use of conventional ~ ase shifters (not 2 shown). Phase shifting is accomplished by varying the ~ phase relationship of the current drivers using conven-4 tional methods known in the communications industry.
Since jamming is normally generated from a particular 6 direction, it may be avoided by use of narrow beams and 7 by steering the signal pattern of the array in a direc-8 tion other than that of the jamming signal source. Thus, 9 the steerable wave pattern of the current driver array makes this arrangement more secure for military purposes 11 than the single current driver embodiment. Since atmo-12 spheric noise is often generated from a particular 13 direction, narrow beams and steering also permit the 14 effects of such noise .o be greatly reduced by not steering the signal pattern in the direction of the 16 noise.
17 5. The Center-Fed Current Driver Array With Tree 18 Terminations (Figures 16 through 18) (U) (S) 19 Due to the physical effects of the earth in terms of shortening the wavelength of a signal propagated through 21 the ground as opposed to the air (typical reductions 22 range from one-half to one-tenth, depending on frequency, 23 conductivity of the earth, and dielectric constant of the 24 earth), the size of the earth antennas which are formed as described herein is significantly less than the size 26 required for conventional aboveground antennas to operate .~ 7 - lZ~53 ~2 (S) 1 at similar frequencies and thus as require ~ ength of the 2 underground current drivers is very small when compared ~ to those typical aboveground antennas. Even so, the 4 length of the current driver is still rather large at low frequencies.
(S) 6One important requirement of the system of the 7 present invention is that the coupling reactance between 8 the current driver and the ground must remain small at 9 all frequencies. It has been found that at lower fre-quencies the current driver capacitance must be increased Il to keep the reactance as low as needed. Capacitance is a 12 function of the current driver conductor dimensions, the 13 number of conductors in parallel, and the conductive 14 characteristics of the surrounding ground. Thus, as the frequency goes down, it becomes necessary to add size to 16 the current driver or to add parallel conductors in order 17 to increase the capacitance for purposes of keeping the 18 reactance low.
(S) 19As illustrated in Figures 10, 13, and 16, the vertical ground currents 15 are generally transmitted 21 from locations along the outer mid section of conducting 22 arms 16 and 18. Thus, the mean path for capacitive 23 coupling tends to be toward the middle of the conducting 24arms 16 and 18. As the length of conducting arms 16 and 2S 18 is increased in order to transmit and receive at lower ~6 frequencies, capacitive coupling tends to occur further --r ~ y ~ ~ r r - Yq ~ r r 12~53 ~2 (S) 1 out along the arms 16 and 18. However, the mean path of 2 the capacitive coupling continues to be at appro~i~ately 3 the mid point of conducting arms 16 and 18.
(S) 4 Since the true frequency of system operation is determined by the current paths, and since the location 6 Of capacitive coupling along the conducting arm 7 identifies the current path, it is the location of 8 capacitive coupling and not entirely the actual current 9 driver length which ultimately defines the operating fre-quencies for a particular current driver configuration.
11 In light of these considerations, if the location of 12 capacitive coupling can be brought closer to the ends of 13 the conducting arms 16 and 18, then the required length 14 of the actual current driver necessarily will be less.
(S) 15 One way of moving the capacitive coupling closer to 16 the ends of conducting arms 16 and 18 is to use the tree 17 terminations generally designated 32, and shown best in 18 Figure 17. Tree terminations 32 can be simply con-19 structed using insulated cable or pipe. The terminations 32 are positioned parallel to each other and are attached 21 to the ends of conducting arms 16 and 18. In the alter-22 native, suitable terminations may be constructed using 23 insulated metal plates (not shown) or a single insulated 24 conductor which is attached to the end of conducting arms 16 and la and extends coaxially therefrom. Terminations 26 32 may be use ~ n the ends of a single current driver or 3 cl 12~ii3 ~;~
(S) 1 they may be used in a current driver array às ~llustrated 2 in Figure 17.
(S) 3 The number of separate conductors 33 used in each 4 tree termination 32 and the length thereof is a function of the capacitance of the conductors 33. As more conduc-6 tors 33 are connected in parallel, the overall length of 7 the tree terminations 32 may be reduced. The actual 8 determination of conductor quantity and termination 9 length is strictly a function of the amount of capacitance necessary to locate the capacitive coupling 11 at a position on conducting arms 16 and 18 which will 12 permit operation of the communication system at the 13 desired frequency.
(S) 14 The spacing of the individual conductors 33 within each tree termination 32 is not critical. In typical 16 underground applications, a spacing of approximately two 17 feet or more between the conductors 33 is adequate.
(S) 18 Without the tree terminations 32, the necessary 19 overall length of the conducting arms 16 and 18 is typically on the order of about one wavelength as mea-21 sured in the ground medium at the system operating 22 frequency. With tree terminations 32 the optimum overall 23 length of conducting arms 16 and 18 may be on the order 24 of two-thirds to three-quarters wavelengths as measured in the ground medium at the system frequency.
~\3 53~2 (S) 1 E~cept for the tree terminations ~ s described 2 above, which have the effect of permitting the overall length of each individual current driver to be shortened 4 somewhat, basic operation of the array illustrated in Figures 17 and 18 is the same as the current driver array 6 previously described in connection with Figures 13 7 through 15. That is to say, the array with tree termina-8 tions 32 also functions in combination with the earth as 9 a vertical plane polarized antenna array by virtue of capacitive coupling between each individual current ll driver 50 and the surrounding earth.
(S) 12 With reference to Figure 18, the signal wave pattern 13 40 is virtually identical to the signal pattern (see 14 Figure 15) of the array described in connect~en with 15 Figures 13 and 14.
16 6. The Wide Band Switched Array (Figure 19) (U) (S) 17 Military communication requirements include systems l8 which operate at VLF, LF, MF, HF, and VHF. With the l9 present invention it is possible to operate at any or all 20 of these frequencies in a single system installation.
Mutual impedance effects would begin occurring at 21 spacings which are only a little less than this.
(S) 22 In contrast, at 400 KHz the wavelength propagated 23 through the ground is in the range of lOO meters or less 24 and thus the half wavelength spacing would be about 50 25 meters. However, current drivers of the type described 26 herein experience essentially no significant mutual ~3 ~
~z~53 ~ 2 (S) 1 coupling even at parallel spacings as close as approxi-2 mately a skin depth. Operation with spacings as close as ~ .5 skin depths have been tested with satisfactory 4 results. A skin depth is the distance at which an electromagnetic wave would attenuate by e 1 , where e is 6 the base of the natural logarithm (2.718). For typical 7 low conductivity soils~ one skin depth is on the order of 8 lO meters at high frequencies and 40 meters or more at 9 low frequencies (see, e.g., Figure 29 for typical values of skin depth). Thus, with the present invention it is ll possible to produce a communication system with a very l2 high power gain and high signal quality using an 13 underground current driver array which occupies a 14 relatively small area. This is one of the principal advantages of the present invention.
(S) 16 With reference to Figure 15, it is seen that the 17 signal pattern 40 of the current driver array is l8 virtually the same as that of the single current driver l9 10 or 50 (see Figures 9 and 12, respectively) so long as the overall current driver a-rray width does not exceed 21 half a wavelength in air at the operating ~requency of 22 the system. If the array width exceeds this value, then 23 the azimuth beam angle will decrease as in conventional 24 antenna arrays located aboveground.
(S) 25 The current driver array has the additional advan-26 tage that its si ~ pattern 40 (see Figure 15) may be 3 ~
`` ~2~3S37;~
(S) 1 steered with the use of conventional ~ ase shifters (not 2 shown). Phase shifting is accomplished by varying the ~ phase relationship of the current drivers using conven-4 tional methods known in the communications industry.
Since jamming is normally generated from a particular 6 direction, it may be avoided by use of narrow beams and 7 by steering the signal pattern of the array in a direc-8 tion other than that of the jamming signal source. Thus, 9 the steerable wave pattern of the current driver array makes this arrangement more secure for military purposes 11 than the single current driver embodiment. Since atmo-12 spheric noise is often generated from a particular 13 direction, narrow beams and steering also permit the 14 effects of such noise .o be greatly reduced by not steering the signal pattern in the direction of the 16 noise.
17 5. The Center-Fed Current Driver Array With Tree 18 Terminations (Figures 16 through 18) (U) (S) 19 Due to the physical effects of the earth in terms of shortening the wavelength of a signal propagated through 21 the ground as opposed to the air (typical reductions 22 range from one-half to one-tenth, depending on frequency, 23 conductivity of the earth, and dielectric constant of the 24 earth), the size of the earth antennas which are formed as described herein is significantly less than the size 26 required for conventional aboveground antennas to operate .~ 7 - lZ~53 ~2 (S) 1 at similar frequencies and thus as require ~ ength of the 2 underground current drivers is very small when compared ~ to those typical aboveground antennas. Even so, the 4 length of the current driver is still rather large at low frequencies.
(S) 6One important requirement of the system of the 7 present invention is that the coupling reactance between 8 the current driver and the ground must remain small at 9 all frequencies. It has been found that at lower fre-quencies the current driver capacitance must be increased Il to keep the reactance as low as needed. Capacitance is a 12 function of the current driver conductor dimensions, the 13 number of conductors in parallel, and the conductive 14 characteristics of the surrounding ground. Thus, as the frequency goes down, it becomes necessary to add size to 16 the current driver or to add parallel conductors in order 17 to increase the capacitance for purposes of keeping the 18 reactance low.
(S) 19As illustrated in Figures 10, 13, and 16, the vertical ground currents 15 are generally transmitted 21 from locations along the outer mid section of conducting 22 arms 16 and 18. Thus, the mean path for capacitive 23 coupling tends to be toward the middle of the conducting 24arms 16 and 18. As the length of conducting arms 16 and 2S 18 is increased in order to transmit and receive at lower ~6 frequencies, capacitive coupling tends to occur further --r ~ y ~ ~ r r - Yq ~ r r 12~53 ~2 (S) 1 out along the arms 16 and 18. However, the mean path of 2 the capacitive coupling continues to be at appro~i~ately 3 the mid point of conducting arms 16 and 18.
(S) 4 Since the true frequency of system operation is determined by the current paths, and since the location 6 Of capacitive coupling along the conducting arm 7 identifies the current path, it is the location of 8 capacitive coupling and not entirely the actual current 9 driver length which ultimately defines the operating fre-quencies for a particular current driver configuration.
11 In light of these considerations, if the location of 12 capacitive coupling can be brought closer to the ends of 13 the conducting arms 16 and 18, then the required length 14 of the actual current driver necessarily will be less.
(S) 15 One way of moving the capacitive coupling closer to 16 the ends of conducting arms 16 and 18 is to use the tree 17 terminations generally designated 32, and shown best in 18 Figure 17. Tree terminations 32 can be simply con-19 structed using insulated cable or pipe. The terminations 32 are positioned parallel to each other and are attached 21 to the ends of conducting arms 16 and 18. In the alter-22 native, suitable terminations may be constructed using 23 insulated metal plates (not shown) or a single insulated 24 conductor which is attached to the end of conducting arms 16 and la and extends coaxially therefrom. Terminations 26 32 may be use ~ n the ends of a single current driver or 3 cl 12~ii3 ~;~
(S) 1 they may be used in a current driver array às ~llustrated 2 in Figure 17.
(S) 3 The number of separate conductors 33 used in each 4 tree termination 32 and the length thereof is a function of the capacitance of the conductors 33. As more conduc-6 tors 33 are connected in parallel, the overall length of 7 the tree terminations 32 may be reduced. The actual 8 determination of conductor quantity and termination 9 length is strictly a function of the amount of capacitance necessary to locate the capacitive coupling 11 at a position on conducting arms 16 and 18 which will 12 permit operation of the communication system at the 13 desired frequency.
