US3594809A

US3594809A - Crossed loop antennas with separating shield

Info

Publication number: US3594809A
Application number: US771577A
Authority: US
Inventors: Rinaldo E De Cola; Alicja D Kulbiej
Original assignee: Thomas International Corp
Current assignee: Thomas International Corp
Priority date: 1968-10-29
Filing date: 1968-10-29
Publication date: 1971-07-20
Anticipated expiration: 1988-07-20

Abstract

An omnidirectional antenna system having two loop antennas disposed at right angles to each other and electrically and magnetically decoupled by a shield between the antennas.

Description

United States Patent Rinaldo E. De Cola Park Ridge;

Alicja D. Kulbiej, Chicago, both 01, ll]. 77 1,577

Oct. 29, 1968 July 20, 1971 Warwick Electronics lnc.

lnventors A pp]. No. Filed Patented Assignee CROSSED LOOP ANTENNAS WITH SEPARATING SHIELD 11 Claims, 5 Drawing Figs.

Int. Cl Olq 7/08, l-lOlq 1/52 Field of Search 343/742,

[56] References Cited UNITED STATES PATENTS 3,031,667 4/1962 Wennerbcrg. 343/788 3,051,903 8/1962 Morrow 343/701 3,440,542 4/1969 Gautney 343/788 3,447,159 5/1969 Stromswold 343/742 Primary Examiner-Eli Lieberman Attorney-Hofgren, Wegner, Allen, Stellman and McCord ABSTRACT: An omnidirectional antenna system having two loop antennas disposed at right angles to each other and electn'cally and magnetically decoupled by a shield between the antennas.

CROSSED LOOP ANTENNAS WITH SEPARATING SHIELD This invention is concerned with an omnidirectional antenna system. More particularly, two loop antennas, bidirectional in nature and disposed at right angles to each other, are decoupled by conductive shielding located between the two antennas.

tennas an equal amount above and below the center frequen- In all known systems of this type, however, it has been found that between the antennas themselves, there is a degree of coupling with is frequency dependent. This coupling produces a phase shift over the band of frequencies to be received with a resulting degradation in the omnidirectivity of the system. This coupling, as a practical matter, may be minimized by precise 90 orientation of the antennas, however, even with accurate alignment, it is impossible to eliminate all couplings between the antennas. Additionally, it should be pointed out that accurate orientation of the antennas is very difficult and also costly from a manufacturing standpoint.

The present invention provides an omnidirectional antenna system in which the coupling between the 90 displaced loop antennas is effectively eliminated whereby substantial omnidirectivity is achieved over an extremely wide band of frequencies while at the same time the cost of the system is reduced by eliminating the extreme accuracy required in the 90 displacement of the antenna.

In accordance with the present invention, two loop antennas are disposed at substantially right angles to one another on opposite sides of a grounded conductive shield which substantially eliminates all coupling therebetween. The physical separation and 90 orientation of the antennas is also effective to enhance the decoupling thereof.

Since the conductive shield effectively decouples the antennas, the system may be rendered omnidirectional over a wide band of frequencies by employing critical remote coupling, stagger tuning the antennas or any other suitable method of obtaining the necessary quadrature relation between the signals in the two antennas.

While achieving the improved wide band omnidirectivity the present invention also reduces the cost of such system by virtue of the fact that the conductive shield effectively decouples the antennas to eliminate the critical 90 displacement factor which is required in the absence of the shield to obtain maximum decoupling.

The present invention may be more fully understood by reference to the following detailed specification and the drawings, in which:

FIG. 1 is a schematic diagram a preferred embodiment of the antenna system of the present invention.

FIG. 2 is a schematic diagram of another embodiment of the antenna system of this invention.

FIG. 3 illustrates the frequency response of the individual loop antennas shown in FIG. 2.

FIG. 4 depicts the physical relationship of the two antennas.

FIG. 5 illustrates the shield used in isolating the antennas.

The invention is illustrated herein as it is used in an amplitude modulated (AM) radio receiver circuit. However, the invention can be used in a transmitter circuit and with coupling circuits other than those specifically shown and described.

