US2860340A - Coupling device for ground antennas - Google Patents

Description

Nov. 1l, 1958 w. M. BARRET r-:T AL

couPLING DEVICE FOR GROUND ANTENNAS Filed March 17, 1951 w\ s l w Q .E i I I I I I I I I I I I I I I INI I I J I I I IIIII .WCSQQY bbku m. @Nk

United States Patent O "i COUPLING DEVICE FOR GROUND ANTENNAS William M. Barret and Clarence H. Clement, Shreveport, La., assignors to rllfhe Engineering Research Corpora-- tion, Shreveport, La., a corporation of Louisiana Application March 17, 1951, Serial No. 216,206

2 Claims. (Cl. 343-822) This invention relates to the art of electrical prospecting, and more particularly to a coupling device for tuning and matching ground antennas to radio transmitters and receivers.

With the more extensive use of ground antennas in radio-prospecting systems for exploring the earth in search of mineral deposits, there has developed an insistent need for an improved means for coupling the transmitter to the ground antenna (or antennas) usually embodied in these prospecting systems. A' less acute need also exists for a more effective means for interconnecting the ground antenna sometimes employed to energize an associated receiver.

The ground-antenna systems used in radio exploration include either one or two bare or insulated resonant wires which are positioned on or near the ground, or below the earths surface. The wires have been tuned to resonance either by adjusting their physical lengths to a predetermined fractional part of the operating wave length in the ground, or by means of series capacitance.

Inasmuch as the entire antenna structure must be adapted for portable operation over various types of ground, for use in both wet and dry weather, and for moderate variations in the operating wave length, it has been found completely impracticable to tune ground antennas by adjusting their physical lengths to meet operating conditions that change from day to day, and sometimes from hour to hour. Aside from minor changes in the operating frequency, the chief problem is brought about by variations in the electrical character of the ground from one locality to another, which variations markedly influence 4the resonant length of a ground antenna. Moreover, the change in the ground wave length from early morning to a sunny midday, or before and after a shower, is sufficient to require a readjustment of the antenna length. When the time of costly field crews is involved, the economics of the situation will not permit such daily, and sometimes hourly, adjustments.

` Series capacitance, if properly utilized, provides a satisfactory means for tuning ground antennas. Unfortunate- 1y, however, series-capacitance tuning has frequently been misapplied in prior-art systems, and this has brought about operating difficulties which increased `as the physical lengths ofthe antennas departed more and more from their respective predetermined ratios to the wave lengths in the ground, as will be pointed out more particularly hereinafter.

When the ratio between the physical length of the antenna and the ground wave length departs considerably from its optimum value, becausel of pronounced changes in the electrical character of the ground, it is found difficult indeed to match a ground-antenna system to its associated transmitter or receiver. In other words, the transfer of radio energy to or from the antenna system is effective only with the type of ground for which the system was designed, and may become highly inefficient with the other types of ground that are to be expected in routine field operations.

2,860,340 Patented Nov. 11, 1958 Nice `Prior-art ground antennas have been coupled to a transmitter or receiver by simple inductive coupling. In the case of a transmitter, power has been transferred to the antenna system through an air-core transformer, the primary winding being connected to the transmitter output and the secondary winding being in series with one or more antennas. With this type of coupling, the primary and secondary circuits can be properly matched only when the physical length of the antennas bears the correct ratio to the operating wave length in the ground. When changing ground conditions disturb this ratio, the efficiency of the coupling suffers, and may become highly inefficient at times.

ln View of the fact that the output of a transmitter utilized in radio exploration presently is limited to a maximum of but 50 watts, it is imperative at all times that as much as possible of the allowable power be effectively propagated into the earth and received therefrom.

The improved coupling device embodied in the instant invention overcomes the defects of the prior-art devices through the introduction of a novel circuit never before applied in the tuning or matching of ground antennas.

