CA1097756A - Multiport cable choke - Google Patents

Abstract

MULTIPORT CABLE CHOKE

ABSTRACT OF THE DISCLOSURE
A unitary broadband high impedance isolation section for a plurality of closely spaced antennas as well as other elec-trical apparatus wherein the respective separate coaxial cables feeding the antennas and the shielded multi-conductor cable for the other electrical apparatus are wound in the same direction on a common core and have the same number of turns, with the shields or outer conductors of all the cables being respectively connected together at the beginning and the end of the windings. The multi-port cable choke thus configured can be provided with respective connectors at each end of the choke for facilitating ease of installation into the electrical system, that is, a single multi-port cable choke can be inserted in the feed lines of multiple antennas and other electrical equipment simply by means of making suitable interconnections by way of the connectors on both sides of the multiport cable choke assembly. Further, the multiport cable choke can be contained in a suitable dielectric housing enveloping the common core and the cable windings and the connectors can be mounted in this housing.

Description

- -~ The present invention relates generally to isolation ¦ apparatuc fGr mulviple antenna installations ard more particularly ¦Ifor cervain ,ns'allations such 2S portable aircral' ccntrol l¦~acilities ha~ring many closely spaced antennas and other auxil'a~y !1 app2ratus such as a wind sensor toge'h2r ~ith 2 plurality of re-spect've radio transceiverC. This equipment is inciuded in a se~f I contained unit which as a consequence o~ relatively close spac ng normally has undesired interaction bet~Jeen the antennas, the vertical'y disposed feed l nes, and the wind sensor convrol cable.
It is well-known that the electrical characteristics of an antenna can be adversely affected when situated in the vicinit~
of other antennas, metal masts, metal surfaces, electrical wiring or transm~ssion lines. For example, the antenna impedance and I radiation c~.aracteristic may be substantially changed due to the l parasitic excitation and reradiation by and from the other conduc-¦~tors.

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1~)~7756 In certain militar~ installations, such as in aircraft control towers where many closely spaced antennas for different frequencies are employed, acceptable antenna performance is diffi-cult to obtain. The various antennas interact with one anGther, resulting in modified impedances and radiation characteristics.
When attempting to analyze the radiation characteristics of a complicated system such as a portable aircraft traffic con-trol facility for military use, and more particularly a facility such as the A~/TS~-97, not only the antennas, but the entire structure consisting of antennas, cables~ ~-ind sensor, console ground and metal masts, have to be regarded as a complex system for radiating and absorbing electromagnetic waves. Accordingly, it is very difficult if not impossible to predict theoretically the radiation characteristics of such a complex structure. It is practical, however, to measure the antenna patterns in the hori-zontal and vertical planes, but the experimental determination of radiating characteristics is a time consuming procedure when the operating bandwidth is great and therefore such a determination m~y have to be based upon measurements made at only a few selected frequen~ies.
One approach to the problem of improving the radiation properties of a multi-antenna system is to locate all of the antennas on a common vertical axis. Such an arrangement provides a substantially omnidirectional pattern in the horizontal plane.
Radiation in the vertical plane~ however, depends upon curren~
distribution, antenna height above grounG an~ operating frequency.
Over all, such an antenna s~stem provides relatively bet~er per- ¦
formance in comparison to ~ny radiating system hav~ng an'ennas mounted in a broadside relationship.

