US2690509A - Omnidirectional loop antenna system - Google Patents

Description

2 Sheets-Sheet 1 Filed Feb. 5, 1951 INVENTOR EMERICK TOTH TO ANTENNA TO ANTENNA 2 ATTORNEYfi Sept. 28, 1954 E. TOTH 2,690,509

OMNIDIRECTIONAL LOOP ANTENNA SYSTEM Filed Feb. 5, 1951 2 Sheets-Sheet 2 INVENTOR EM ER l CK TOTH ATTORNEYj Patented Sept. 28, 1954 i EHQE OMNIDIRECTIONAL LooP ANTENNA SYSTEM Emerick Toth, Takoma Park, Md.

Application February 5, 1951, Serial No. 209,502

2 Claims.

(Granted under Title 35, U. S. Code (1952),

sec. 266) This invention relates to omni-directional antenna systems.

More particularly this invention relates to omni-directional antenna systems made up of a plurality of directive antenna elements.

It is generally known that both the dipole and loop type antennas have directional characteristics. The three dimensional figure representing the response pattern of these antennas is toroidal or doughnut-shaped. In the case of the loop antenna, the axis of the toroid is perpendicular to the plane of the loop, while with the dipole the said axis coincides with the dipole.

It is also known in the art (see U. S. patent to Weagant 2,280,562) that by taking two similar loop antennas or dipoles and relating the dipole or loop antenna elements so that their directive patterns would be in a 90 degree space phase relation and then displacing the phase of the voltage induced in one of the antenna elements so that a spherical or omni-directional response pattern will result if the voltages thus obtained are added together for most conditions of wave polarization.

The problem of getting the 90 degree time phase displacement between the voltages originating in the directive antenna elements was in the U. S. patent to Weagant 2,280,562 accomplished by loosely inductively coupling the resonant circuit of which one directive element is a part to the resonant circuit of which the other directive element is a part, and then taking the output across one of the purely reactive elements therein.

The prior art methods have been found unsatisfactory when more than a single frequency is to be simultaneously received, or when the receiver is not exactly tuned to the received frequency. Thus, for example, when pulsed energy, or amplitude or frequency modulated waves are to be received wherein the received wave has a substantial bandwidth, the response pattern for all the frequencies other than those in the vicinity of the carrier frequency were substantially directive because the proper phase and amplitude relationships were obtained only at or near the carrier frequency.

The present invention is an improvement over the prior art methods in that omni-directional response is obtained over a substantially larger band of frequencies.

One aspect of the present invention basically consists of coupling the inductive-capacitive circuits associated with each of the directional antennas so that the degree of coupling is in the neighborhood of critical coupling.

Another aspect of the present invention is in broadening the response still further by increasing the coupling slightly above the critical value and then changing the sensitivity of one of the antenna elements, and then adding reactance as necessary to compensate for any change so that the circuits are tuned substantially the same as before.

One object of the present invention is therefore to provide an antenna system comprising a plurality of directive elements wherein the circuits associated therewith give the antenna system omni-directional characteristics over a band of frequencies.

Still another object of the present invention is to provide an omni-directional antenna system over a band of frequencies made up of directive antenna elements by means of relatively simple circuitry associated therewith.

These and other objects will become apparent to those skilled in the art upon reference to the specification to follow and the attached drawings wherein:

Figure 1a is a perspective view of the directive toroidal response characteristic of the loop and dipole antenna.

Figure 1b is a right cross-sectional view of the toroidal characteristic of Figure 1a.

Figure 2 is one embodiment of the present invention.

Figures 3-5 show other embodiments of the present invention.

Figure 6 is a simplified embodiment of the present invention.

Figures 7a., b, and c disclose various response curves obtained when the coupling is varied between the resonant circuits associated with each antenna element of the embodiment of Figure 2.

Figure 7d is the response characteristic for the embodiment of Figure 5.

In the specification to follow, the same reference characters connote similar elements throughout.

As is well known in the art, a dipole or single wire antenna a (see Figure 1a) has directive properties in that it is responsive to a maximum degree to an electric field parallel to it, and is not responsive at all to an electric field perpendicular to it. In such case, the three-dimensional figure in Figure 1a represents the response pattern of such an antenna.

Substantially the same toroidal response pattern is present for the loop antenna b where the axis of the pattern is perpendicular to the plane of the loop.