(S) 14 The spacing of the individual conductors 33 within each tree termination 32 is not critical. In typical 16 underground applications, a spacing of approximately two 17 feet or more between the conductors 33 is adequate.
(S) 18 Without the tree terminations 32, the necessary 19 overall length of the conducting arms 16 and 18 is typically on the order of about one wavelength as mea-21 sured in the ground medium at the system operating 22 frequency. With tree terminations 32 the optimum overall 23 length of conducting arms 16 and 18 may be on the order 24 of two-thirds to three-quarters wavelengths as measured in the ground medium at the system frequency.
~\3 53~2 (S) 1 E~cept for the tree terminations ~ s described 2 above, which have the effect of permitting the overall length of each individual current driver to be shortened 4 somewhat, basic operation of the array illustrated in Figures 17 and 18 is the same as the current driver array 6 previously described in connection with Figures 13 7 through 15. That is to say, the array with tree termina-8 tions 32 also functions in combination with the earth as 9 a vertical plane polarized antenna array by virtue of capacitive coupling between each individual current ll driver 50 and the surrounding earth.
(S) 12 With reference to Figure 18, the signal wave pattern 13 40 is virtually identical to the signal pattern (see 14 Figure 15) of the array described in connect~en with 15 Figures 13 and 14.
16 6. The Wide Band Switched Array (Figure 19) (U) (S) 17 Military communication requirements include systems l8 which operate at VLF, LF, MF, HF, and VHF. With the l9 present invention it is possible to operate at any or all 20 of these frequencies in a single system installation.
21 Figure 19 illustrates one means for accomplishing this 22 type of wide band operating capability using the princi-23 ples of the present invention.
(S) 24 The embodiment of Figure 19 includes a control 25 center 70 which contains a transmitter/receiver (not 26 shown) a$ well as a\conventional power splitter (not ~29~3 ~2 (S) l shown) together with other electronic con~rol circuitry 2 (not shown) for controlling the system operation as 3 hereinafter more fully described. The control center 70 4 is connected to the various current driver elements generally designated at 72-76 by means of coaxial cable 6 14.
(S) 7 In the illustrated embodiment, various types of 8 current driver elements 72-76 are used in combination.
9 For example, the current driver elements 72 comprise center-fed current drivers which are similar in their ll construction and basic operation to the center-fed 12 current driver embodiment previously described in con-l3 nection with Figures 10 through 12. The current drivers 14 72 each have a pair of conducting arms 78 and 80 which are connected to an impedance matching circuit 17 which 16 may be, for example, a bal~n. The co~lducting arms 78 and l7 80 consist of No. 12 copper wire or other suitable wire, 18 cable or pipe as previously described. The length of 19 each conducting arm 78 and 80 is approximately 300 meters. At this length, current drivers 72 are desi~ned 21 to operate in the frequency range of 40 KHz to 100 KHz.
(S) 22 Current driver 73 is also a center-fed current 23 driver. The current driver 73 includes two identically 24 configured conducting arms generally designated at 82 and 84 which are placed end-to-end and which are connected to 26 a balun 17 for purpo~ses of matching the impedance of the ~Z9S372 (S) l conducting arms 82 and 84 to the impedance o ~ he trans-2 mitter/receiver (not shown) located at the control center ~ 70. For purposes of simplifying the illustration and 4 description, only the elements of conducting arm 82 are described in detail and it will be understood that the 6 opposite conducting arm 84 is identically constructed.
(S) 7Conducting arm 82 consists of four lengths of cable 8 85-88 which, in the illustrated embodiment, may be, for 9 example, No. 12 copper wire. Each length of cable varies. For example, the length of cable 85 is approxi-ll mately two meters, cable 86 is approximately two and 12 one-half meters, cable 87 is approximately four and 13one-half meters and cable 88 is approximately 286.5 14 meters. The lengths of cable 85-88 are interconnected by lS a series of switching devices 90-94. Each switching 16 device 90-93 may be, for example, a conventional elec-17 trical relay which can be activated from the control 18 center 70 to connect or disconnect the various lengths of l9 cable 85-88.
(S) 20 The switching elements 90-93 may also be conven-21tional pneumatic switches, relays or other suitable types 22of conventional switching devices. When using electrical 23relays as switches it is necessary to use RF chokes in 24the control lines at the current driver switching lo-25cations. The pneumatic switches with dielectric control l.,~t,'~
lZ~i372 ( ) 1 lines avoid the use of RF chokes with resulting increased 2 electromagnetic pulse (EMP) hardness.
(S) 3 The tree termination generally designated 94 is 4 interconnected between the switching elements 92 and 93 and forms part of~he conducting arm 82, along with the 6 lengths of.cable 85-88.
(S) 7 In use, the various lengths of cable 85-88 together 8 with the tree termination 94 may be combined in various 9 ways through the use of switching elements 90-93 to vary the length of the conducting arms 82 and 84 for purposes 11 of permitting the current driver 73 to be used over a 12 wide band of frequencies. For example, the length of 13 cable 85 is designed for operation in the range of 15 MHz 14 to 30 MHz. By using the switching element 90 to add the length of cable 86 to the length of cable 85, the current lfi driver 73 can be operated in the range of 8 MHz to 17 15 MHz. Similarly, by using switching element 91 to add 18 the length of cable 87 to the lengths of cables 86 and 19 85, the current driver can be operated in the range of 3 MHz to 8 MHz. When switching element 92 is operated to 21 add the tree termination 94 to the three lengths of cable 22 85-87, system operation is in the range of 1.5 MHz to 23 3 MHZ.
(S) 24 Also, it should be noted that other current drivers such as the current drivers 74, 75 and 76 may be designed ~6 to include lengths ~f cable, switching elements and tree i3~2 (S) l terminations which are the same as the cable lengths 2 85-87, tree termination 94 and switching elements 90-92 3 of current driver 73, with the net effect Oc creating a 4 current driver array consisting of essentially eight identical current driver elements, depending on how each 6 current driver element is switched. As previously ex-7 plained, use of eight current drivers in an array will 8 increase the signal gain by a factor of eight in the 9 particular range of operating frequencies.
(S) 10 ~y adding the length of cable 88 through the use of 11 switching element 93, the current driver 73 becomes 12 approximately the same in its length as current driver 13 elements 72, and is thus capable of operating as part of 14 a parallel array in the frequency range of 40 KHz to 100 KHz.
(S) 16 Other possible switching arrangements are illus-17 trated in connection with the current driver elements 75 18 and 76. For example, current driver 75 includes a 19 further tree termination generally designated at 96 which includes five conductors 97, each of which is approxi-21 mately eighteen meters in length. When the tree termina-22 tion 96 is switched so as to form a part of the conduct-23 ing arm of the current driver element 75, the current 24 driver 75 is operational in the frequency range of 350 KHz to 500 KHz. And, as illustrated by current driver 26 element 76, a further extension 98 can be added onto the lZ~53 o 2 \
(S) 1 tree termination 96 to further increase the length of the 2 current driver 76, thus permitting operation at even ~ lower frequency ranges.
(S) 4 ~rom the foregoing illustrative embodiment, it is apparent that the principles of the present invention can 6 be advantageously used to design a highly flexible and 7 efficient underground or surface deployed communication 8 system capable of wide band operation, and in conjunction 9 with enhanced signal processing, capable of performance at data rates closely comparable to conventional 11 aboveground wireless communication systems. The current 12 drivers used in the array can be either end-fed or 13 center-fed members which, when properly matched by the 14 baluns 17 to the impedance of he transmitter/receiver, will effectively operate with the earth as vertical plane 16 polarized antennas. Moreover, by properly combining the 17 various current driver elements of the array, signal gain 18 can be materially enhanced.
9 7. The Unidirectional Buried Current Driver (Figures 20-22? (U) (S) 21 There are a number of significant advantages to 22 having an underground communication system which includes 23 an earth antenna having the capability of end fire or 24 unidirectional operation. These advantages include (1) 25 reduced effects from noise and interference (jamming) ~6 originating from di~ tions _ot~ tl~ =D~ desired t~
12~53 ~ 2 (S) 1 communication direction; (2) increased transmit signal 2 gain both from each current driver element, and from each 3 array which utili~es this configuration; and (3) reduced 4 production of interference in other communication units not in the desired communication direction. Figure 20 6 illustrates one means for accomplishing this type o~
7 unidirectional operating capability using the principles 8 Of the present invention.
(S) 9 The embodiment of Figure 20 includes a current driver 200 having conducting arms 226 and 228 positioned 11 in end-to-end alignment and connected at their adjacent 12 ends to a balun 17 which provides impedance matching as 13 well as balance-to-unbalance transformer action. Current 14 driver 200 is positioned beneath the surface of the earth 11 in the manner previously described so as to produce 16 ground currents 208 which are induced through current 17 driver 200 by means of a transmitter/receiver (not 18 shown).
(S) 19 In the illustrated embodiment, it is seen that conducting arm 226 is substantially shorter than conduct-21 ing arm 228. In this configuration the communication 22 system functions as a unidirectional antenna. It has 23 been found that this communication system functions best 24 as a unidirectional antenna when the length ratio between 25 conducting arm 228 and conducting arm 226 is 2 to 1 or 26 greater.
~;~953 ~ ~
(S) 1 It is apparent that the current loop ~ th formed by 2 currents 208 is not centered about the balun 17, but rather the current loop is significantly elongated in the direction of the longer conducting arm 228. Thus, the 5 current loop formed by currents 208 through the earth and 6 through current driver 200 causes the current driver 200 7 and the earth 11 to function as a traveling wave antenna 8 in which the long conducting arm 228 acts as a "slow-wave 9 structure" which effectively controls the propagation lO direction of the wave produced by the basic loop antenna.
(S) 11 Figure 21 illustrates a typical signal wave pattern l2 230 of the unidirectional current driver arrangement.
13 The azimuth wave pattern for this current driver embodi-14 ment comprises an unbalanced figure eight pattern with the main or front beam being in line with the conducting l6 arms of current driver 200, and propagating in the l7 direction of the longer arm 228. The 3 dB beam angle for l8 a single element as illustrated in Figures 20 and 21 is l9 40-60 degrees wide, depending upon frequency. As the communication system operating frequency increases, 21 signal gain is found to increase and beam angle is found 22 to become narrower.
(S) 23 The front-to-back lobe ratio for the unidirectional 24 signal produced by a current driver 200 which has a 2 to 25 1 conducting arm length ratio as illustrated in Figure 21 26 is typically about 10 dB. In this configuration, the ~--t X
(S) l signal bandwidth tends to be so ~ than the 2 previously described bidirectional signal and thus the ~ unidirectional system experiences a more uniform perfor-4 mance over the 2 to 30 MHz operating range. In addition, in the unidirectional operating configuration the lower 6 cutoff frequency is controlled by the length of the 7 shorter conducting element 226, and it has been found 8 that the transmit signal gain for this configuration is 9 approximately 3 to lO dB higher than that in the bidirectional configuration, the exact amount depending ll upon the system operating frequency.
(5) 12 The unidirectional current driver configuration may 13 be utilized in array form as illustrated in Figure 22.
14 The configuration of the array of Figure 22 is substan-lS tially identical to the center-fed loop array illustrated 16 in Figures 13 through 15, with the exception that the 17 length ratio of the conducting arms is modified as 18 explained above. As with the bidirectional current l9 driver array, the current drivers in the array (four in this example illustration) are fed with power of the same 21 amplitude and phase. This is accomplished by using a 22 conventional power splitter 30 connected by coaxial 23 cables 14 to a 4-to-1 balun 17 for each current driver 24 200. It is important that the interconnecting coaxial 25 cables 14 between baluns 17 and power splitter 30 be of ~2953 J2 (S) l equal length so that the current drivers 200 are fed with 2 the same phase angle.