10~7756 Where, however, a broadside array is desirable, notwith-standing the advantages gained by a vertical in-line array, it is possible to reduce the strength of the induced feed line currents by careful arrangement of radiators and avoiding resonant lengths 5 of feed lines; however, in a case of systems operat~ng over a broad frequency ran~e, e.g., over two octaves for example, it is very difficult to pick optimum lengths of cables, etc.
One means which is helpful in optimizing broadband radiating systems is to insert separate high impedance broadband 10 cable chokes ir. series with the antenna feed lines. When connected in series with feed lines, the high impedance property of the cable choke can tend to suppress feed line current flowing on the ~utside of the feed line induced by the impressed electrical field.
Illustrative examples of such apparatus is taught for example, in 15 U.S. Patent 3,879,735 of Donn V. Campbell and James J. Arnold, entitled "Broadband Antenna System With Isolated Independent Radi-ators"; and U.S. Patent 3,961,331 of Donn V. Campbell, entitled "Lossy Cable ~hoke Broadband Isolation Means for Independent Antennas'l, both assigned to the assignee of the present invention.
20 The cable chokes illustrated therein consist of a high ind~ctance made by winding a plurality of coaxial cables in the shape of a helix and with ~everal separate cable chokes being provided ln those cases in which d~fferent frequency ranges are involved. For example, one choke may impede feed line current at frequenc~es be-25 tween 30 and 80 M~z while another choke may be designed for the~requency range of 200-40~ MHz. At VHF frea~uencies, the cho~e ¦would normally be wound on a magnetic core such 2S ferr~te in order to maximize the inductance of the cable choke wh~le at ~-HF fre-encies, the magnetic core is usuall~ deleted.

SUMMARY
Instead of employlng a separate choke for each feed line, it has been found practical to utilize one or more multiport cable chokes in the feed line in accordance with the teaching of the sub~ect invention. Briefly, the subject invention is directed 5 to the improvement comprising the winding of a plurality of coaxial cables and any other shielded multiconductor cable(s) respectively adapted for connection to a plurality of antennas and auxiliary apparatus, in the same direction on a common core with the same number of turns and having the shields or outer conductors of all lO the cables commonly connected together at each end of the winding ad~acent to the core. Additionally, the choke including the core and windings may be included in a non-magnetic, non-conducting housin~ having end walls including connector means respecti~ely for each of the coaxial cables and shielded multi-conductor 15 cables(s). These chokes preferably are inserted a quarter wave length from the antennas for maximum effect. When a broad band of frequencies, or a plurality of separate frequencles, is lnvolved, one can establish the quarter wave spacing on the basis of the center of the band or the arithmetic mean freauency, as the case 20 may be. The chokes are each tuned to the geometrical mean fre-quency when the fre~uency bandwidth is of the order of an octave or less. For example, if the bandwidth extends between frequencies fl or f2, or if the two separate frequencies fl and f2 are sepa~ate by not more than an octave, the chokes ~lould be tuned to approx-25 imately f~ = ~ . If two separate widely spaced frequencies areinvolved, one multiport choke could be tuned to frequency fl and t other multiport cnoke tuned to frequency ~2 ~0~7756 B~I~F D~SC~IPTION OF THE ~RAWINGS
Figure 1 ls illustrative of prior art practice whereby separate cable chokes are utilized for each antenna of an array, Figure 2 is a diagram illustrating relative amplitude of surface current as measured along the length of a typical mast or 5 line connected to a half wave dipole antenna;
Figure 3 is a diagram illustrating the measured current amplitude along the same structure as shown in Figure 2 when cable chokes are positioned at quarter wave length intervals;
Figure 4 is a schematic diagram broadly illustrative of 10 the concept of the subject invention;
Figure 5 is a plan view of one embodiment of the subject invention;
Figure 6 is a plan vlew of another embodiment of the subject inventlon equivalent to the one shown in Figure 5;
Figure 7 is an electrical schematic view further illus-trative of the concept of the sub~ect invention;
Figure 8 is a plan view of yet another embodiment of the subject invention;
Figure 9 is a plan view of an embodiment of the sub;ect 20 invention equivalent to the embodiment shown in Figure 8;
Figure 10 is a perspective view of the housing for the sub~ect inventionj Figure 11 is a perspective view of an embodiment of the subject invention which is adapted to be located ~n the houslng 25 shown in Figure 10;
Figure 12 is an electrical equivalent circu~t diagram ~f the subject invention;

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lQ~7756 Flgure 13 is a diagram i11ustrat'~e Or the reactance vs.
frequency characteristic of the sub~ect inventionj Figure 14 is a diagram illustrative of a radio system for a plurality of radio apparatus operating on different frequency 5 bands and employing more than one multiport cable choke according to the sub~ect invention; and Figure i5 is a diagram illustrative of a retransmission station employing the sub~ect invention for improving isolation be-tween receiving and sending radio apparatus.