A right cross-section of the pattern 0 would be a figure 8 as shown in Figure 1b formed by two circular figures e and 1.

Two similar antennas whose outputs are added together and placed so that their responsive patterns are at right angles in space would at first throught appear to give an omni-directional system. It can be shown, however, that the system would only be non-directive when the phase of the voltage produced by one of the antennas is displaced 90 degrees with respect to the other. To make the pattern omni-directional over a band of frequencies, however, not only must the phase of one of these voltages remain displaced 90 degrees relative to the other, but the relative amplitudes of the voltages must remain the same regardless of frequency over at least a restricted band.

To obtain a relatively simple circuit which will shift the phase in the neighborhood of 90 degrees and at the same time maintain the same relative amplitudes for a band of frequencies would appear to be no easy matter. However, the present invention accomplishes this result by the use of relatively simple double tuned coupled circuits as shown in Figures 2, 3 and 4 wherein the degree of coupling used becomes important, as well as the relative gains of the antennas.

Basically one aspect of the present invention comprises two inductive-capacitive circuits each including one of the directive antennas which are coupled together so that one circuit is separately tuned to a particular frequency which falls within the band of frequencies to be received, and the other circuit is separately tuned to the same frequency.

The degree of coupling between the two circuits is at least equal to or greater than critical coupling at the above mentioned frequency. By critical coupling is meant that degree of coupling between two circuits where the square of the coupling impedance is equal to the product of the equivalent series resistances for the two circuits.

At critical coupling the phase and amplitude voltage relationships are proper to give the antenna system substantially omni-directional characteristics over a narrow band of frequencies for the usual values of circuit Q.

In the embodiment of Figure 2, two similar loop antennas are placed at right angles to each other and are respectively coupled to tuned circuit I and 2' through an impedance matching transformer 3 which serves also to coupled tuned circuit 2 to tuned circuit I. Tuned circuits I and 2' include respectively tuning condensers I and 8 coupled across inductances 56' which in the example shown are also part of the secondary windings of transformer 3. The coupling between tuned circuits I and 2' is adjusted to be at least equal to critical coupling. (The desirability and effect of varying the degree of coupling above critical coupling will be later discussed.) Condensers I and 8 are preferably adjusted so that the circuits I and. 2 and the impedance respectively reflected therein from the primary circuits of transformer 3, which includes antennas I and 2, resonate to the same desired frequency.

The voltage across tuning condenser l in circuit I is coupled to a circuit to which the system is to be associated. Condenser I is shown coupled between the grid 9 and cathode It of an electron discharge device I? since the an tenna system there shown is used as a receiving antenna system. If antenna I-2 were to be than one.

used as a transmitting antenna system, circuit I would be coupled to the output of a transmitter device.

The embodiment of Figure 3 is similar to the embodiment of Figure 2 except that the tuned circuits I and 2' are coupled directly together across a mutual coupling impedance comprising a condenser I2, instead of by a transformer 3 as in Figure 2. Accordingly, separate uncoupled transformers 3 and 3 are utilized respectively to couple the antennas I2 to tuned circuits I and 2. The circuit is preferably tuned so that condensers l, inductance 5", the reactance reflected from the primary circuit of transformer 3' (which includes antenna I), and a capacity equal to /2 the value of condenser I2, form a circuit resonant to the desired frequency. Likewise, condenser 8", inductance 6", the reactance reflected from the primary circuit of transformer 3 (which includes loop antenna 2), and a capacity the value of capacity I2 form a circuit also resonant to the desired frequency. The coupling between circuits I and 2 is adjusted to at least critical coupling.

The embodiment of Figure 4 is similar to the embodiment of Figure 3 except that a series condenser I2 is used to couple circuits I and 2' together. Condenser I2 is of such value that it offers a low impedance to the operating frequency. Condensers I and 8 are preferably adjusted so that condensers i and 8 resonate respectively with inductance 58 and the reactance reflected from the primary circuits of transformers i'4".

The embodiment of Figure 5 is similar to the embodiment of Figure 2 except that the size of antenna 2 has been reduced so that the voltage at its terminals is less than that at antenna I. The decrease in the reactance of antenna 2 is compensated for by adding an inductance II equal to the decrease of inductance of antenna 2. Instead of reducing the size of the loop, the number of turns in the loop could be decreased instead if loop 2 comprised several turns rather The advantage of the embodiment 5 over the embodiment of Figure 2 is that it has substantial ominidirectivity over a larger band of frequencies for reasons which will be later explained.