(S) ~ The operating parameters for a unidirectional current driver array are substantially identical with those previously discussed with respect to the 6 bidirectional current driver array configuration. Thus, 7 for wide beam operation the basic requirement is that the 8 entire width of the parallel array be no more than half a 9 wavelength in air at the system's operating frequency.
(S) 10 Likewise, the degree of improvement in system ll performance of a unidirectional current driver array as 12 compared with a single unidirectional current driver 3 system is substantially identical to the improvement in 14 performance experienced in the bidirectional current driver situation. Thus, with the current drivers 200 16 connected and balanced as illustrated in Figure 22, and 17 with proper parallel spacing between the current drivers 18 200, the power gain and efficiency of the current driver l9 array becomes a direct multiple of the number of current drivers contained in the array.
~S) 21 Still further, as is the case in the bidirectional 22 current driver situation, the signal pattern 234 of the 23 unidirectional current driver array is virtually the same 24 as the waveform 230 (Figure 21) of the single unidirectional current driver, so long as the overall ~6 current driver array width does not exceed half a L~; ~ ij lZ9~i37;~
(S) 1 wavelength in air at the operating frequency of the 2 system. The unidirectional current driver array of 3 Figure 22 also provides the advantage of steerability 4 through the use of conventional phase shifters.
(S) 5 Because of its greatly improved gain as compared to 6 that of the bidirectional current driver arrangement, 7 coupled with the further flexibility provided in the 8 steerable current driver array arrangement, the uni-9 directional current driver configuration becomes particu-larly desirable in applications requiring strong, 11 steerable directional signals, or in applications 12 requiring the ability to minimize the effects of inter-13 ference either from other sources, or as produced by the 14 current dri~er communication sys~em itself.
lS 8. The CancelIation Current Barrier (Figures 23 16 through 26) (U) (S) 17 By reference to Figures 6 and 7, it is possible to 18 identify several additional factors which reduce the 19 operating efficiency of the underground communication system disclosed herein. In particular, it can be seen 21 that even though the current driver is positioned in 22 proximity to the surface of the earth, some vertical 23 plane loop currents still exist between the current 24 driver 10 and the surface of the earth 11. These cur-rents travel in directions opposite to the loop currents ~6 beneath the current driver 10, and thus they cancel the `~ 3, 2 (S) 1 effect of some of ~he currents below the current driver, 2 thus reducing the system performance. This problem 3 becomes especially true during wet weather conditions 4 when increased conductivity in the earth above current driver lO permits increased current flow in that region.
6 Thus, one further improvement to the communication system 7 of the present invention would be to prevent the flow of 8 loop currents in the ground above current driver 10.
(S) 9 ~y reference to Figure 7 it is apparent that the currents on each side of current driver lO effectively 11 cancel each other out. Nevertheless, the energy utilized 12 to produce these cancellation currents is simply wasted, 13 co~nprising another source of communication system ineffi-14 ciency. Therefore, it would be another important im-provement to the communication system to provide a means 16 for preventing the flow of the side currents 23 as 17 illustrated in Figure 7.
(S) 18 Upon examination of Fi~ures 6 and 7 it becomes 19 readily apparent that one method of eliminating cancellation currents above the currellt driver lO is to 21 position the current driver near the surface of the 22 earth. As the depth of earth above current driver 10 23 increases, so does the volume of cancellation currents 24 flowing above that member. Thus, selection of the amount of earth over current driver 10 can be used as a L, ';2 ~9537;~
(S) 1 mechanism for inhibiting the cancellation currents which 2 may otherwise result.
(S) ~ In addition to the problems with cancellation 4 currents as described above, current, driver operation typically creates induced ground currents near the center 6 of the current driver. These induced ground currents act 7 to effectively short out the current driver, and their effect is especially apparent at high frequencies such as 9 those in excess of 20 MH~, because of the low per unit length capacitive reactance of the current driver at 11 these frequencies. As a result, it would be a further 12 improvement in the communication system to provide a 13 means for removing these unwanted center ground currents.
(s)14 One preferred embodiment of a means for overcoming the cancellation current problem described above is lfi illustrated in Figures 23 and 24. By reference to Figure 17 23 it is seen that a layer of low conductivity material 18 138 is placed over the current driver 100 so as to extend 19 between the current driver and the surface of the earth 11 along substantially the entire length of the current 21 driver 100. Layer 138 can be comprised of any low 22 conductivity material such as rocks or asphalt. By this 23 inhibiting means, ground loop currents are substantially 24 prevented from forming in the region between the top of current driver 100 and the surface of the earth 11.
26 Alternatively, the layer 138 can comprise material such ~, 3 lZ95372 (S) 1 as rocks positioned directly above the ~ ~driver 2 100, with asphal~ or some other water resistant material ~ being placed adjacent the surface of the earth. The 4 asphalt or water resistant material in this configuration serves to keep the region above the current driver dry 6 and low in conductivity in all seasons and weather 7 conditions.
(S) 8Another layer of porous low conductivity material 9 140 is positioned directly beneath the center portion of current driver 100, so as to prevent ¢r greatly reduce 11 the formation of induced ground currents near the center 12 of the current driver. Connected to the lower portion of 13the porous material 140 is a drain 142. The drain 142 is 14 provided so as to preclude collection of water in the areas containing material 140, thus preventing formation 16 of higher conducting paths through such water.
(S) 17By reference to Figure 24, it is seen that the low 18 conductivity covering 138 may be positioned so as to 19 extend outwardly from current driver 100 in the region around the central portion of the current driver. By 21 positioning the low conductivity material 138 in this 22 manner, side cancellation currents are substantially 23 eliminated.
(S) 24 Another preferred embodiment of the cancellation current barrier is illustrated by reference to Figure 25.
~6 In this embodiment~ a single trench of s~bstantially 129~372 (S) 1 uniform depth may be formed, with an initial layer of low 2 conductivity material 144 placed in the bottom thereof ~ along appro~imately the center two-thirds of the trench's 4 length. A current dri~er lOO comprised of insulated wire, or similar conductive material having reasonable 6 flexibility~ is placed within the trench so that its 7 center portion is positioned upon the low conductivity 8 material 144. The end portions of current driver lOO are 9 placed directly upon the earth. An additional layer of low conductivity material 146 is then placed over current 11 driver lOO. Optionally, an asphalt or other low 12 conductivity, water resistant covering may comprise low 13 conductivity material 146, or the water resistant materi-14 al may optionally be extended over the low conductivity material 146 near the top of the trench. Drainage for 16 this embodiment is provided by means of a nonconducting 17 pipe 148 or similar conventional drainage means extending 18 downward and away from a drain inlet position in the l9 lower portion of the trench and adjacent low conductivity material 144.
(S) 21 Still another preferred embodiment for the cancella-22 tion current barrier of the present invention is illus-23 trated in Figure 26. The configuration of Figure 26 24 comprises a small mound within which is positioned a double level trench. The lower level of the trench is 26 filled with p ~ us, low conductivity material 152 such as 1'~9S3 ~ ~ ---(S) l rock. A drain 154 positioned in the lower portion of the 2 lower trench is comprised of a low conductivity pipe ~ which provides a means for draining water away from the 4 lower portions of the trench. A current driver lO0 is positioned within the trench so that the outer portions 6 of the current driver's extended conducting arms 102 and 7 104 are positioned on the earth 11, while the central 8 portion of current driver 100 is positioned adjacent to 9 the low conductivity material 152. An additional layer of low conductivity material 156 such as porous rock is ll positioned above the current driver along its entire 12 length. Positioned upon the upper layer of low 13conductivity material 156 is a layer of asphalt 158 or 14 similar water resistant material. This mound configura-tion provides a means whereby water contacting the mound l6 surface is drained away without passing downwardly 17 through the porous, low conductivity materials 156 and 18152. The drain 154 provides a means whereby any water l9 which may enter the trench area is carried away from the vicinity of the current driver lO0.
(S) 21By reference to Figure 27, it becomes apparent that 22 the cancellation current barrier arrangement may also be 23 successfully utilized in array current driver applica-24 tions. Specifically, the configuration of the cancella-tion current barrier of Figure 27 may comprise any of the 26 embodiments described above. In addition, the outward C3S3 ~ 2 (S) 1 extending portions 160 of the low co ~ ity material 2 138 are joined together so as to define a continuous 3 e~tension between adjacent current drivers. By this 4 means, transmission of side cancellation currents is substantially prevented.
(S) 6 Through use of the cancellation current barrier 7 described above, capacitive coupling may be encouraged to 8 occur in preferential coupled regions, and communication 9 system efficiency may be further improved by amounts as much as 4 to 6 dB. In addition, the drainage and water 11 protection features af the cancellation current barrier 12 provide an important means for maintaining uniform system 13 operation in substantially any weather condition, as well 14 as for providing increased system operating life by reducing the exposure of the current driver 100 and 16 associated system elements to the corrosive and otherwise 17 damaging effects of extended contact with water.
18 9. Design ConsiderationS (U) (U) 19 The foregoing embodiments illustrate many of the wide variety of ways in which the principles of the 21 present invention can be practiced. Following is a brief 22 summary of the principal design criteria that may be 23 varied to meet the design considerations imposed by a 24 given system.
(S) 25 The first step is to determine the length of the 26 underground current driver. Current driver length is 5~
~Z~53 ~Z
(S) 1 inversely proportional to the system's signa ~frequency.
2 As described above, depending upon whether tree termina-~ tions are utilized on the current driver ends, total 6 current driver length will range from three-quarters to one signal wavelength as measured in the ground medium at 6 the desired frequency. Signal wavelength in a ground 7 medium is graphically illustrated as a function of signal 8 frequency in Figure 28. Graph line 34 represents the 9 wavelength in meters as a function of frequency in rich agricultural earth having a conductivity of 10 2 mho per 11 meter. Graph line 36 indicates the wavelength in meters 12 as a function of frequency in rocky land having a 13 conductivity of 2 x 10 3 mho per meter. The area 38 16 between lines 34 and 36 describes the signal wavelength as a function of signal frequency in the majority of 1~ typical kinds of ground media. The equation for deter-17 mining the wavelength represented by lines 34 and 36 of 18 Figure 28 is 22 ~ ( ) (1) ~6 \ _ -~ ~ z95372 1 where ~ = wavelength i~ the eàrth (meters) ~ = 1 for large depth and ~
3 for current driver at or above the surface ~ = permeability of the earth (4~ x 10 7 weber/amp-meter) E = ~0 ~r = permittivity of 7 earth (farad/meter) 8 ~ = permittivity of free space (8.85 x 10 12 farad/meter) ~ = relative permittivity of earth r (farad/meter) 11 o = conductivity of earth (mho/meter) 12 ~ = 2~ f 13 f = frequency (Hz).
(S) In the case of a communication system operating at a 16 frequency of 400 KHz, from equation (1) the signal 17 wavelength is equal to approximately 100 meters when 18 propagated through low conductivity ground at that frequency. Thus, total current driver length will range from 75 to 100 meters, depending on whether tree termina-21 tions are used at the ends of the current driver.
(S) 22 Another principal consideration in the design of the 23 present invention is the depth of current driver burial.
24 As previously indicated, there is approximately a 3 dB
drop in power gain when the current driver is buried 26 approximately one meter below the earth's surface as compared to deployment on the ground surface. This loss . ~ .. , .. , .. , . ~ . , . , , . . ~, .. .. .
.~ . . . . .