DESCRIPTION OF THE PREFERRED EMBODIME~TTS
An antenna is a conductor so constructed as to either radiate electromagnetic energy, to collect electromagnetlc energy, or both. A transmlttlng antenna converts electrical energy into 15 electromagnetic waves called radio waves ~Jhich radiate away from the antenna at speeds near the velocity of light. A receiving antenna converts electromagnetic waves which it intercepts into electrical energy and applies this energy to electronic circuits for interpretation. Some antennas are adapted to serve both func-20 tions and accordingly the electrical and physical features are de-termined by the use to which they are put. Such ~eatures will vary with operating fre~uency, power handling capability, plane of polarization, and desired radiation field pattern. The physical size of an antenna is determined by its operatin~ frequency and 25 power handlin~ capability whlle its shape and height are deter-mined by the desired radiation field pattern. Such apparatus is well known to those skilled in the art and is well documented in all the literature dealing with fundamentals of radio trans~.ission.

The sub~ec' invention is directed to an im~roved means 3o for reducing the interaction of closely spaced antenn2s, auxiliary apparatus, and the respective feed lines therefor.

ll 10.-7756 Referring now to the drawings, reference is first made to Figure 1 whlch is ~llustrative of prior art practice wherein a plurality of radio apparatus 10, 12, and 14 are coupled to re-spective dipole antennas 16, 18 and 20, mounted in a broadside 5 array by means of the feed lines 22, 24 and 26. As shown in Figure 1 single port chokes 17, 19 and 21, constructed in the man-ner shown in Figure 6 of the earlier-mentioned patent #3,879,73~, preferably should be located as close as possible to the point of connectlon of the feed lines to the corresponding antenna radiating 10 elements (for example, the dipole ar~s in a center-fed dipole antenna) to create a high impedance point at said point of con-nection and establish the correct electrical length of the radi-ating elements. The single port choke could be replaced by a quarter wave coaxial sleeve choke with the sleeve surrounding the 1~ feed llne and the end of the sleeve remote from the antenna con-nected to the shield of the feed line. In addition, each of the feed lines 22, 24 and 26 includes a separate series connected cable choke 28, 30, 32. The feed lines 22, 24 and 26 are comprised of coaxial transmission lines and the cable chokes 28, 30 ~nd 32 are 20 formed fro~ a portion of the coaxial cable wound in the shape of a helix as shown for example in the above referenced U.S, Patent 3,879,735, and are configured for the operating frequencies of the respective radio app~ratus 10, 12 and 14 with which they are util,zed.
F~gure 2 illustrates the undesirable build up of surface currents along a typical single coaxia~ antenna feed line 11 con-nected between a dipole antenna 13 and ground and including a single cable choke 23 indicated by a cross in Figure 2 (corre-sponding to chokes 17, 19 and 21 of Figure 1) located at one end 3o of the dipole to define the half wave length ol the dipole. In Il l~g7756 dditlon to the dipole radiating current, spurious current peaks ~ccur at half wave length intervals along the feed line 11.
By positioning a plurality of single port cable chokes 25 along feed line 11 at quarter wave intervals, starting at a 5 quarter wave length from the antenna choke 23, as indicated in Figure 3 the measured antenna feed line current is substantially ¦eliminated, except for a negligible current peak ~ust below the ~ipole. It has been found that, at a distance of about one wave length from the dipole, further chokes usually are not required.
10 In some instances, one single port choke 23 can provide adequate reduction of feed line current. The larger the inductance of the choke, the greater is the reduction of shield currents along the feed line.
Referring now to Figure 4, there is disclosed in ~lock 15 diagrammatic form the basic concept of the sub~ect invention.
