Antenna with pattern directivity control

July 21, 1970

J. P. SHELTON, JR., ET AL

3,521,284

ANTENNA WITH PATTERN DIRECTIVITY CONTROL

Filed Jan. 12, 1958

Sheets-Sheet 1

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July 21, 1970

J. P. SHELTON, JR., ET AL 3,521,284

ANTENNA WITH PATTERN DIRECTIVITY CONTROL

Sheets-Sheet 2

Filed Jan. 12, 1958

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<img file="US3521284A_D0003.tif" />

J. Paul Snelton, Jr, /.eland D. Strom

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ATTORNEYS

3,521,284

Patented July 21, 1970

United States Patent Office

3,521,284 ANTENNA WITH PATTERN DIRECTIVITY CONTROL John Paul Shelton, Jr., Bethesda, Md. (6727 Baron Road, McLean, Va. 22102), and Leland D. Strom, 1830 Massachusetts Ave., McLean, Va. 22102 Filed Jan. 12, 1968, Ser. No. 697,392 Int. CI. HOlq 1/24, 21/00 U.S. Cl. 343—702 20 Claims

ABSTRACT OF THE DISCLOSURE

A wide band antenna array, for minimizing the effect of multi-path radiation on a television image, has an omnidirectional, loop-like pattern and a dipole-like pattern. The array is excited so the separate patterns produce a resultant pattern having a broad, substantial null behind a main lobe and a constant frequency response over the VHF television band for both patterns.

The present invention relates generally to wave energy transducers and, more particularly, to a wave energy transducing array wherein an omnidirectional radiation pattern is combined with a pattern including a plurality of nulls.

A prime requisite of transducers for converting television electromagnetic waves, in strong signal areas, to electric signals, i.e., television antennas, is elimination of reflected or multi-path energy impinging on the transducer. In regions of strong television signal strength wherein field strengths exceed 5,000 microvolts per meter, such as business or densely populated areas of a city, multiple reflections from tall buildings and other structures frequently result in ghost images on a television receiver screen.

To eliminate ghost images from the screen of a television receiver, television receiving antennas adapted to be utilized in strong signal areas should generally be designed so that the pattern thereof has one relatively narrow main lobe and substantially no side or back lobes. In many instances, the substantial elimination of a back lobe is as important as the elimination of side lobes because energy forming ghost images is generally defined by a family of confocal ellipses with foci at the transmitting and receiving antennas. Each ellipse represents the locus of ghost forming objects which will produce a given separation between image and ghost. Hence, the apparent source of reflections from objects located behind the receiving antenna, i.e., on the side of the antenna opposite from the transmitting source, is closer to the antenna than reflections received at right angles to the propagation direction of signals from the transmit-, ting antenna. Because signal energy coupled to an antenna is inversely proportional to the square of the distance between the antenna and an apparent energy source, reflections from points behind the receiving antenna often have a greater influence in creating ghost images than signals reflected from structures at right angles to the receiving antenna.

A very frequently utilized antenna of the prior art for reception of strong television, signals is the well-known rabbit-ear configuration. A rabbifcear antenna is actually an adjustable dipole having a radiation pattern with a pair of nulls, presuming the rabbit-ear elements are properly adjusted. The nulls in the pattern of a rabbit-ear antenna, however, are frequently eliminated by improper adjustment of the elements, whereby ghost images from multi-path reception cannot be obviated.

A further, and perhaps more significant, problem even with properly adjusted rabbit-ear, dipole antennas is that the two nulls of the dipole pattern are located at right angles to the direction of maximum antenna sensitivity. The pattern includes a back lobe, i.e., a lobe pointing away from the transmitting source, that is generally vir_P tually equal in amplitude to the forward or main pattern lobe amplitude. Therefore, multi-path radiation reaching the antenna in response to reflections from structures located behind the antenna are generally coupled to a receiver via a pattern lobe having the same gain as the lobe receiving direct, unreflected radiation. The rabbitear configuration, as a result, has a tendency to couple multi-path radiation derived from points behind the antenna to the receiver with a relatively large amplitude and augment the ghost image.

A further disadvantage with the typical rabbit-ear configuration is the degrading aesthetic qualities thereof. The unsightly appearance of a pair of elongated metal rods, forming the rabbit-ear dipole elements, mounted on a plastic base has often been criticized. While many at20 tempts to design rabbit-ear antennas having more pleasing appearances have been made, none of these antennas, to our belief, can be considered attractive.