Each loop antenna in this system is similarly constructed so that the gain and other characteristics of the antennas are identical. An omnidirectional antenna pattern is achieved by the superposition of two equal gain patterns upon one another. Two bidirectional, i.e. figure eight, patterns having a common center point and displaced at right angles to each other provide a circular, omnidirectional pattern upon the vector addition of the patterns.

FIG. illustrates a preferred embodiment of the present invention wherein the loop antennas l0 and 12 are remotely critically coupled to produce signals in phase quadrature relation. Each of the

antennas

10 and 12 are shunted by a variable ganged capacitor 14 and 116 respectively and have their-low impedance ends coupled to ground through a common inductor 20. The antennas are physically separated and oriented at to each other, as shown in FIG. 41. An electrically conductive shield 18 (shown in dotted line in FIG. 1) is located between the antennas and is grounded at 115. A coil 22 forms a transformer secondary for antenna 12. One end of coil 22 is grounded and the other end is coupled to the input of RF amplifier 23. Coil 22 picks up the signals in antenna 12 which are then amplified in RF amplifier 23 from which the signals are coupled to conventional receiving apparatus (not shown).

Each antenna of the omnidirectional system comprises a number of turns of wire would on a rod or cylindrical core member 56 of a magnetic ferrite material so that the combined inductance of the antenna is approximately 490 microhenries. The ferrite rod provides a low reluctance path for the magnetic field, minimizing stray fields outside the ferrite and forming a high Q inductance. A ferrite rod antenna by itself is very directional. The antenna coil extends over substantially the entire length of the core 56 and is tightly wound thereon. Each antenna has a circular cross section and is supported on grounded shield 18 by L-shaped brackets 58, as shown in FIG. 1. The antennas are placed at 90 to each other, such that centerlines, through, and perpendicular to, the respective circular cross sections, are skewed and substantially at 90 to one another. Shield R8 is composed of laminated parallel'layers of a conductor, as a metal 60, and paper 61, as shown in FIG. 5.

A pair of single loop antennas, if disposed perpendicularly to each other are effectively decoupled only if they are disposed symmetrically about a common center point. Since it is impossible to so dispose the ferrite rod antennas shown in FIG. 4, the 90 orientation cannot completely decouple the antennas. Additionally, the plural elongated loops of such antennas produced even further coupling problems. By disposing a grounded conductive shield such as shield 18 between the antennas, however, complete decoupling is achieved and the attendant problems of coupling are eliminated.

in order to derive the necessary phase quadrature relationship between the signals received by each antenna, the inductor 20 is provided. Inductor 20 is selected with regard to the Q of the antenna circuits such that critical coupling is achieved. This critical coupling, by definition, causes a phase quadrature component of the signal from one antenna to appear in the Other. Thus, secondary or takeoff winding 22 couples to the RF amplifier both the signal received by antenna 12 and the signal received by antenna 10 shifted 90 i.e., the signals are in the required phase quadrature relationship, and a completely omnidirectional system is achieved. This system is essentially free of any frequency dependent coupling between the antennas, and the 90 orientation of the antennas is rendered less critical since the effect thereof on directivity is less than the degradation caused by coupling between the antennas in nonisolated systems wherein slight variations in the 90 orientation causes a substantial increase in the coupling.

Although a single, common inductor 20 is shown in FIG. 1, it is to be understood that separate critically coupled inductors connected in series with each antenna could also be used.

FIG. 2 illustrates another embodiment of the present invention wherein the phase quadrature relationship between the antenna signals is achieved by stagger tuning the antennas.

Antennas 10' and 12' are the same as in FIG. 1, and are shunted by variable ganged capacitors l4 and 16' respectively. The antennas are physically separated, again as in FIG. 4. The conductive shield 18 is located between the antennas as in FIG. 1. Output coils l9 and 21 are coupled to the antennas l and 12' respectively and have one end connected to ground. Coils l9 and 21 form transformer secondaries for antennas l0 and I2 and couple the signals received thereby to the

input bases

24 and 26 of

RF transistor amplifiers

28 and 30. The emitters 32 and 34 of the PNP transistors are connected together as are the

respective collectors

36 and 38. A combined output signal is developed across load resistor 40 connected form ground to the common collectorjunction.