Single port chokes 17, 19 and 21 are connected ad~acent the re-spective antennas, 16, 18 and 20, as shown in Figure 1. The plurality o~ radlo apparatus 10, 12 and 14 of ~igure 4 are coupled to their respective antennas 16, 18 and 20 through unitary multi-20 port cable chokes 34A and 34B, two embodiments of which are shownin Figures ~ and 6. Figure 5 is illustrative of one embodlment of the sub~ect invention wherein the three feed lines 22, 24 and 26 are wound side by side in the same dlrection with the same number of turns on a common core 33 comprised preferab'y of a cylinder of 25 ferrous material such as ferrite, but, when desired3 can be made from non-~errous non-conducting material. Each of the multiport cable cho~es 34 are inserted a quarter wave ~ength from the cor-respon~ing single port chokes 17, 19 and 21 at the respecti~e an-tennas 16, 18 and 20, as sho~n in ~igure 4 and the interval be-3o tween chokes 34A an~ 34B is a ~uarter wave length. In some cases,one multiport choke along the feed lines may not suffice. Usually, 1~ 10~7756 one can reduce spurious feed line currents to a negligible amount ¦after proceeding about one wave length along the feed line; in ¦other words, no more than four mult$port chokes 34 are normally . ¦recuired.
5 ¦ The shields or outer conductors of the coaxial cables ¦22, 24 and 26 are electrically connected together at both ends of ¦the respective windings such as at points A and B as shown in ¦Figure 5. When so connected, the radio frequency potential of ¦all three windings is the same at end A and likewise the potential 10 ¦of the three windings is the same at point B, although the poten-¦tial at point A may differ from the potential at point B. The ¦configuration shown in Figure 6 is similar in all respects except ¦that a toroidal core 35 is depicted; as in Figure 5, all coaxial ¦cable windings are wound in the same direction with the same num-15 ber of turns and the shields are interconnected at the ends of the windings.
Referring now to the configuration shown in Figure 7, there is contemplated in addition to a pair of radio apparatus 36 and 38 coupled to respective antennas 40 and 42, the use of auxil-20 ~ary apparatus 44 which may be, for example, a wind sensor orother devices associated with a portable aircraft traffic control ~acility coupled to respecti~e control and/or metering apparatus 46 through a shielded multi-conductor cable 48.
Owing to the proximity of the feed line 48 for the aux-25 iliary apparatus 44 of Figure 7 to the feed lines 50 and ~2 forantennas 40 and 42, the possibility exists that, in the absence of choke means, in the feed lines 50 and 52, spurious currents could appear on feed lines 50 and 52 and induce spurious sh~eld currents in feed line 48. Moreover, spurious currents could also 3o be induced in all three ~eed lines 48, 50, and 52 ol ~igure 7 owing ~o the presence of additional radiation sources in the vicinity.
hese undesirable shield currents in feed line 48 could then be oupled lnto auxlllary apparatus 44 or device 46, thereby adversely nfluencing the operation of devices 44 and 46. Similarly, when no hoke means is used in the feed lines, it is possible that spurious urrents appearing alon6 feed line 48 could be induced into antenna eed lines 50 and 52.
As in the case of Figure 4, and for reasons pointed out n the description of Figure 4, single port chokes 17' and 19' are nserted adjacent the antennas 40 and 42; in addition, a multiport hoke 49 is inserted at the point of connection of the feed line 48 o the auxiliary apparatus. The multiport cable choke 34' of ?igure 7 (in the case illustrated, a three-port choke) is positionet L
quarter wave length from the chokes 17'~ 19', and 49'. The feed ine 48 ~s illustrated in Flgure 7 as including three separate con-~uctors; however, the number of such conductors is not restricted to three.
Although only one multiport cable choke 34' is shown in the feed lines of Flgure 7, two or more such multiport cable chokes 20 can be used, as indlcated ln Figure 4, and such cable chokes would ~e spaced apart from one another by a ~uarter wave length, and the ~ultiport cable choke nearest cable chokes 17', 19', and 49' would be spaced one quarter wave length ~rom each of the latter three cho~es.
Schematically, the multiport cable choke 34' is shown in two forms in Figures 8 and 9 wherein the two coaxial feed lines 50 and ~2 are wound together with the shielded multiconductor cable 48 on a common core com~osed of ferrous or non-ferrous, non-conducting material. In Figure 8 there is disclosed a cylin-3o ~rical core 5~ whereas in Figure 9 a toroidal core 55 is shown.