The utility of the invention and its operation will be understood from the description that follows and the accompanying drawings, of which:

g Fig. l is a diagrammatic drawing of the preferred form of apparatus for coupling a pair of ground antennas to a translation device, which may be a radio transmitter or receiver. v

Fig. 2 is a diagrammatic drawing of the preferred form of apparatus for coupling a single ground antenna to a translation device.

if the translation device be a transmitter, then radiofrequency energy may be fed through the coaxial cable lt, or other suitable feed line, to the link coil 2, and thence by induction to the antenna coils 3 and 4, to the currentindicating meters 5 and 6 and variable condensers 7 and 8, and nally to the ground antennas 9 and l0, To avoid spurious radiation, an electromagnetic shield 11 is provided, which may be connected by the wire 12 to the midpoint of the antenna circuit, which is at zero potential, and the shield 11 may also be grounded by the wire 13 to the earth as indicated. The wires 14 and 15 connect the variable condenser 16 in parallel with the antenna coils 3 and 4.

The preferred form of apparatus is particularly adapted to tuning and loading ground antennas whose lengths approximate an odd quarter-wave multiple of the operating wave length in the ground. The antennas may be bare or insulated, positioned either on the ground, near the ground, or buried in the earth. The device will tune and load two antennas as shown, with or without the ground connection 13, or alternatively, it will tune and load either antenna, with the other antenna removed and connection 13 in place. Since the electrical characteristics of ground are often different under each antenna, separate tuning of each antenna, with the other antenna removed and ground connection 13 in place, is required to avoid a tedious trial and error procedure. ln the absence of the ground connection 13, the device may stili be used to tune and match a single antenna provided the shield 11 is conductively or capacitively coupled to the earth.

Supply line 1 preferably is a two-conductor coaxial cable with its surrounding shield grounded to the shield 11. The lengthvof the cable may be made sutlicient to remove the transmitting equipment a suitable distance from the radiating elements of the antenna system.

In operating the preferred apparatus to tune and load two groundantennas, first one antenna, for example antenna 10, is removed and antenna 9 is tuned by means of the variable condenser` 7 until the meter 5 indicates resonance by a maximum reading. Antenna 10 is then connected, antenna 9 is disconnected, and the same tuning procedure is repeated, using the tuning means t5 and resonance-indicating means 6. Antenna 9 is next reconnected, and without making any further adjustmentsVV of the variable condensers 7 or 8, the variable condenser 16 is varied until the substantially equal readings of meters 5 and 6, reach amaximum value. The two ground antennas 9 and l@ then are in sharp resonance at thek operating frequency, and the antenna system is properly matched to the transmitter. With a single antenna, the operating procedure is the same, except that it is unnecessary to tune the inactive antenna circuit. lt is obvious that, if but single-antenna operation is contemplated, the inactive circuit components may be deleted, and the variable condenser 16 connected by the wiresV 14 and S to the ends of thel active antenna coil. This simplified arrangement is illustrated diagrammatically in Fig. 2, where the components are numbered in accordance with the like components in Fig. l.

The foregoing tuning and matching procedure is repeated each time the apparatus is set up at a diierent location, and at the same location the adjustments should be checked whenever a change is suspected in the electrical properties of the ground, which generally is due to a change in its moisture content. The necessity for maintaining uniform tuning and matching is caused chiefly by the requirement in most radio prospecting that the amplitude of the waves radiated along the ground be maintained at constant values at various distant points at or near the ground, and that the receiver output be linearly related to the field intensity of the waves arriving at the receiving antenna. Any appreciable change in the antenna tuning at the transmitting and receiving antennas will bring about corresponding changes in their respective radiation and reception patterns, and any change in matching the antennas to their associated translation devices will cause similar changes in the transmitted and received power. These efects may, in the case of priorart devices, introduce such uncertainties in the operation that the results of a survey may, at times, be subject to grave errors. lt is very evident, therefore, that a satisfactory coupling device must provide a means for overcoming these uncertainties.

With the herein disclosed coupling device, such uncertainties may be eliminated when operating over any type ground normally met with in practice. It is only necessary to tune the antenna or antennas as already described, and match the antenna system to the transmitter (or receiver) with the variable condenser )1.6.

in this connection it is to be noted in Fig. l that a separate tuning means, which in the preferred embodiment of the invention is a variable condenser, is provided to permit the individual tuning of each antenna. This arrangement maintains electrical symmetry of the preferred double-ended antenna system, and this in turn makes for uniformity in the systems radiation or reception pattern. in contradistinction to this balanced arrangement, priorart devices have made use of but a single tuning means positioned in one or the other antenna circuit, so that the electrical midpoint of the system, and the current distribution therein, would vary with each adjustment of the tuning means, and accordingly with each change in the electrical properties of the ground. The resulting asymmetry would cause variations in the radiation and reception patterns of prior-art antenna systems, as well as variations in the coupling coefhcient, or matching, between the antenna systems and their associated translation devices. Each of these variations would introduce uncertainties in the operation which could not be overcome with the former systems.