In order that the system be omnidirectional, the signals at the

base inputs

24 and 26 must be in phase quadrature. In this embodiment of the invention the phase quadrature relation is achieved by stagger tuning the resonant circuits of each anterina in the manner shown in the frequency response curves of FIG. 3. The resonant circuit comprised of capacitor 14 and the inductance of antenna is tuned to a frequencyf below the center frequency f., as indicated by curve A. The resonant circuit comprised of capacitor 16' and the inductance of antenna 12' is tuned to a frequencyf which is above f by an amount equal to the difference between f and f See curve B.

By appropriately tuning the resonant circuits in this manner the signals appearing in takeoff coils l9 and 21 will have the requisite phase quadrature relationship and the system will be omnidirectional. As in the system shown in FIG. 1, the complete decoupling of the antennas by the use of the shield 18 eliminates the degradation caused by phase variations due to coupling between antennas and thus better omnidirectivity is achieved.

We claim:

I. An omnidirectional antenna system comprising:

conductive shield means;

first and second loop antennas having a plurality of turns,

said antennas having bidirectional signal receiving patterns; said first and second loop antennas being disposed on opposite sides of said conductive shield means and positioned at right angles to each other such that the center turn of each antenna fomi; a crosslike projection in the plane ofsaid conductive shield means, said conductive shield means functioning to decouple said loop an tennas from each other; and

circuit means for combining signals leceived by said two antennas.

2. The omnidirectional antenna system of claim I wherein the antennas are substantially equal distances from the conductive shield.

3. The omnidirectional antenna system of claim 1 including means for tuning each antenna to resonance.

4. The omnidirectional antenna system of claim 3 wherein the first antenna is tuned to a frequency above the resonant frequency of the system and the second antenna is tuned to a frequency below said system resonant frequency, the stagger tuned antenna having frequencies of timing equally separated above and below the system resonant frequency.

5. The omnidirectional antenna system of claim 4 including means for physically connecting each antenna to the conductive shield.

6. The omnidirectional antenna system of claim 4 wherein the loops are symmetrical providing similar frequency response curves when the antennas are tuned to their resonant frequency.

7. The omnidirectional antenna system of claim 1 wherein each bidirectional antenna has an equal gain.

8. The apparatus of claim 1 wherein said circuit means comprises, means for establishing a phase quadrature relation between the signals received by said antennas and means for combining the phase-quadrature related signals.

9. The apparatus of claim 8 wherein said means for establishing said phase quadrature relation comprises means for remotely, critically couplin said antennas. 10. The apparatus of claim wherein said means for critically coupling said antennas comprises a common inductor connected in series with both of said antennas.

11. The apparatus of claim 9 wherein said means for combining said phase-quadrature related signals comprises a secondary winding coupled to one of said antennas and amplifier means coupled to said secondary winding.

Claims

1. An omnidirectional antenna system comprising: conductive shield means; first and second loop antennas having a plurality of turns, said antennas having bidirectional signal receiving patterns; said first and second loop antennas being disposed on opposite sides of said conductive shield means and positioned at right angles to each other such that the center turn of each antenna forms a crosslike projection in the plane of said conductive shield means, said conductive shield means functioning to decouple said loop antennas from each other; and circuit means for combining signals received by said two antennas.

2. The omnidirectional antenna system of claim 1 wherein the antennas are substantially equal distances from the conductive shield.

4. The omnidirectional antenna system of claim 3 wherein the first antenna is tuned to a frequency above the resonant frequency of the system and the second antenna is tuned to a frequency below said system resonant frequency, the stagger tuned antenna having frequencies of tuning equally separated above and below the system resonant frequency.

9. The apparatus of claim 8 wherein said means for establishing said phase quadrature relation comprises means for remotely, critically coupling said antennas.

10. The apparatus of claim 9 wherein said means for critically coupling said antennas comprises a common inductor connected in series with both of said antennas.