While satisfactory television antennas have been designed and are utilized in attic and rooftop locations, 25 these antennas usually require a multiplicity of relatively large elements, spaced from each other by significant distances. In consequence, multi-element antennas cannot be utilized in the living quarters of a residence, such as an apartment building or a room in a single family dwell³⁰ ing inaccessible to a lead-in from an attic or rooftop.

The advent of truly portable receivers weighing less than 20 pounds and color receivers which are readily moved from place to place present additional problems of ghost image rejection and space with regard to tele^3o vision receiving antennas. With color receivers the effects of ghost images on the viewer are even greater than with black and white receivers. Objects causing ghost image reflections are found at many different locations in and ₄₀ about the typical residence so that with presently available portable type receivers it is usually necessary to substantially modify existing antennas each time the receiver is moved. Of course, the use of relatively large antennas is out of the question with truly portable tele₄₅ vision receivers.

According to the present invention, the problems of prior art antennas designed for strong signal areas are substantially obviated by providing a television receiving transducer or antenna having, in the horizontal plane, a ₅₀ substantially omnidirectional pattern and a second pattern with a plurality of nulls, e.g. a dipole-like pattern. The antenna is excited so that fields produced by the two patterns do not interact in the antenna, whereby the elements of the antenna that produce the two fields are 55 electrically orthogonal and mutually decoupled. Because the antenna elements are decoupled from each other, the responses thereof are independent and have no effect on each other.

The responses of the antenna elements producing the βθ two patterns are combined in a directional coupler to provide a resultant pattern for the array having substantially no side lobes, an appreciable front-to-rear lobe ratio, and a relatively broad rear null. An antenna array having such a pattern meets the requirements for sub55 stantial elimination of ghost images in a television receiver, as indicated supra. By designing the amplitude of the omnidirectional pattern to equal approximately 0.8 times the maximum amplitude of a loop type pattern having a pair of nulls and oppositely directed and phased 70 main lobes, a resultant pattern with a substantial null (—20 db relative to the maximum amplitude of a resultant main lobe) over an angle of 100° is attained.

3,521 ³

To achieve broad band operation over the entire VHF television band, extending between 54 and 216 megahertz (mHz.), the antenna is designed to have a resonant frequency above the highest VHF television carrier, frequency. Thereby, the array has a monotonic amplitude _gversus frequency response over the entire VHF band, and the design of a combiner is simplified.

According to one preferred embodiment of the invention, the omnidirectional pattern is attained with a horizontal loop, while the pattern element having a plurality of nulls is derived from a dipole having its center aligned with the loop center. These antenna elements have been found to be admirably suited to provide the desired characteristics. By selecting a resonant frequency of 250 mHz. for the dipole and loop, an attractive and aesthetically ₁₅pleasing array of relatively small size is obtained.

To achieve uniform frequency response of the loop and dipole over the entire VHF television band is a problem since the loop has a power versus frequency response directly proportional to a fourth power of fre- 20 quency (/⁴) when operated below resonance, while the dipole power versus frequency response is proportional to frequency squared (/²). Therefore, energy coupled from the loop and dipole to the receiver must be with different frequency responses. Energy is fed from the loop 25 and dipole to the receiver via a pair of coupled transmission lines comprising a four port directional coupler. Connected to opposite terminals of one of the lines comprising the directional coupler are the television receiver and the loop antenna element, while the dipole and a 30 matching resistor are connected to opposite sides of the other line. The directional coupler is designed so that energy coupled from the dipole is fed to the terminals of the television receiver as a monotonic function of frequency squared. The direct connection between the ter- 35 minals of the television receiver and the omnidirectional antenna element results in direct coupling of the receiver and the loop with no substantial change in the frequency response between them. Hence, the coupler modifies the frequency response from the receiver to the loop and 40 dipole so that energy is coupled to the terminals of the television receiver from both antenna elements as a monotonic function of frequency to the fourth power over the entire VHF television band.

The loop-dipole antenna also provides reception in the 45 UHF band, without ghost elimination properties because the directional coupler achieves 0 db coupling at a frequency of approximately 450 mHz., i.e. substantially all of the dipole energy at 450 mHz. is coupled to the television receiver. Since virtually all UHF television trans- 50 missions are in the lower portion of the UHF spectrum, i.e. between 470 and 647 mHz., and are unlikely to reach into the higher portions of the spectrum, the present antenna functions as an efficient one-wavelength dipole radiator for UHF television reception. 55