With the improved coupling device illustrated diagrammatically in Fig. l, electrical symmetry is maintained about the midpoint of the antenna system, which may in fact be grounded by the wire 13, and a like and uniform current distribution is maintained in each antenna, so that the configuration ofthe radiation or reception pattern may be kept within close limits for any ordinary ground by adjusting the tuning means 7 and 3. Moreover, the range of the matching means 16 may easily be made sufficient to match the antenna system to the translation device, when operating with the various types of ground met with in practice, and thereby provide at all times a highly effective transfer of energy from the the transmitter to the antenna system, or from the antenna system to the receiver.

When receiving waves, the visually or aurally indicated output of the connected receiver may be substituted for the resonance-indicating means 5 and 6.

The herein disclosed coupling device is particularly suited to tuning and matching the ground-antenna systems described in applicant Barrets copending applications Serial No. 584,960, filed March 26, 1945, Patent No. 2,661,466; Serial No. 757,242, filed June 26, 1947, Patent No. 2,585,907; Serial No. 48,376, tiled September 9, 1948, Patent No. 2,637,996; and Serial No. 187,792, filed September 30, 1950.

Although the preferred form of apparatus disclosed in the instant application is adapted particularly to the tuning and matching of current-fed quarter-wave groundk antennas, or current-fed ground antennas having lengths that approximate other odd quater-wave multiples of the ground wave length, it will be understood by those versed in the art that the type of feed and the antenna lengths may be Varied, and that various other modiications may be made in the apparatus hereinbefore disclosed, and in the method of operating the same, without departing from the spirit of the invention as defined in the following claims.

What is claimed as new and useful is:

l. in apparatus for geophysical exploration, in combination with a ground antenna; a tuning and matching device comprising, an inductance, a variable capacitance having one side connected in series to one end of said inductance, the other side of said capacitance being connected to said antenna, said variable capacitance being adapted to tune said antenna to resonance at the operating frequency, means in circuit with said capacitance and said inductance for indicating resonance of said antenna, variable capacitance means connected in parallel with said inductance for tuning the same, and means for electromagnetically coupling said inductance to a translation device; shielding means enclosing said tuning and matching device and said coupling means; and means for connecting the other end of said inductance to said shielding means, said shieldingmeans being adapted to be selectively connected to ground.

2. In apparatus for geophysical exploration, in combination with a pair of ground antennas in dipole configuration; a tuning and matching device comprising, a rst variable capacitance haivng one side thereof connected to one of said pair of antennas, said irst capacitance being adapted to tune said one of said pair of antennas to resonance at the operating frequency, a second variable capacitance having one side thereof connected to the other of said pair of antennas, said second capacitance being adapted to tune said other of said pair of antennas to resonance at the operating frequency, an.

inductance having one end thereof connected to the other side of said rst capacitance, the other end of said inductance being connected to the other side of said second capacitance, means in circuit with said rst capacitance for indicating resonance of said one of said pair of antennas, means in circuit with said second capacitance for indicating resonance of said other of said pair of antennas, variable capacitance means connected in parallel with said inductance for tuning the same, and means for electromagnetically coupling said inductance to a translation device; shielding means enclosing said tuning and matching device and said coupling means; and means for con- 5 necting said inductance at the electrical midpoint thereof 2,143,658 to said shielding means, said shielding means being 2,197,494 adapted to be selectively connected to ground. 2,205,881 2,585,907 References Cited in the file of this patent 5 2,661,466

UNITED STATES PATENTS 1,597,420 Austin Aug. 24, 1926 1,875,952 Taylor Sept. 6, 1932 326, 327

1,944,564 Lewis Jan. 23, 1934 10 6 Morris et al Ian. 10, 1939 Erben Apr. 16, 1940 Franz June 25, 1940 Barret Feb. 19, 1952 Barret Dec. 1, 1953 OTHER REFERENCES Radio Amateurs Handbook, 26th edition, 1949, pages

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