~ C~
~L2~5372 (S) 1 is principally due to the electromagne~ t~l~d cancella-2 tion effects of the vertical ground current paths 15 (see ~ Figure 1), as previously mentioned. Although reduction 4 in power gain continues to increase as the current driver is buried deeper, the rate of reduction is much less 6 significant than the initial reduction experienced as 7 between surface deployment and burial just beneath the 8 earth's surface.
(U) 9 Penetration or skin depth is graphically illustrated as a function of signal frequency in Figure 29. Graph 11 line 48 represents the penetration depth in meters as a 12 function of frequency in rich agricultural earth having a 13 conductivity of 10 2 mho per meter. Graph line 49 14 indicates the penetration depth in rocky land having a conductivity of 2 x 10 3 mho per meter. The area 47 16 between lines 48 and 49 describes the penetration depth 17 in the majority of typical kinds of ground media. The 18 equation for tetermining the penetration depth represent-l9 ed by lines 48 and 49 of Figure 29 is 2l ~ , 1 22 ~ I /1+ 2 ' ~ (2) 23 V ~ V ~ E
(~, C~
12~t53 ~2 (U) where ~ = is the penetr ~ -2 (meters) ~ ~ = 2~ f 4 ~ = the relative permeabilitv Of the surrounding earth (weber/
amp-meter) 6 a = the conductivity of the surrounding earth (mho/meter) E = the dielectric constant of the 8 surrounding earth.
(S) It is well-known that for low frequencies in the l! range of 50 to 150 KH7, equation (2) may be simplified to ~ 503 3 (3) ~9 22 Thus, using as an example a system operating at 150 KHz, 23 and assuming the conductivity of the surrounding earth to 24 be 10 3 mho per meter, from equation t3) the penetration depth woult be about 41 meters. It has been determined 26 that the maximum effective current loop depth is on the order of one sXin depth~
1~53 ~2 \
10 . Summary ( U ) (S) 2 The current driver configurations embodied in the 3 present invention offer the following advantages, among 4 others: (1) the underground positioning of the current driver provides system hardness and survivability in all 6 cases except a direct hit by a nuclear weapon, while 7 permitting data transmission on a wide frequency band 8 and, when using enhanced signal processing, at data rates 9 comparable to conventional aboveground systems; (2) because of its underground positioning and the use of 11 capacitive coupling with the earth to produce a vertical 12 plane polarized antenna configuration, the communication 13 system of the present invention permits communication at 14 frequencies ranging from the VLF range to the VHF range;
(3) by placing the current drivers underground and by 1~ using two or more current drivers in a parallel array 17 configuration, significant improvement in signal power 18 gain and communication system operating efficiency are 19 achieved, while requiring only a relatively small underground area for the placement of the current driver 21 array; (4) by use of the tree terminations, coupling is 22 improved and the mean location of capacitive coupling 23 between the current driver and the earth is extended 24 toward the outer ends of conducting arms, with the result that the required current driver length for operation at 26 a given frequency i`s~ significantly reduced; (5) because . ' ~ : '` -53 ~2 (S) 1 of increased bandwidth and directional` steering the 2 system has potential for very low vulnerability to ~ jamming and other adverse effects of interfering 4 electromagnetic radiation; (6) by configuring the current driver such that one end is shorter than the other, a 6 unidirectional vertical plane polarized antenna can 7 easily and quickly be formed, having improved signal gain 8 and narrowed beam width for further improvement in the 9 ability to avoid signal interference; (7) a means is provided whereby the undesirable effects of cancellation 11 currents can be minimized and the current driver can be 12 protected for all weather high quality operation; (8) the 13 communication system described herein is also adaptable 14 for use in a surface deployment mode upon the surface of the ground, in near surface, aboveground deployment, or lfi in a shallow trench so that the system may be easily 17 transported; (g) camouflage of the system is a simple 18 matter, and decoying through use of dummy current drivers 19 is also economically feasible; (10) environmental impacts 20 are minimized and surrounding aesthetic conditions are 21 preserved; and (11) overall costs are significantly 22 reduced, permitting redundancy of installation to further 23 increase system hardness.
(U) 24 The invention may be embodied in other specific 25 forms without departing from its spirit or essential 26 characteristics. ~e described embodiments are to be (~3 `" 1;~537~ `
(U) 1 considered in all ~espects only as illustrative and not2 restrictive and the scope of the invention is, therefore, 8 indicated by the appended claims rather than by the 4 foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be 6 embraced within their scope.
(S) 7 ~6 C~ ~
(S) 24 The embodiment of Figure 19 includes a control 25 center 70 which contains a transmitter/receiver (not 26 shown) a$ well as a\conventional power splitter (not ~29~3 ~2 (S) l shown) together with other electronic con~rol circuitry 2 (not shown) for controlling the system operation as 3 hereinafter more fully described. The control center 70 4 is connected to the various current driver elements generally designated at 72-76 by means of coaxial cable 6 14.
(S) 7 In the illustrated embodiment, various types of 8 current driver elements 72-76 are used in combination.
9 For example, the current driver elements 72 comprise center-fed current drivers which are similar in their ll construction and basic operation to the center-fed 12 current driver embodiment previously described in con-l3 nection with Figures 10 through 12. The current drivers 14 72 each have a pair of conducting arms 78 and 80 which are connected to an impedance matching circuit 17 which 16 may be, for example, a bal~n. The co~lducting arms 78 and l7 80 consist of No. 12 copper wire or other suitable wire, 18 cable or pipe as previously described. The length of 19 each conducting arm 78 and 80 is approximately 300 meters. At this length, current drivers 72 are desi~ned 21 to operate in the frequency range of 40 KHz to 100 KHz.
(S) 22 Current driver 73 is also a center-fed current 23 driver. The current driver 73 includes two identically 24 configured conducting arms generally designated at 82 and 84 which are placed end-to-end and which are connected to 26 a balun 17 for purpo~ses of matching the impedance of the ~Z9S372 (S) l conducting arms 82 and 84 to the impedance o ~ he trans-2 mitter/receiver (not shown) located at the control center ~ 70. For purposes of simplifying the illustration and 4 description, only the elements of conducting arm 82 are described in detail and it will be understood that the 6 opposite conducting arm 84 is identically constructed.
(S) 7Conducting arm 82 consists of four lengths of cable 8 85-88 which, in the illustrated embodiment, may be, for 9 example, No. 12 copper wire. Each length of cable varies. For example, the length of cable 85 is approxi-ll mately two meters, cable 86 is approximately two and 12 one-half meters, cable 87 is approximately four and 13one-half meters and cable 88 is approximately 286.5 14 meters. The lengths of cable 85-88 are interconnected by lS a series of switching devices 90-94. Each switching 16 device 90-93 may be, for example, a conventional elec-17 trical relay which can be activated from the control 18 center 70 to connect or disconnect the various lengths of l9 cable 85-88.
(S) 20 The switching elements 90-93 may also be conven-21tional pneumatic switches, relays or other suitable types 22of conventional switching devices. When using electrical 23relays as switches it is necessary to use RF chokes in 24the control lines at the current driver switching lo-25cations. The pneumatic switches with dielectric control l.,~t,'~
lZ~i372 ( ) 1 lines avoid the use of RF chokes with resulting increased 2 electromagnetic pulse (EMP) hardness.
(S) 3 The tree termination generally designated 94 is 4 interconnected between the switching elements 92 and 93 and forms part of~he conducting arm 82, along with the 6 lengths of.cable 85-88.
(S) 7 In use, the various lengths of cable 85-88 together 8 with the tree termination 94 may be combined in various 9 ways through the use of switching elements 90-93 to vary the length of the conducting arms 82 and 84 for purposes 11 of permitting the current driver 73 to be used over a 12 wide band of frequencies. For example, the length of 13 cable 85 is designed for operation in the range of 15 MHz 14 to 30 MHz. By using the switching element 90 to add the length of cable 86 to the length of cable 85, the current lfi driver 73 can be operated in the range of 8 MHz to 17 15 MHz. Similarly, by using switching element 91 to add 18 the length of cable 87 to the lengths of cables 86 and 19 85, the current driver can be operated in the range of 3 MHz to 8 MHz. When switching element 92 is operated to 21 add the tree termination 94 to the three lengths of cable 22 85-87, system operation is in the range of 1.5 MHz to 23 3 MHZ.
(S) 24 Also, it should be noted that other current drivers such as the current drivers 74, 75 and 76 may be designed ~6 to include lengths ~f cable, switching elements and tree i3~2 (S) l terminations which are the same as the cable lengths 2 85-87, tree termination 94 and switching elements 90-92 3 of current driver 73, with the net effect Oc creating a 4 current driver array consisting of essentially eight identical current driver elements, depending on how each 6 current driver element is switched. As previously ex-7 plained, use of eight current drivers in an array will 8 increase the signal gain by a factor of eight in the 9 particular range of operating frequencies.
(S) 10 ~y adding the length of cable 88 through the use of 11 switching element 93, the current driver 73 becomes 12 approximately the same in its length as current driver 13 elements 72, and is thus capable of operating as part of 14 a parallel array in the frequency range of 40 KHz to 100 KHz.
(S) 16 Other possible switching arrangements are illus-17 trated in connection with the current driver elements 75 18 and 76. For example, current driver 75 includes a 19 further tree termination generally designated at 96 which includes five conductors 97, each of which is approxi-21 mately eighteen meters in length. When the tree termina-22 tion 96 is switched so as to form a part of the conduct-23 ing arm of the current driver element 75, the current 24 driver 75 is operational in the frequency range of 350 KHz to 500 KHz. And, as illustrated by current driver 26 element 76, a further extension 98 can be added onto the lZ~53 o 2 \
(S) 1 tree termination 96 to further increase the length of the 2 current driver 76, thus permitting operation at even ~ lower frequency ranges.
(S) 4 ~rom the foregoing illustrative embodiment, it is apparent that the principles of the present invention can 6 be advantageously used to design a highly flexible and 7 efficient underground or surface deployed communication 8 system capable of wide band operation, and in conjunction 9 with enhanced signal processing, capable of performance at data rates closely comparable to conventional 11 aboveground wireless communication systems. The current 12 drivers used in the array can be either end-fed or 13 center-fed members which, when properly matched by the 14 baluns 17 to the impedance of he transmitter/receiver, will effectively operate with the earth as vertical plane 16 polarized antennas. Moreover, by properly combining the 17 various current driver elements of the array, signal gain 18 can be materially enhanced.
9 7. The Unidirectional Buried Current Driver (Figures 20-22? (U) (S) 21 There are a number of significant advantages to 22 having an underground communication system which includes 23 an earth antenna having the capability of end fire or 24 unidirectional operation. These advantages include (1) 25 reduced effects from noise and interference (jamming) ~6 originating from di~ tions _ot~ tl~ =D~ desired t~
12~53 ~ 2 (S) 1 communication direction; (2) increased transmit signal 2 gain both from each current driver element, and from each 3 array which utili~es this configuration; and (3) reduced 4 production of interference in other communication units not in the desired communication direction. Figure 20 6 illustrates one means for accomplishing this type o~
7 unidirectional operating capability using the principles 8 Of the present invention.
(S) 9 The embodiment of Figure 20 includes a current driver 200 having conducting arms 226 and 228 positioned 11 in end-to-end alignment and connected at their adjacent 12 ends to a balun 17 which provides impedance matching as 13 well as balance-to-unbalance transformer action. Current 14 driver 200 is positioned beneath the surface of the earth 11 in the manner previously described so as to produce 16 ground currents 208 which are induced through current 17 driver 200 by means of a transmitter/receiver (not 18 shown).