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11 1~977S6 11 `
The cables 48, 50 and 52 are wound in the same direction a~out the core 55 and are located ad~acent one another; furthermore, each of the cables has the same number of turns on the core. It should also be pointed out that the cables 48, 50 and 52 are covered with 5 suitable insulation so as to prevent short circuits between ad~a-cent turns. As in the case for the other embodiments shown in ~igu~es 5 and 6, at the be~inning and end of the winding, i.e., at points A and B, the shields or outer conductors are electrically connected together for the purposes set forth above. Accordingly, 10 since the three cables 48, 50 and 52 are wound in the same direc-tion on a common core 54 with the same number of turns, it follows that the impedance characteristic of the entire assembly will be unitary.
The single multiport cable choke concept is a dlstinct 15 improvement over the prior art due to the fact that the following disadvantages accrue with the use of separate cable chokes as shown in Figure 1. First, the capacitive coupling between ad~a-cent shields, i.e., outer conductors, and the fact that the RF
potential of the shields at the points where they are connected to 20 the separate chokes wlll in general cause external feed line cur-rents to be induced on the shield which in turn will interfere with the normal operation of the separate antennas. ~econdly, the fact that the chokes are wound on separate cores the effective resonant frequency o~ the respective chokes will tend to be dlf-25 ferent and will be a function of the spacing and placement of thecho~es with respect to each Gther, whereas the multiport caole choke of the subject invention will have a well established uni-tary resonant frequency.

~ 10_17~6 Referring now to Figures 10 and 11, there is disclosed a physical embodiment of the present invention. Reference numeral ~6 des~gnates a generally cylindrical dielectric housing having a pair of end walls 58 and 60. A first plurality of coar.ial connec-5 tors 62, 64 and 66 and a multipin connector 6~ are mounted onthe end wall 58 while a like number of coaxial connectors 70, 72 and 74 as well as a corresponding multipin connector 75 are .~ounted on the other end wall 60. Three coaxial cables 76, 78 and 80 as well as a set o electrical conductors 82 inside of a 10 braided shield 83 (shown partially cut-away) are wound ad~acent one another on a toroidal ferrite core 84 and being held in posi-tion on the core by means of a piece of electrical tape 86. Thus, for example, the coaxial cable 76 terminates in opposing coaxial connectors 62 and 7~, the coaxial cable 78 in opposing coaxial 15 connectors 64 and 70 and coaxial cable 80 in coaxial connectors 66 and 72. The se' of electrical conductors 82 accordingly ter-minates in the pair of multipin connectors 68 and 75. The fact that a~l of the coaxial cables and electrical conductors have the same number of turns and the coaxial cables and the shielded 20 cable have their outer conductors commonly connected together as by connections 88 and 87, ad~acent their respecti~e connectors exhibits an equi~alent circuit as shown in Figure 12 and having a reactance vs. frequency characteristic such as sh~wn in ~igure 13, wherein a single resonant frequency fO is established.
The number of turns used in a given choke depends on such factors as the core permeability, operating frequency and required bandwidth. In general, bandwidth is inversely propor-tional to the self-capacitance C (figure 12) of the ~-ir.ding.
5reatest bandwidth is obtained by minimiz ng the self-capacitance.
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Another ~actor to be consldered ~n the design Or a cable choke according to the sub~ect invention is the power loss in the magnetic core and surrounding dielectric material. When connected in series with the transmission line, it is possible 5 that high RF voltages can develop across the choke and consider-able power could be dissipated in the choke.
For operation in the frequency range of 30-70 MHz, the parallel resistance ~ of the multiport cable choke is in the order of 10,000 ohms; however, as long as the radlo ~requency voltage 10 across the choke is less than 100 volts, rms power loss is less than one watt. The reactance of the multiport cable choke, as is well known, is a function of frequency and varies in the same way as the reactance of a parallel tank circuit being positive ~or frequencies below resonance and negative for frequencies above 15 resonance.
~ andwidth ls deflned somewhat arbitrarily by the frequency range within which the cable choke's reactance exceeds a certain minimum value. For example, suppose that the minimum acceptable choke reactance is 1,000 ohms in a particular appli-20 catlon. With such a specification, the VHF multiport cable chokeaccording to the sub~ect invention has a broadband width extending from approximately 37 MHz to above 70 ~Hz.
Figures 14 and 1~ are included to indicate the use of multiport cable cho~e tuned to different resonant requencies to 25 reduce antenna interaction and feed line radiation ~here different ~requency bands are employed by separate radio apparatus. More particularly, a VHF-AM radio 88, a VHF-FM radio 90, and a UHF-radio g2 are coupled ~o respective antennas 94, 96 and 98 through two multiport cable chokes 100 and 102 and which would be con- !
3o~ ~igured, for example, in the manner shown in Figures ~ and 6.