The circuit shown in Figure 6 is a simplified circuit which is similar to the embodiments of Figures 2 and 5 except that the impedance matching transformer coupling the antennas I and 2 to the tuned circuits I and 2' has been omitted. Accordingly circuit I' there shown includes a series circuit of loop antenna I, an inductance 5 which is inductively coupled to the inductance 6 of tuned circuit 2', and a tuning condenser I. The resistance I3 shown in circuit I represents the total resistance of the circuit and includes the resistance of the winding of inductance 5 etc.

Likewise, circuit 2' comprises a series circuit of an inductance 6 which is coupled to inductance 5 of circuit I, antenna 2 and tuning condenser 8. Resistance I4 shown in circuit 2 represents the total resistance in circuit 2'.

The voltage e7 across condenser I is coupled to load device I1. The embodiment of Figure 6 because of its simplicity will be used to explain the theory of operation of the present invention, it being understood that the embodiments of Figures 3-5 operate in a similar manner. If circuit 2 were to be open circuited then circuit I Would be tuned to a frequency f0 preferably the carrier frequency. If the circuit I were open circuited then circuit 2 would be tuned to the same frequency fo.

Condenser voltage 21 is made up of two component voltages er and er. In order to render the antenna system non-directive for a given frequency, the phase of voltage 67 must lead or lag e1" by 90 degrees, and the relative amplitudes of er and er" must be the same as the relative amplitudes of the voltages induced by the signal in two substantially identical antennas (i. e., antennas having similar response) whose directivity patterns are 90 space degrees apart having sinusoidal response patterns as shown in Figure b.

The curves of Figur 7 illustrating the various response characteristics of the double-tuned circuit of Figure 6 will now be referred to together with Figure 6 to explain the theory of operation of the invention herein claimed used as a receiving antenna. It should be understood that by a similar analysis, the operation of the antenna system when used as a transmitting antenna system will become apparent.

As was previously explained, the main object of the instant invention is to provide a substantially omnidirectional antenna system. If the net voltage e7 across condenser I (which is coupled to load device I'I) remains constant irrespective of the direction from which the incoming radio waves strike antennas I and 2 then the antenna system is omnidirectional.

The voltage component e1" developed across condenser I of circuit I is produced by the volttage e" present at the terminals of receiving antenna 2. The voltage component e1 developed across condenser I of circuit l' is produced by the voltage 6 present at the terminals of receiving antenna I.

Assume that the net voltage 6 and e" induced respectively in antennas I and 2 are equal at frequency fo (the frequency to which circuit I is tuned if circuit 2 were open circuited and vice versa). Now, antenna I, and thus voltage e is directly applied in circuit I. The voltage 6 originating in antenna 2 is coupled to circuit I in Figure 6 by the inductive coupling between inductances 5 and 6. If the coupling is equal to critical coupling (for the circuit shown in Figures 2 and 6 this is present when the mutual impedance= /R1s R14) it has been found that the voltage e7" across condenser I which originates from antenna 2 is 90 degrees out of phase with the voltage 27' across condenser I which originates from antenna I and the amplitudes of the voltages are equal.

Figure 7a shows the amplitude variation of the voltages er and er across condenser I as the frequency is varied above or below frequency ft for the condition of critical coupling. In this connection it should be noted that the e7 curve represents the primary circuit frequency response curve of a double tuned circuit while the av curve represents the secondary circuit frequency response curve of the same double tuned circuit.

Between the center frequency f0 and frequencies f1 and f2 respectively below and above center frequency, the amplitude differences between er and er" are not in general sufficiently different to cause much change in the omni-directive characteristics of the antenna system. Although the phase difference between the voltages also varies from 90 degrees on either side of center frequency f0, this change is not substantial enough within the range of frequencies from h to f2 to seriously vary the omni-directive characteristics of the antenna system.

If the coupling is reduced much below critical coupling two undesirable conditions result. First, as shown in Figure 7b, the amplitude difference between er and er" becomes pronounced even at the center frequency in; also for a givenfrequency band extending between f1 and f2 above and below the center frequency ,fo, the phase difference between ew and er" varies more from the 90 degree phase condition than for the case of critical coupling.