According to a further embodiment of the present invention, the omnidirectional pattern and the pattern having a plurality of nulls is obtained by utilizing a turnstile antenna array. The turnstile array includes four relatively orthogonal output terminals, each of which is excited 60 with energy including real and quadrature components. The real component exciting each of the turnstile terminals is in phase to achieve omnidirectional characteristics, while the quadrature energy applied to opposite terminals is of opposite polarity for dipole mode excitation 65 of the turnstile array. The diameter of the turnstile array is relatively small compared to a resonant frequency slightly in excess of the resonant frequency of the highest television VHF channel. By dimensioning the turnstile elements to be relatively small compared to the designed 70 resonant frequency, i.e., the diameter of the turnstile array is between a quarter and a half wave length of the designed resonant frequency, the currents in a loop about the extremities of the turnstile elements are all in phase to achieve a loop type, omnidirectional response. 75 ,284

Since the dipole and loop modes are excited in phase quadrature, the vertical patterns thereof do not interact but the horizontal patterns combine to provide substantially the same pattern as is achieved with the loop and dipole elements, as discussed supra.

By selecting the resonant frequency of the turnstile antenna to be in excess of the highest frequency in the VHF band, both excitation modes of the turnstile are substantially monotonic as the fourth power of frequency over the entire band of interest. Thereby, conventional coupling elements, such as hybrids and quadrature couplers, can be utilized between the turnstile elements and the terminals of the television receiver. There is no necessity to utilize coupling networks having differing frequency versus amplitude responses for the two excitation modes of the turnstile elements.

A distinct advantage of the turnstile antenna is that both elements comprising the antenna, as well as the coupling network thereof, can be mounted within the confines of a television receiver having a dielectric casing. The selection of the antenna null can be accomplished electrically, utilizing a relatively simple and inexpensive switch between the turnstile elements and the coupling network.

It is, accordingly, an object of the present invention to provide a new and improved wave energy transducer having a radiation pattern with substantially no side lobes and a relatively wide null to the rear of the main lobe.

Another object of the present invention is to provide a new and improved wide band antenna wherein reception of multipath radiation is minimized.

It is a further object of the present invention to provide a new and improved antenna particularly adapted for television reception purposes wherein ghost images are substantially eliminated from a picture on the receiving screen.

An additional object of the present invention is to provide a new and improved television receiving antenna of relatively small size, enabling it to be carried on the receiver or in proximity to it and having ghost image elimination properties superior to prior art antennas adapted to be mounted on a receiver.

A further object of the present invention is to provide a new and improved, relatively small and inexpensive television receiving antenna that is attractive in an aesthetic sense.

A further object of the present invention is to provide a new and improved antenna adapted to be incorporated integrally within a television receiver, and having substantial ghost image elimination properties.

Yet another object of the present invention is to provide a new and improved wide band antenna adapted to be excited simultaneously with currents to produce an omnidirectional pattern and a pattern having a plurality of nulls, to provide a resultant pattern having substantially no side lobes and a relatively wide null to the rear of the main lobe.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a top view of an antenna and the coupling network therefor, in accordance with a preferred embodiment of the present invention;

FIG. 2 is a polar plot of the responses of the several elements comprising the antenna of FIG. 1, as well as a pattern of the entire array comprising the two elements;

FIG. 3 is a top view of a turnstile antenna comprising a second embodiment of the present invention;

FIG. 4 is a circuit diagram of the excitation network

3,521,284 utilized for driving the antenna elements of FIG. 3 to provide the patterns of FIG. 2; and

FIG. 5 is a perspective view, with a portion in phantom, illustrating the antenna of FIGS. 3 and 4 as a unitary portion of a television receiver.

Reference is now made to FIG. 1 of the drawings, illustrating a top view of half wave dipole 11 and horizontal circular loop 12. Half wave dipole antenna 11 includes a pair of quarter wave length radiators 13 and 14 extending in opposite directions from a common midpoint, coincident with the center of loop 12.

Each of elements 13 and 14 of half wave dipole 11 is designed to be resonant at a frequency greater than the upper frequency boundary of the VHF television band, 216 mHz. A typical resonant frequency selected for half wave dipole 11 is 250 mHz. The diameter of loop antenna 12 is selected so that all of the current in the loop is in phase, as is achieved with loops having relatively small diameters, i.e., diameters between one-quarter and one-half of a wave length of the loop resonant frequency of 250 mHz. By selecting the length of dipole 11 and loop 12 in accordance with the criteria stated, the efficiencies of these elements in transferring power into a pair of terminals in response to horizontally polarized electromagnetic radiation, such as subsists in television transmissions, are respectively monotonic functions of frequency squared and frequency to the fourth power over the entire VHF television spectrum.