(S) 19 In the illustrated embodiment, it is seen that conducting arm 226 is substantially shorter than conduct-21 ing arm 228. In this configuration the communication 22 system functions as a unidirectional antenna. It has 23 been found that this communication system functions best 24 as a unidirectional antenna when the length ratio between 25 conducting arm 228 and conducting arm 226 is 2 to 1 or 26 greater.
~;~953 ~ ~
(S) 1 It is apparent that the current loop ~ th formed by 2 currents 208 is not centered about the balun 17, but rather the current loop is significantly elongated in the direction of the longer conducting arm 228. Thus, the 5 current loop formed by currents 208 through the earth and 6 through current driver 200 causes the current driver 200 7 and the earth 11 to function as a traveling wave antenna 8 in which the long conducting arm 228 acts as a "slow-wave 9 structure" which effectively controls the propagation lO direction of the wave produced by the basic loop antenna.
(S) 11 Figure 21 illustrates a typical signal wave pattern l2 230 of the unidirectional current driver arrangement.
13 The azimuth wave pattern for this current driver embodi-14 ment comprises an unbalanced figure eight pattern with the main or front beam being in line with the conducting l6 arms of current driver 200, and propagating in the l7 direction of the longer arm 228. The 3 dB beam angle for l8 a single element as illustrated in Figures 20 and 21 is l9 40-60 degrees wide, depending upon frequency. As the communication system operating frequency increases, 21 signal gain is found to increase and beam angle is found 22 to become narrower.
(S) 23 The front-to-back lobe ratio for the unidirectional 24 signal produced by a current driver 200 which has a 2 to 25 1 conducting arm length ratio as illustrated in Figure 21 26 is typically about 10 dB. In this configuration, the ~--t X
(S) l signal bandwidth tends to be so ~ than the 2 previously described bidirectional signal and thus the ~ unidirectional system experiences a more uniform perfor-4 mance over the 2 to 30 MHz operating range. In addition, in the unidirectional operating configuration the lower 6 cutoff frequency is controlled by the length of the 7 shorter conducting element 226, and it has been found 8 that the transmit signal gain for this configuration is 9 approximately 3 to lO dB higher than that in the bidirectional configuration, the exact amount depending ll upon the system operating frequency.
(5) 12 The unidirectional current driver configuration may 13 be utilized in array form as illustrated in Figure 22.
14 The configuration of the array of Figure 22 is substan-lS tially identical to the center-fed loop array illustrated 16 in Figures 13 through 15, with the exception that the 17 length ratio of the conducting arms is modified as 18 explained above. As with the bidirectional current l9 driver array, the current drivers in the array (four in this example illustration) are fed with power of the same 21 amplitude and phase. This is accomplished by using a 22 conventional power splitter 30 connected by coaxial 23 cables 14 to a 4-to-1 balun 17 for each current driver 24 200. It is important that the interconnecting coaxial 25 cables 14 between baluns 17 and power splitter 30 be of ~2953 J2 (S) l equal length so that the current drivers 200 are fed with 2 the same phase angle.
(S) ~ The operating parameters for a unidirectional current driver array are substantially identical with those previously discussed with respect to the 6 bidirectional current driver array configuration. Thus, 7 for wide beam operation the basic requirement is that the 8 entire width of the parallel array be no more than half a 9 wavelength in air at the system's operating frequency.
(S) 10 Likewise, the degree of improvement in system ll performance of a unidirectional current driver array as 12 compared with a single unidirectional current driver 3 system is substantially identical to the improvement in 14 performance experienced in the bidirectional current driver situation. Thus, with the current drivers 200 16 connected and balanced as illustrated in Figure 22, and 17 with proper parallel spacing between the current drivers 18 200, the power gain and efficiency of the current driver l9 array becomes a direct multiple of the number of current drivers contained in the array.
~S) 21 Still further, as is the case in the bidirectional 22 current driver situation, the signal pattern 234 of the 23 unidirectional current driver array is virtually the same 24 as the waveform 230 (Figure 21) of the single unidirectional current driver, so long as the overall ~6 current driver array width does not exceed half a L~; ~ ij lZ9~i37;~
(S) 1 wavelength in air at the operating frequency of the 2 system. The unidirectional current driver array of 3 Figure 22 also provides the advantage of steerability 4 through the use of conventional phase shifters.
(S) 5 Because of its greatly improved gain as compared to 6 that of the bidirectional current driver arrangement, 7 coupled with the further flexibility provided in the 8 steerable current driver array arrangement, the uni-9 directional current driver configuration becomes particu-larly desirable in applications requiring strong, 11 steerable directional signals, or in applications 12 requiring the ability to minimize the effects of inter-13 ference either from other sources, or as produced by the 14 current dri~er communication sys~em itself.
lS 8. The CancelIation Current Barrier (Figures 23 16 through 26) (U) (S) 17 By reference to Figures 6 and 7, it is possible to 18 identify several additional factors which reduce the 19 operating efficiency of the underground communication system disclosed herein. In particular, it can be seen 21 that even though the current driver is positioned in 22 proximity to the surface of the earth, some vertical 23 plane loop currents still exist between the current 24 driver 10 and the surface of the earth 11. These cur-rents travel in directions opposite to the loop currents ~6 beneath the current driver 10, and thus they cancel the `~ 3, 2 (S) 1 effect of some of ~he currents below the current driver, 2 thus reducing the system performance. This problem 3 becomes especially true during wet weather conditions 4 when increased conductivity in the earth above current driver lO permits increased current flow in that region.
6 Thus, one further improvement to the communication system 7 of the present invention would be to prevent the flow of 8 loop currents in the ground above current driver 10.
(S) 9 ~y reference to Figure 7 it is apparent that the currents on each side of current driver lO effectively 11 cancel each other out. Nevertheless, the energy utilized 12 to produce these cancellation currents is simply wasted, 13 co~nprising another source of communication system ineffi-14 ciency. Therefore, it would be another important im-provement to the communication system to provide a means 16 for preventing the flow of the side currents 23 as 17 illustrated in Figure 7.
(S) 18 Upon examination of Fi~ures 6 and 7 it becomes 19 readily apparent that one method of eliminating cancellation currents above the currellt driver lO is to 21 position the current driver near the surface of the 22 earth. As the depth of earth above current driver 10 23 increases, so does the volume of cancellation currents 24 flowing above that member. Thus, selection of the amount of earth over current driver 10 can be used as a L, ';2 ~9537;~
(S) 1 mechanism for inhibiting the cancellation currents which 2 may otherwise result.
(S) ~ In addition to the problems with cancellation 4 currents as described above, current, driver operation typically creates induced ground currents near the center 6 of the current driver. These induced ground currents act 7 to effectively short out the current driver, and their effect is especially apparent at high frequencies such as 9 those in excess of 20 MH~, because of the low per unit length capacitive reactance of the current driver at 11 these frequencies. As a result, it would be a further 12 improvement in the communication system to provide a 13 means for removing these unwanted center ground currents.
(s)14 One preferred embodiment of a means for overcoming the cancellation current problem described above is lfi illustrated in Figures 23 and 24. By reference to Figure 17 23 it is seen that a layer of low conductivity material 18 138 is placed over the current driver 100 so as to extend 19 between the current driver and the surface of the earth 11 along substantially the entire length of the current 21 driver 100. Layer 138 can be comprised of any low 22 conductivity material such as rocks or asphalt. By this 23 inhibiting means, ground loop currents are substantially 24 prevented from forming in the region between the top of current driver 100 and the surface of the earth 11.
26 Alternatively, the layer 138 can comprise material such ~, 3 lZ95372 (S) 1 as rocks positioned directly above the ~ ~driver 2 100, with asphal~ or some other water resistant material ~ being placed adjacent the surface of the earth. The 4 asphalt or water resistant material in this configuration serves to keep the region above the current driver dry 6 and low in conductivity in all seasons and weather 7 conditions.
(S) 8Another layer of porous low conductivity material 9 140 is positioned directly beneath the center portion of current driver 100, so as to prevent ¢r greatly reduce 11 the formation of induced ground currents near the center 12 of the current driver. Connected to the lower portion of 13the porous material 140 is a drain 142. The drain 142 is 14 provided so as to preclude collection of water in the areas containing material 140, thus preventing formation 16 of higher conducting paths through such water.
(S) 17By reference to Figure 24, it is seen that the low 18 conductivity covering 138 may be positioned so as to 19 extend outwardly from current driver 100 in the region around the central portion of the current driver. By 21 positioning the low conductivity material 138 in this 22 manner, side cancellation currents are substantially 23 eliminated.
(S) 24 Another preferred embodiment of the cancellation current barrier is illustrated by reference to Figure 25.
~6 In this embodiment~ a single trench of s~bstantially 129~372 (S) 1 uniform depth may be formed, with an initial layer of low 2 conductivity material 144 placed in the bottom thereof ~ along appro~imately the center two-thirds of the trench's 4 length. A current dri~er lOO comprised of insulated wire, or similar conductive material having reasonable 6 flexibility~ is placed within the trench so that its 7 center portion is positioned upon the low conductivity 8 material 144. The end portions of current driver lOO are 9 placed directly upon the earth. An additional layer of low conductivity material 146 is then placed over current 11 driver lOO. Optionally, an asphalt or other low 12 conductivity, water resistant covering may comprise low 13 conductivity material 146, or the water resistant materi-14 al may optionally be extended over the low conductivity material 146 near the top of the trench. Drainage for 16 this embodiment is provided by means of a nonconducting 17 pipe 148 or similar conventional drainage means extending 18 downward and away from a drain inlet position in the l9 lower portion of the trench and adjacent low conductivity material 144.
(S) 21 Still another preferred embodiment for the cancella-22 tion current barrier of the present invention is illus-23 trated in Figure 26. The configuration of Figure 26 24 comprises a small mound within which is positioned a double level trench. The lower level of the trench is 26 filled with p ~ us, low conductivity material 152 such as 1'~9S3 ~ ~ ---(S) l rock. A drain 154 positioned in the lower portion of the 2 lower trench is comprised of a low conductivity pipe ~ which provides a means for draining water away from the 4 lower portions of the trench. A current driver lO0 is positioned within the trench so that the outer portions 6 of the current driver's extended conducting arms 102 and 7 104 are positioned on the earth 11, while the central 8 portion of current driver 100 is positioned adjacent to 9 the low conductivity material 152. An additional layer of low conductivity material 156 such as porous rock is ll positioned above the current driver along its entire 12 length. Positioned upon the upper layer of low 13conductivity material 156 is a layer of asphalt 158 or 14 similar water resistant material. This mound configura-tion provides a means whereby water contacting the mound l6 surface is drained away without passing downwardly 17 through the porous, low conductivity materials 156 and 18152. The drain 154 provides a means whereby any water l9 which may enter the trench area is carried away from the vicinity of the current driver lO0.
(S) 21By reference to Figure 27, it becomes apparent that 22 the cancellation current barrier arrangement may also be 23 successfully utilized in array current driver applica-24 tions. Specifically, the configuration of the cancella-tion current barrier of Figure 27 may comprise any of the 26 embodiments described above. In addition, the outward C3S3 ~ 2 (S) 1 extending portions 160 of the low co ~ ity material 2 138 are joined together so as to define a continuous 3 e~tension between adjacent current drivers. By this 4 means, transmission of side cancellation currents is substantially prevented.
(S) 6 Through use of the cancellation current barrier 7 described above, capacitive coupling may be encouraged to 8 occur in preferential coupled regions, and communication 9 system efficiency may be further improved by amounts as much as 4 to 6 dB. In addition, the drainage and water 11 protection features af the cancellation current barrier 12 provide an important means for maintaining uniform system 13 operation in substantially any weather condition, as well 14 as for providing increased system operating life by reducing the exposure of the current driver 100 and 16 associated system elements to the corrosive and otherwise 17 damaging effects of extended contact with water.