ll lQ97756 As indicated in connection with the embodiments o~ ~igures 4 and 7, high impedance chokes 17", 19", and 21" are inserted ad~acent antennas 94, 96, and 98, respectively.
Referring now to Figure 15, there is disclosed another 5 application for the sub~ect invention wherein one or more multi-port cable chokes 104a to 104c and lC6a to 106c spaced at quarter wave intervals are used to improve electrical isolation between radio apparatus 108 and 110 included in a retransmission station which ~or example receives signals on antenna 112 and re-transmits 10 radio signals from antenna 114. Reference numeral 116 denotes a harness including various audio/control wiring/radio frequency cables lnterconnecting the radio apparatus 108 and 110.
Thus what has been shown and described ls a multiport cable choke which by virtue of its high impedance property mini-15 mizes feed line radiation while at the same time providing connec-tions as may be required for remote control power supply or sensing apparatus and radio frequency signal transmission while at the same tlme reducing weight and complexity of the radio system.
Addltionally, when desired, such apparatus ls adapted to improve 20 electrical isolation between radio apparatus irrespective of their ~ tl~n ~ c~n~ h ~A~nA- ~e~ n- , ~d o r~r~h

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
AS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination, a plurality of electromagnetic wave transmission lines each interconnecting one group of elec-trical apparatus and another group of electrical apparatus, and at least one multiport choke means for preventing buildup of un-desired radio frequency currents along the external portions of said wave transmission lines, said multiport choke means comprising a single core having a plurality of windings of the same number of turns adjacently wound on said core in the same direction, said windings each comprising an electrical conductor means with a shield member, each of said windings being continuous with a corresponding one of said wave tranmission lines, said multiport choke means further including a direct electrical connection adjacent said core between said choke shield members at each end of said choke windings.

2. The combination of claim 1 wherein said single core is a toroidal core.

3. The combination of claim 1 wherein said single core is of generally cylindrical shape.

4. The combination of claim 1 including connector means for said wave transmission line and connector means at both ends of the windings of said multiport choke means for interconnecting corresponding wave transmission means and shielded electrical con-ductor means.

5. The combination of claim 4 wherein said multiport choke means is contained within a dielectric housing having said conductor means mounted thereto.

6. The combination of claim 1 wherein each of one of said groups of electrical apparatus comprises antenna means.

7. The combination of claim 1 wherein said one multiport choke means is positioned substantially a quarter wave length distant from said one group of electrical apparatus.

8. The combination of claim 7 further including a high impedance choke positioned in each of said wave transmission lines at the point of connection of said corresponding transmission line to the respective electrical apparatus and disposed a quarter wave length from said one multiport choke means.

9. The combination of claim 1 wherein each of said one group of electrical apparatus comprises a receiving antenna and a receiver and each of said second group of apparatus comprises a transmitter and a transmitting antenna.

10. The combination of claim 1 wherein each of said one group of electrical apparatus is an antenna and each of the other of said group of electrical apparatus is a radio receiver.

11. The combination of claim 1 comprising a plurality of said multiport choke means spaced at quarter wave length in-tervals along said wave transmission lines.

12. The combination of claim 11 further including a high impedance choke positioned in each of said wave transmission lines at the point of intersection of said corresponding trans-mission line to the respective electrical apparatus and disposed a quarter wave length from the multiport choke means nearest said electrical apparatus.

13. The combination of claim 12 wherein one of said transmission lines includes a plurality of electrical conductors within the shield member and the high impedance choke associated therewith is a multiport choke.

14. The combination of claim 11 wherein said trans-mission lines propagate wave energy at more than one frequency and wherein the spacing of said multiport choke means is based onthe arithmetic mean of the frequencies involved.

15. The combination of claim 1 wherein said core is made of ferrous material.

16. The combination of claim 1 wherein said core is made of non-ferrous material.