If the coupling is increased above critical coupling then the response curve of Figure results. Here the amplitude difference between the curves of er and er" also becomes pronounced but it has been found that the greater the coupling the less becomes the phase difference between at and 61 from the desired degree phase relation condition for the same band of frequencies.

If the portions of curve er of Figure 70 between the peak 0 and d were brought down to the level of the portion between the peak of curve e7, or if the level of curve e7 was raised to meet the level of the portion of curve an" between the peaks as shown in Figure 7d, then better omni-directivity would result over a given band of frequencies because the proper phase and amplitude conditions are present over a greater frequency range. Of course, if the circuits were overcoupled too much above critical coupling, the sensitivity or response would drop off so materially as to be unsatisfactory from the sensitivity standpoint. The seriousness of this loss of overall sensitivity depends on the particular conditions under which the present invention is used.

A considerable advantage may be achieved from a small amount of overcoupling from the critical coupling points without much loss of overall sensitivity.

The selectivity curves of 61' and c7" are raised or lowered by increasing or decreasing voltage e or e". This is accomplished by increasing or decreasing the overall gain or sensitivity of one of the antennas.

If a loopantenna is the directive element, this can be accomplished by decreasing the size of loop 2 (see Figure 5) or increasing the size of loop I. Varying the number of turns of the loop is another way to change the overall gain or sensitivity of the loop.

In an embodiment such as shown in Figure 2 utilizing a transformer 3 to couple the directive antennas I-2 respectively to the tuning circuits I and 2', the selectivity curves e7 or e7 can be raised or lowered by varying the step-up ratio (transfer efficiency) of the transformer.

Changing the gain or sensitivity of an antenna may result in a change of its impedance so that reactance (inductance H in Figure 5) must be added to compensate for the change of impedance so that circuits I and 2 are still separately tuned to the same frequency.

It is to be noted that in the embodiment of Figure 5 just described, even though one of the antennas is not substantially identical to the other the condition for omni-directivity previously mentioned that the relative amplitudes of varying the values of the resistances in these respective circuits. (I. e., vary resistances l3 and I4 shown in Figures 2, 3, 4 and 6.) If the circuit is in a condition of critical coupling it can be shown that a condition slightly above critical coupling may be obtained by slightly decreasing the value of either resistances 13 or M.

The embodiments of Figures 2-5 include transformer coupling between the loop antennas and the tuned circuits 1 and 2 to increase the system sensitivity, and for impedance matching purposes. Varying the step up ratio of transformer 3 also presents a convenient way to utilize the same antennas for widely separated frequency bands since changing the step up ratio of transformer 3 varies the impedance reflected into circuits I and 2' and thus the tuning condenser therein can thereby tune the associated circuits to resonance over several frequency hands.

If a completely omnidirectional (spherical) response pattern is not desired, then the antennas may be placed at an angle other than 90 degrees which will give the system an amount of directivity depending on the degree to which the antennas are displaced from the 90 degree relation.

Many modifications may be made of the specific embodiment disclosed without deviating from the scope of the broadest aspect of the present invention.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In an omni-directional antenna system comprising first and second directive antennas, one antenna having a greater gain than the other and having the axes of their directivity patterns in degree space phase relation, first and second parallel-resonant tuned circuits coupled respectively to said first and second antennas and forming therewith respective resonant circuits tuned to the same given frequency, mutual coupling means coupling said parallel-resonant circuits together so that the degree of coupling therebetween is greater than critical coupling.

2. An omni-directional antenna system comprising first and second directive antennas, one antenna efiectively having a greater gain than the other and having the axes of their directivity patterns in 90 degree space phase relation, first and second parallel-resonant tuned circuits coupled respectively to said first and second antennas and forming therewith respective resonant circuits tuned to the same given frequency, mutual coupling means coupling said parallel-resonant circuits together so that the degree of coupling therebetween is greater than critical coupling, and means coupling only the voltage developed across the parallel-resonant circuit associated with the antenna having the larger gain to a utilization circuit.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,280,562 Weagant Apr. 21, 1942 2,488,612 Tunick Nov. 22, 1949 2,520,984 Williams et a1 Sept. 5, 1950 FOREIGN PATENTS Number Country Date 362,530 Great Britain Dec. 10, 1931 419,783 Great Britain Nov. 19, 1934 532,164 Great Britain Jan. 20, 1941

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