18 9. Design ConsiderationS (U) (U) 19 The foregoing embodiments illustrate many of the wide variety of ways in which the principles of the 21 present invention can be practiced. Following is a brief 22 summary of the principal design criteria that may be 23 varied to meet the design considerations imposed by a 24 given system.
(S) 25 The first step is to determine the length of the 26 underground current driver. Current driver length is 5~
~Z~53 ~Z
(S) 1 inversely proportional to the system's signa ~frequency.
2 As described above, depending upon whether tree termina-~ tions are utilized on the current driver ends, total 6 current driver length will range from three-quarters to one signal wavelength as measured in the ground medium at 6 the desired frequency. Signal wavelength in a ground 7 medium is graphically illustrated as a function of signal 8 frequency in Figure 28. Graph line 34 represents the 9 wavelength in meters as a function of frequency in rich agricultural earth having a conductivity of 10 2 mho per 11 meter. Graph line 36 indicates the wavelength in meters 12 as a function of frequency in rocky land having a 13 conductivity of 2 x 10 3 mho per meter. The area 38 16 between lines 34 and 36 describes the signal wavelength as a function of signal frequency in the majority of 1~ typical kinds of ground media. The equation for deter-17 mining the wavelength represented by lines 34 and 36 of 18 Figure 28 is 22 ~ ( ) (1) ~6 \ _ -~ ~ z95372 1 where ~ = wavelength i~ the eàrth (meters) ~ = 1 for large depth and ~
3 for current driver at or above the surface ~ = permeability of the earth (4~ x 10 7 weber/amp-meter) E = ~0 ~r = permittivity of 7 earth (farad/meter) 8 ~ = permittivity of free space (8.85 x 10 12 farad/meter) ~ = relative permittivity of earth r (farad/meter) 11 o = conductivity of earth (mho/meter) 12 ~ = 2~ f 13 f = frequency (Hz).
(S) In the case of a communication system operating at a 16 frequency of 400 KHz, from equation (1) the signal 17 wavelength is equal to approximately 100 meters when 18 propagated through low conductivity ground at that frequency. Thus, total current driver length will range from 75 to 100 meters, depending on whether tree termina-21 tions are used at the ends of the current driver.
(S) 22 Another principal consideration in the design of the 23 present invention is the depth of current driver burial.
24 As previously indicated, there is approximately a 3 dB
drop in power gain when the current driver is buried 26 approximately one meter below the earth's surface as compared to deployment on the ground surface. This loss . ~ .. , .. , .. , . ~ . , . , , . . ~, .. .. .
.~ . . . . .
~ C~
~L2~5372 (S) 1 is principally due to the electromagne~ t~l~d cancella-2 tion effects of the vertical ground current paths 15 (see ~ Figure 1), as previously mentioned. Although reduction 4 in power gain continues to increase as the current driver is buried deeper, the rate of reduction is much less 6 significant than the initial reduction experienced as 7 between surface deployment and burial just beneath the 8 earth's surface.
(U) 9 Penetration or skin depth is graphically illustrated as a function of signal frequency in Figure 29. Graph 11 line 48 represents the penetration depth in meters as a 12 function of frequency in rich agricultural earth having a 13 conductivity of 10 2 mho per meter. Graph line 49 14 indicates the penetration depth in rocky land having a conductivity of 2 x 10 3 mho per meter. The area 47 16 between lines 48 and 49 describes the penetration depth 17 in the majority of typical kinds of ground media. The 18 equation for tetermining the penetration depth represent-l9 ed by lines 48 and 49 of Figure 29 is 2l ~ , 1 22 ~ I /1+ 2 ' ~ (2) 23 V ~ V ~ E
(~, C~
12~t53 ~2 (U) where ~ = is the penetr ~ -2 (meters) ~ ~ = 2~ f 4 ~ = the relative permeabilitv Of the surrounding earth (weber/
amp-meter) 6 a = the conductivity of the surrounding earth (mho/meter) E = the dielectric constant of the 8 surrounding earth.
(S) It is well-known that for low frequencies in the l! range of 50 to 150 KH7, equation (2) may be simplified to ~ 503 3 (3) ~9 22 Thus, using as an example a system operating at 150 KHz, 23 and assuming the conductivity of the surrounding earth to 24 be 10 3 mho per meter, from equation t3) the penetration depth woult be about 41 meters. It has been determined 26 that the maximum effective current loop depth is on the order of one sXin depth~
1~53 ~2 \
10 . Summary ( U ) (S) 2 The current driver configurations embodied in the 3 present invention offer the following advantages, among 4 others: (1) the underground positioning of the current driver provides system hardness and survivability in all 6 cases except a direct hit by a nuclear weapon, while 7 permitting data transmission on a wide frequency band 8 and, when using enhanced signal processing, at data rates 9 comparable to conventional aboveground systems; (2) because of its underground positioning and the use of 11 capacitive coupling with the earth to produce a vertical 12 plane polarized antenna configuration, the communication 13 system of the present invention permits communication at 14 frequencies ranging from the VLF range to the VHF range;
(3) by placing the current drivers underground and by 1~ using two or more current drivers in a parallel array 17 configuration, significant improvement in signal power 18 gain and communication system operating efficiency are 19 achieved, while requiring only a relatively small underground area for the placement of the current driver 21 array; (4) by use of the tree terminations, coupling is 22 improved and the mean location of capacitive coupling 23 between the current driver and the earth is extended 24 toward the outer ends of conducting arms, with the result that the required current driver length for operation at 26 a given frequency i`s~ significantly reduced; (5) because . ' ~ : '` -53 ~2 (S) 1 of increased bandwidth and directional` steering the 2 system has potential for very low vulnerability to ~ jamming and other adverse effects of interfering 4 electromagnetic radiation; (6) by configuring the current driver such that one end is shorter than the other, a 6 unidirectional vertical plane polarized antenna can 7 easily and quickly be formed, having improved signal gain 8 and narrowed beam width for further improvement in the 9 ability to avoid signal interference; (7) a means is provided whereby the undesirable effects of cancellation 11 currents can be minimized and the current driver can be 12 protected for all weather high quality operation; (8) the 13 communication system described herein is also adaptable 14 for use in a surface deployment mode upon the surface of the ground, in near surface, aboveground deployment, or lfi in a shallow trench so that the system may be easily 17 transported; (g) camouflage of the system is a simple 18 matter, and decoying through use of dummy current drivers 19 is also economically feasible; (10) environmental impacts 20 are minimized and surrounding aesthetic conditions are 21 preserved; and (11) overall costs are significantly 22 reduced, permitting redundancy of installation to further 23 increase system hardness.
(U) 24 The invention may be embodied in other specific 25 forms without departing from its spirit or essential 26 characteristics. ~e described embodiments are to be (~3 `" 1;~537~ `
(U) 1 considered in all ~espects only as illustrative and not2 restrictive and the scope of the invention is, therefore, 8 indicated by the appended claims rather than by the 4 foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be 6 embraced within their scope.
(S) 7 ~6 C~ ~
Claims (71)
1. A wireless communication system including a transmitter/
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means comprising at least two conductors positioned in proximity to the earth's surface, said conductors each comprising means for encouraging capacitive coupling of said conductors to the earth along a first portion of the length of said conductors such that a closed current loop is formed by said conductors in combination with the earth, said conductors of the current driver means having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagating through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna;
means for connecting the transmitter/receiver to the current driver means; and means for inhibiting cancellation currents along a second portion of the length of the conductors.
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means comprising at least two conductors positioned in proximity to the earth's surface, said conductors each comprising means for encouraging capacitive coupling of said conductors to the earth along a first portion of the length of said conductors such that a closed current loop is formed by said conductors in combination with the earth, said conductors of the current driver means having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagating through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna;
means for connecting the transmitter/receiver to the current driver means; and means for inhibiting cancellation currents along a second portion of the length of the conductors.
2. A wireless communication system as defined in claim 1 wherein the current driver means is protected along substantially its entire length by an external sheath of electrical insulating material.
3. A wireless communication system as defined in claim 1 further comprising means connected to the transmitter/receiver and to the current driver means for adjusting the impedance of said current driver means and the earth as measured above one wavelength at the operating frequency so as to correlate said impedance with the impedance of the transmitter/receiver.
4. A wireless communication system as defined in claim 1 wherein the current driver means is deployed substantially parallel to the earth's surface.
5. A wireless communication system as defined in claim 1 wherein the current driver means is configured such that current will be capacitively coupled to the earth in a preferential coupled region.
6. A system as defined in claim 1 wherein the length of one of said conductors is less than the length of the other of said conductors such that the electromagnetic signal transmitted outwardly in the direction of said other conductor has a greater signal gain than the electromagnetic signal transmitted outwardly in the direction of said one conductor.
7. A wireless communication system including a transmitter/
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means comprising at least first and second conductors positioned in proximity to the earth's surface, said conductors each comprising means for encouraging substantial capacitive coupling between at least a portion of said conductors and the earth in a preferential coupled region so as to establish a closed current loop formed by said conductors in combination with the earth, said conductors having sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagating through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna;
means for connecting the transmitter/receiver to the current driver means;
means connected to the current driver means for adjusting the impedance presented to the transmitter/receiver by the current driver means so as to correlate said impedance with the impedance of the transmitter/receiver; and means for connecting the adjusting means to the transmitter/
receiver.
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means comprising at least first and second conductors positioned in proximity to the earth's surface, said conductors each comprising means for encouraging substantial capacitive coupling between at least a portion of said conductors and the earth in a preferential coupled region so as to establish a closed current loop formed by said conductors in combination with the earth, said conductors having sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagating through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna;
means for connecting the transmitter/receiver to the current driver means;
means connected to the current driver means for adjusting the impedance presented to the transmitter/receiver by the current driver means so as to correlate said impedance with the impedance of the transmitter/receiver; and means for connecting the adjusting means to the transmitter/
receiver.
8. A wireless communication system as defined in claim 7 wherein the current driver means is protected along substantially its entire length by an external sheath of electrical insulating material.
9. A system as defined in claim 7 wherein the conductors are positioned upon the earth's surface along at least a portion of their length.
10. A wireless communication system as defined in claim 7 wherein the current driver means is configured such that current will be capacitively coupled to the earth in a preferential coupled region.
11. A system as defined in claim 7 wherein the current driver means further comprises conductive tree terminations attached to the end of at least one of the conductors, said tree terminations being protected about substantially their entire outer surface by an external sheath of electrical insulating material.
12. A system as defined in claim 7 wherein each of the conductors are divided into one or more lengths and wherein the current driver further comprises switching means selectively connecting the various lengths of each said conductor such that the overall length of each said conductor can be varied for purposes of system operation at various frequencies and for purposes of controlling signal gain, propagation direction and waveshape corresponding to the different possible current driver lengths.
13. A system as defined in claim 7 further comprising a plurality of current driver means which are spaced in parallel to form a current driver array, said parallel spacing between current drivers being at least half a skin depth measured in the earth at the system operating frequency.
14. A system as defined in claim 13 wherein at least one of the conductors in at least one of the current driver means in the array is divided into a plurality of lengths, with switching means selectively connected to the various lengths of each said divided conductor in order to vary the overall length of each divided conductor so as to provide for system operation at any one of a plurality of different frequency ranges, and for purposes of controlling signal gain, propagation direction and waveshape corresponding to the different possible current driver lengths.
15. A system as defined in claim 13 further comprising: a power splitter interconnected between said transmitter/receiver and each said current driver in the array, the power splitter being connected to each of the current drivers in the array through substantially equal lengths of coaxial cable, and the power splitter providing power of the same amplitude and phase to each of the said current drivers.
16. A system as defined in claim 13 wherein the overall width of the current driver array is as much as half a signal wavelength in air at the system's operating frequency without experiencing substantial signal beam compression.
17. A system as defined in claim 13 wherein one or more of the said current drivers comprise conductive tree terminations attached to the ends thereof.
18. A system as defined in claim 7 wherein the first and second conductors are positioned in end-to-end configuration, the current driver means further comprising:
a dielectric partition connected between the adjacent ends of said first and second conductors; and a coaxial cable having first and second conductors connected at one end to the adjusting means, the first conductor being connected through said dielectric partition to one of said conductors, and the second conductor being connected to the other said conductor.
a dielectric partition connected between the adjacent ends of said first and second conductors; and a coaxial cable having first and second conductors connected at one end to the adjusting means, the first conductor being connected through said dielectric partition to one of said conductors, and the second conductor being connected to the other said conductor.
19. A system as defined in claim 7 wherein the first and second conductors are positioned in end-to-end configuration and wherein the adjusting means is positioned between the adjacent ends of said first and second conductors.
20. A system as defined in claim 18 or 19 wherein said conductors are constructed from pipe.
21. A system as defined in claim 18 or 19 wherein said conductors are made from wire cables.
22. A system as defined in claim 18 or 19 wherein the length of one of said conductors is less than the length of the other of said conductors such that the electromagnetic signal transmitted outwardly in the direction of said other conductor has a greater signal gain than the electromagnetic signal transmitted outwardly in the direction of said one conductor.
23. A system as defined in claim 7 wherein said adjusting means comprises a balun.
24. A system as defined in claim 7 further comprising means for inhibiting cancellation currents, said means for inhibiting cancellation currents comprising low conductivity materials positioned so as to lie at least partially in the flow paths of said cancellation currents to inhibit flow of said cancellation currents therethrough.
25. A system as defined in claim 24 further comprising means connected adjacent the low conductivity materials for preventing collection of water in areas adjacent to the current driver.
26. A system as defined in claim 7 wherein the current driver means is buried beneath the surface of the earth at a depth of no more than one skin depth measured in the earth at the system operating frequency.
27. A wireless communication system including a transmitter/
receiver for generating and processing electromagnetic signals, the system comprising:
a current driver comprising first and second conductors placed end-to-end and in substantially axial alignment, said conductors being coated on substantially their entire exterior surface by a sheath of electrical insulating material and said conductors being deployed substantially parallel to, and in proximity to, the surface of the earth along at least a first portion of their length, said insulated sheath encouraging capacitive coupling between said first portion and the earth so as to establish a closed current loop through said first portion and the earth, said conductors having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagating through the earth, whereby the earth and said current driver function as a vertical plane polarized antenna;
means for connecting the transmitter/receiver to the current driver;
means for inhibiting cancellation currents along at least a second portion of each of said conductors, said means for inhibiting cancellation currents comprising means for reducing capacitive coupling between said second portion of the conductors and the earth;
a balun connected to the current driver for adjusting the impedance presented to the transmitter/receiver by the current driver so as to correlate said impedance with the impedance of said transmitter/receiver; and a coaxial cable interconnecting the balun to the transmitter/receiver.
receiver for generating and processing electromagnetic signals, the system comprising:
a current driver comprising first and second conductors placed end-to-end and in substantially axial alignment, said conductors being coated on substantially their entire exterior surface by a sheath of electrical insulating material and said conductors being deployed substantially parallel to, and in proximity to, the surface of the earth along at least a first portion of their length, said insulated sheath encouraging capacitive coupling between said first portion and the earth so as to establish a closed current loop through said first portion and the earth, said conductors having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagating through the earth, whereby the earth and said current driver function as a vertical plane polarized antenna;
means for connecting the transmitter/receiver to the current driver;
means for inhibiting cancellation currents along at least a second portion of each of said conductors, said means for inhibiting cancellation currents comprising means for reducing capacitive coupling between said second portion of the conductors and the earth;
a balun connected to the current driver for adjusting the impedance presented to the transmitter/receiver by the current driver so as to correlate said impedance with the impedance of said transmitter/receiver; and a coaxial cable interconnecting the balun to the transmitter/receiver.
28. A system as defined in claim 27 wherein the current driver is buried beneath the surface of the earth at a depth of no more than one skin depth measured in the earth at the system operating frequency.
29. A system as defined in claim 27 wherein the current driver further comprises conductive tree terminations attached to the end of at least one of the conductors so as to provide for system operation at lower frequencies, said tree terminations being protected along substantially their entire length by an external sheath of electrical insulating material.
30. A system as defined in claim 27 wherein the length of one of the conductors is less than the length of the other of said conductors such that the electromagnetic signal transmitted outwardly in the direction of said other conductor has a greater signal gain than the electromagnetic signal transmitted outwardly in the direction of said one conductor.
31. A system as defined in claim 29 or 30 wherein the current driver is buried beneath the surface of the earth at a depth of no more than one skin depth measured in the earth at the system operating frequency.
32. A system as defined in claim 27 wherein the inhibiting means comprises low conductivity material positioned to lie at least partially in flow paths of said cancellation currents and to inhibit flow of said cancellation currents therethrough.
33. A system as defined in claim 32 further comprising means connected adjacent the low conductivity materials for preventing collection of water in areas adjacent to the current driver.
34. A system as defined in claim 27 further comprising one or more current drivers spaced one from the other in parallel to form a current driver array.
35. A system as defined in claim 34 further comprising a power splitter interconnected between each current driver in the array and said transmitter/receiver, the power splitter being connected to each balun in the array through substantially equal lengths of coaxial cable, and the power splitter providing power of equal magnitude and phase to each of said baluns in the array.
36. A system as defined in claim 34 or 35 wherein the overall width of the said array is as much as half the signal wavelength in air at the system's operating frequency without experiencing substantial signal beam compression.
37. A method of sending and receiving electromagnetic signals using a wireless communication system comprising a transmitter/
receiver for generating and processing electromagnetic signals and a current driver means comprising at least two conductors connected to said transmitter/receiver, wherein the method comprises the steps of:
positioning the conductors in proximity to the surface of the earth;
inhibiting capacitive coupling along at least a first portion of the length of the conductors so as to reduce cancellation currents;
capacitively coupling at least a second portion of said conductors to the earth so that said capacitive coupling between said second portion and earth is encouraged and is substantially greater than the capacitive coupling between said first portion and earth, whereby said conductors and earth together provide a closed current loop; and selecting the length of the conductors to establish a closed current loop having a perimeter which is greater than one wavelength of the electromagnetic signals when propagating through the earth, whereby Raid current driver means and the earth function as a vertical plane polarized antenna.
receiver for generating and processing electromagnetic signals and a current driver means comprising at least two conductors connected to said transmitter/receiver, wherein the method comprises the steps of:
positioning the conductors in proximity to the surface of the earth;
inhibiting capacitive coupling along at least a first portion of the length of the conductors so as to reduce cancellation currents;
capacitively coupling at least a second portion of said conductors to the earth so that said capacitive coupling between said second portion and earth is encouraged and is substantially greater than the capacitive coupling between said first portion and earth, whereby said conductors and earth together provide a closed current loop; and selecting the length of the conductors to establish a closed current loop having a perimeter which is greater than one wavelength of the electromagnetic signals when propagating through the earth, whereby Raid current driver means and the earth function as a vertical plane polarized antenna.
38. A method as defined in claim 37 wherein the capacitive coupling step comprises capacitively coupling the conductors to the earth in a preferential coupled region.
39. A method as defined in claim 37 wherein the inhibiting step comprises the step of positioning low conductivity material so as to lie at least partially in flow paths of the cancellation currents and to inhibit flow of cancellation currents therethrough.
40. A method as defined in claim 39 further comprising the step of removing water from the proximity of the current driver so as to prevent prolonged contact between said current driver and said water.
41. A method as defined in claim 37 wherein the communication system includes a plurality of current drivers and wherein the method further comprises the step of placing the current drivers parallel one to the other and as close as half a skin depth between adjacent current drivers, thus forming an array of current drivers having an overall width of as much as one-half the signal wavelength in air at the system's operating frequency without experiencing substantial signal beam compression.
42. A method as defined in claim 41 further comprising the step of providing each current driver of said array with power that is substantially equal in phase and magnitude.
43. A method as defined in claim 37 further comprising the steps of:
dividing at least one of the conductors of the current driver into a plurality of lengths; and selectively interconnecting one or more of the various lengths of the conductors to provide the current driver with the capability of being used at various frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
dividing at least one of the conductors of the current driver into a plurality of lengths; and selectively interconnecting one or more of the various lengths of the conductors to provide the current driver with the capability of being used at various frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
44. A method as defined in claim 37 further comprising the step of steering the signals produced by the vertical plane polarized antennas.
45. A method as defined in claim 37 further comprising the step of burying the current driver beneath the surface of the earth at a depth of less than one skin depth.
46. A method as defined in claim 37 further comprising the step of configuring the current driver such that the length of one of its extended conductors is less than the length of its other extended conductor, whereby the electromagnetic signal transmitted outwardly in the direction of said other conductor has a greater gain than the electromagnetic signal transmitted outwardly in the direction of said one conductor.
47. A method as defined in claim 37 further comprising the step of applying a sheath of electrical insulating material along substantially the entire exterior portion of each of the conductors.
48. A method as defined in claim 36 further comprising the step of applying a sheath of electrical insulating material along substantially the entire exterior portion of each of the conductors.
49. A method as defined in claim 37 further comprising the step of orienting the current driver means so as to be substantially parallel with the surface of the earth.
50. A method of sending and receiving electromagnetic signals using a wireless communication system comprising a transmitter/
receiver for generating and processing electromagnetic signals, a current driver means comprising at least two conductors connected to said transmitter/receiver and means connected to the transmitter/
receiver and to the conductor for adjusting the impedance of the conductors, wherein the method comprises the steps of:
positioning the conductors in proximity to the surface of the earth along at least a portion of their length;
adjusting the impedance present to the transmitter/
receiver by the current driver means at the operating frequency to correlate the impedance of the current driver means with the impedance of the transmitter/receiver;
capacitively coupling said conductors to the earth in a preferential coupled region of each conductor's length so as to provide a closed current loop; and selecting the length of the conductors to establish a closed current loop having a perimeter which is greater than one wavelength of the electromagnetic signals when propagating through the earth, whereby said current driver means and the earth function as a vertical plane polarized antenna.
receiver for generating and processing electromagnetic signals, a current driver means comprising at least two conductors connected to said transmitter/receiver and means connected to the transmitter/
receiver and to the conductor for adjusting the impedance of the conductors, wherein the method comprises the steps of:
positioning the conductors in proximity to the surface of the earth along at least a portion of their length;
adjusting the impedance present to the transmitter/
receiver by the current driver means at the operating frequency to correlate the impedance of the current driver means with the impedance of the transmitter/receiver;
capacitively coupling said conductors to the earth in a preferential coupled region of each conductor's length so as to provide a closed current loop; and selecting the length of the conductors to establish a closed current loop having a perimeter which is greater than one wavelength of the electromagnetic signals when propagating through the earth, whereby said current driver means and the earth function as a vertical plane polarized antenna.
51. A method as defined in claim 50 further comprising the step of inhibiting electrical coupling along portions of the length of the conductors so as to reduce cancellation currents.
52. A method as defined in claim 51 wherein the inhibiting step comprises the step of positioning low conductivity material so as to lie at least partially in flow paths of the cancellation currents and to inhibit flow of cancellation currents therethrough.
53. A method as defined in claim 52 comprising the step of removing water from the proximity of the current driver so as to prevent prolonged contact between said current driver and said water.
54. A method as defined in claim 50 wherein the capacitive coupling step comprises capacitively coupling the conductors to the earth in a preferential coupled region.
55. A method as defined in claim 50 wherein the communication system includes a plurality of current drivers and wherein the method further comprises the step of placing the current drivers parallel one to the other and as close as half a skin depth between adjacent current drivers, thus forming an array of current drivers having an overall width of as much as one-half the signal wavelength in air at the system's operating frequency without experiencing substantial signal beam compression.
56. A method as defined in claim 55 further comprising the step of providing each current driver of said array with power that is substantially equal in phase and magnitude.
57. A method as defined in claim 50 further comprising the steps of:
dividing at least one of the conductors of the current driver into a plurality of lengths; and selectively interconnecting one or more of the various lengths of the conductors to provide the current driver with the capability of being used at various frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
dividing at least one of the conductors of the current driver into a plurality of lengths; and selectively interconnecting one or more of the various lengths of the conductors to provide the current driver with the capability of being used at various frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
58. A method as defined in claim 50 further comprising the step of steering the signals produced by the vertical plane polarized antennas.
59. A method as defined in claim 50 further comprising the step of configuring the current driver such that the length of one of its extended conductors is less than the length of its other extended conductor, whereby the electromagnetic signal transmitted outwardly in the direction of said other conductor has a greater gain than the electromagnetic signal transmitted outwardly in the direction of said one conductor.
60. A method as described in claim 50 further comprising the step of positioning the conductors upon the surface of the earth along at least a portion of their length.
61. A method of sending and receiving electromagnetic signals using a wireless communication system comprising a transmitter/
receiver for generating and processing electromagnetic signals, and a current driver comprising first and second conductors connected to said transmitter/receiver, wherein the method comprises the steps of:
positioning the conductors so as to be substantially parallel to, and in proximity to, the earth's surface along at least a portion of their length;
capacitively coupling each of the conductors to the earth along at least a portion of the conductor's length;
interconnecting an adjusting means to the conductors and through a first coaxial cable to the transmitter/receiver;
adjusting the impedance of the current driver means and the earth as measured above one wavelength at the operating frequency to correlate said impedance with the impedance of the transmitter/receiver;
inhibiting electrical coupling along portions of the length of the conductors so as to eliminate cancellation currents;
and inducing an electrical current into the earth through at least a portion of the capacitively coupled portion of the conductors to establish a closed loop current having a perimeter which is greater than one wavelength of the electromagnetic signals when propagating through the earth, such that said current driver and the earth function as a vertical plane polarized antenna.
receiver for generating and processing electromagnetic signals, and a current driver comprising first and second conductors connected to said transmitter/receiver, wherein the method comprises the steps of:
positioning the conductors so as to be substantially parallel to, and in proximity to, the earth's surface along at least a portion of their length;
capacitively coupling each of the conductors to the earth along at least a portion of the conductor's length;
interconnecting an adjusting means to the conductors and through a first coaxial cable to the transmitter/receiver;
adjusting the impedance of the current driver means and the earth as measured above one wavelength at the operating frequency to correlate said impedance with the impedance of the transmitter/receiver;
inhibiting electrical coupling along portions of the length of the conductors so as to eliminate cancellation currents;
and inducing an electrical current into the earth through at least a portion of the capacitively coupled portion of the conductors to establish a closed loop current having a perimeter which is greater than one wavelength of the electromagnetic signals when propagating through the earth, such that said current driver and the earth function as a vertical plane polarized antenna.
62. A method as defined in claim 61 wherein the inhibiting step comprises the step of positioning low conductivity material so as to lie at least partially in flow paths of the cancellation currents and to inhibit flow of cancellation currents therethrough.
63. A method as defined in claim 62 further comprising the step of removing water from the proximity of the current driver so as to prevent prolonged contact between said current driver and said water.
64. A method as defined in claim 61 wherein the communication system includes a plurality of current drivers and wherein the method further comprises the step of placing the current drivers parallel one to the other and as close as half a skin depth between adjacent current drivers, thus forming an array of current drivers having an overall width of as much as one-half the signal wavelength in air at the system's operating frequency without experiencing substantial signal beam compression.
65. A method as defined in claim 64 further comprising the step of providing each current driver of said array with power that is equal in phase and magnitude.
66. A method as defined in claim 61 further comprising the step of burying the current driver beneath the surface of the earth at a depth of less than one skin depth.
67. A method as defined in claim 61 further comprising the steps of:
dividing at least one of the conductors of the current driver into a plurality of lengths; and selectively interconnecting one or more of the various lengths of the conductors to provide the current driver with the capability of being used at various frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
dividing at least one of the conductors of the current driver into a plurality of lengths; and selectively interconnecting one or more of the various lengths of the conductors to provide the current driver with the capability of being used at various frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
68. A method as defined in claim 61 further comprising the step of configuring the current driver such that the length of one of its extended conductors is less than the length of its other extended conductor, whereby the electromagnetic signal transmitted outwardly in the direction of said other conductor has a greater gain than the electromagnetic signal transmitted outwardly in the direction of said one conductor.
69. A wireless communication system including a transmitter/
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means having at least a first conductor and a second conductor, each of said conductors being divided into one or more lengths, said conductors being deployed in proximity to the earth's surface along at least a portion of their length;
switching means for selectively connecting the various lengths of said conductors such that the overall length of said conductors can be varied for the purpose of system operation at various frequencies and for purposes of controlling signal gain, propagation direction and waveshape;
means for connecting the transmitter/receiver to the current driver;
means for inhibiting capacitive coupling between said current driver means and the earth along at least a first portion of said conductor so as to reduce cancellation currents;
means connected to the current driver means for adjusting the impedance presented to the transmitter/receiver by the current driver means so as to correlate said impedance with the impedance of the transmitter/receiver;
means for connecting the adjusting means to the transmitter/receiver; and said conductors each comprising means for capacitively coupling said conductors to the earth along at least a second portion of their length such that capacitive coupling along said second portion is substantially greater than the capacitive coupling along said first portion, whereby said current driver means in combination with said earth provides a closed current loop, said conductors of the current driver means having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagated through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna.
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means having at least a first conductor and a second conductor, each of said conductors being divided into one or more lengths, said conductors being deployed in proximity to the earth's surface along at least a portion of their length;
switching means for selectively connecting the various lengths of said conductors such that the overall length of said conductors can be varied for the purpose of system operation at various frequencies and for purposes of controlling signal gain, propagation direction and waveshape;
means for connecting the transmitter/receiver to the current driver;
means for inhibiting capacitive coupling between said current driver means and the earth along at least a first portion of said conductor so as to reduce cancellation currents;
means connected to the current driver means for adjusting the impedance presented to the transmitter/receiver by the current driver means so as to correlate said impedance with the impedance of the transmitter/receiver;
means for connecting the adjusting means to the transmitter/receiver; and said conductors each comprising means for capacitively coupling said conductors to the earth along at least a second portion of their length such that capacitive coupling along said second portion is substantially greater than the capacitive coupling along said first portion, whereby said current driver means in combination with said earth provides a closed current loop, said conductors of the current driver means having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagated through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna.
70. A wireless communication system including a transmitter/
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means having at least two conductors positioned in proximity to the earth's surface along at least a portion of their length, said current drivers comprising conductive tree terminations attached to the ends thereof;
means for inhibiting cancellation currents;
means for connecting the transmitter/receiver to the current driver means;
means connected to the current driver means for adjusting the impedance presented to the transmitter/receiver by the current driver means so as to correlate said impedance of the current driver means with the impedance of the transmitter/receiver;
means for connecting the adjusting means to the transmitter/receiver; and said conductors each comprising means fox encouraging capacitive coupling between the earth and said conductors along at least a portion of their length so as to provide in combination with the earth a closed current loop, said conductors of the current driver means having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagated through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna.
receiver for generating and processing electromagnetic signals, the system comprising:
current driver means having at least two conductors positioned in proximity to the earth's surface along at least a portion of their length, said current drivers comprising conductive tree terminations attached to the ends thereof;
means for inhibiting cancellation currents;
means for connecting the transmitter/receiver to the current driver means;
means connected to the current driver means for adjusting the impedance presented to the transmitter/receiver by the current driver means so as to correlate said impedance of the current driver means with the impedance of the transmitter/receiver;
means for connecting the adjusting means to the transmitter/receiver; and said conductors each comprising means fox encouraging capacitive coupling between the earth and said conductors along at least a portion of their length so as to provide in combination with the earth a closed current loop, said conductors of the current driver means having a sufficient overall length so that the closed current loop has a perimeter which is greater than one wavelength of said electromagnetic signals when propagated through the earth, whereby the earth and said current driver means function together as a vertical plane polarized antenna.
71. A method of sending and receiving electromagnetic signals using a wireless communication system comprising a transmitter/
receiver for generating and processing electromagnetic signals and a current driver means comprising at least two conductors connected to said transmitter/receiver, wherein the method comprises the steps of:
positioning the conductors in proximity to the surface of the earth along at least a portion of their length;
dividing at least one of the conductors of the current driver into a plurality of lengths;
capacitively coupling said conductors to the earth along a first portion of their length so that said conductors and earth together provide a closed current loop having a perimeter which is greater than one wavelength of the electromagnetic signals when propagated through the earth, whereby said current driver means and the earth function together as a vertical plane polarized antenna;
inhibiting capacitive coupling along a second portion of the lengths of the conductors so as to reduce cancellation currents in the region of said second portion; and selectively interconnecting one or more of the various lengths of said conductors to provide the current driver with the capability of being used at various signal frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
receiver for generating and processing electromagnetic signals and a current driver means comprising at least two conductors connected to said transmitter/receiver, wherein the method comprises the steps of:
positioning the conductors in proximity to the surface of the earth along at least a portion of their length;
dividing at least one of the conductors of the current driver into a plurality of lengths;
capacitively coupling said conductors to the earth along a first portion of their length so that said conductors and earth together provide a closed current loop having a perimeter which is greater than one wavelength of the electromagnetic signals when propagated through the earth, whereby said current driver means and the earth function together as a vertical plane polarized antenna;
inhibiting capacitive coupling along a second portion of the lengths of the conductors so as to reduce cancellation currents in the region of said second portion; and selectively interconnecting one or more of the various lengths of said conductors to provide the current driver with the capability of being used at various signal frequencies and of producing various signal types and waveshapes depending upon the number of lengths connected together.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000597466A CA1295372C (en) | 1989-04-21 | 1989-04-21 | Wireless communication system using current formed underground vertical plane polarized antennas |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000597466A CA1295372C (en) | 1989-04-21 | 1989-04-21 | Wireless communication system using current formed underground vertical plane polarized antennas |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1295372C true CA1295372C (en) | 1992-02-04 |
Family
ID=4139941
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000597466A Expired - Fee Related CA1295372C (en) | 1989-04-21 | 1989-04-21 | Wireless communication system using current formed underground vertical plane polarized antennas |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1295372C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8395878B2 (en) | 2006-04-28 | 2013-03-12 | Orica Explosives Technology Pty Ltd | Methods of controlling components of blasting apparatuses, blasting apparatuses, and components thereof |
-
1989
- 1989-04-21 CA CA000597466A patent/CA1295372C/en not_active Expired - Fee Related
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8395878B2 (en) | 2006-04-28 | 2013-03-12 | Orica Explosives Technology Pty Ltd | Methods of controlling components of blasting apparatuses, blasting apparatuses, and components thereof |
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