The responses of dipole 11 and loop 12 are combined and fed to terminals 17 of television receiver 18 so that the efficiency of both elements is substantially the same. Otherwise, nulling effects introduced by dipole 11 on the omnidirectional pattern of loop 12 would be variable over differing portions of the VHF television spectrum. If the response of the loop and dipole elements differs as a function of frequency over the VHF band, the same null pattern would not exist for each received television channel and ghost images would be variable as a function of a station being tuned.

To compensate for the differing frequency versus power efficiency characteristics of dipole 11 and loop 12, coupler 19 is provided. Coupler 19 is a symmetrical directional coupler, the theory of which is fully described in the Proceedings of the Institute of Electrical Engineers, May 1955, pp. 383-392.

Directional coupler 19 includes a pair of coupled conductors 21 and 22 surrounded by shielded cable 23. Conductor 21 is directly connected to quarter wave length element 14 of dipole 11 by balanced line 24, while conductor 22 is connected to one of the terminals of loop 12 by balanced line 25. The remaining terminals of loop 12 and the terminal at the end of quarter wave length element 13 are connected via lines 24 and 25 to shielded cable 23 of coupling network 19. To the other ends of leads 21 and 22 of coupling network 19 are connected matching resistor 26 and lead-in wire 27, the ends of which are connected to the antenna input terminals 17 of television receiver 18.

Coupler 19 is designed so that reactive coupling between leads 21 and 22 results in a monotonic attenuation of energy between leads 24 and 27 that is directly proportional to frequency squared over the VHF television band. Since energy coupled between lines 24 and 27 is a function of frequency squared over the entire VHF television spectrum, the efficiency in transferring power from the dipole 11 to the terminals 17 of television receiver 18 is a function of frequency to the fourth power. Since a direct connection exists between the terminals of loop 12 and antenna terminals 17 of receiver 18 via lines 22, 25 and 27, and the frequency response of loop 12 is a monotonic function of frequency to the fourth power over the entire VHF band, the frequency responses of both the dipole and loop elements, in combination with coupler 19, are proportional to monotonic functions of frequency to the fourth power over the entire VHF television band.

To eliminate the effects of horizontally polarized radiation components, as may be derived from multi-path radiation, on the signal coupled to terminal 17 of receiver 18 and decouple the effects of currents in the loop and dipole from each other, the responses of loop 12 and dipole 11 in the vertical plane are isolated. The responses of dipole 11 and loop 12 in the vertical plane are isolated by exciting the loop and dipole with signals displaced in phase from each other by 90°, as is attained by suitable selection of the length of lines 24 and 25, or the insertion of a lumped parameter phase shifter in one of the lines. The relative phase shift introduced by lines 24 and 25 between coupler 19 and dipole 11, as well as loop 12, is governed by the phase shift of coupler 19 between the excitation end thereof connected to line 27 and the ends connected to lines 24 and 25.

For design purposes in eliminating multi-path radiation causing ghost images on the picture of receiver 18, it is necessary for the maximum response of dipole 11 to exceed that of loop 12 by a predetermined amount, as indicated by the radiation pattern plots of FIG. 2. To achieve the desired maximum amplitude of the patterns, a resistor is inserted in one of the lines 24 or 25. In the present specifically considered embodiment, it is assumed that the radiation efficiency of dipole 11 is in excess of the radiation efficiency of loop 12, whereby resistor 28 is connected in shunt with the leads comprising balanced line 24.

Giving further consideration to the antenna radiation pattern of FIG. 2, in the horizontal plane the pattern of loop 12 comprises circle 31 having radius B, while the horizontal pattern of dipole 11 comprises two circles of equal diameter A and a common point of tangency at the center of circle 31. Patterns 31 and 32 of loop 12 and dipole 11 are combined to provide the resultant pattern indicated by the cardioid-like pattern 33. Main lobe 34 of pattern 33 has an amplitude equal to the sum of the diameter of one of the circles of pattern 32 and the radius of pattern 31 since the loop and dipole currents are in phase along the line ¢=0. In contrast, the rear, or secondary, lobe 35 of pattern 33 has an amplitude equal to the difference between the diameter of one of the circles 32 and the radius of circle 31 because the loop and dipole currents are 180° displaced along the line θ=π. There is a very low amplitude response in the side lobes of pattern 33, whereby multi-path radiation reaching the antenna is severely attenuated compared to radiation reaching the antenna directly from a radiating source, presuming that the maximum amplitude of lobe 34 is ditected at a site of television transmission.

To attain a front-to-back ratio of 20 db, i.e., a ratio of the maximum amplitude of the power in lobe 34 to the maximum amplitude of the power in lobe 35, equal to 13.3, it can be shown that B=0.819A. The same selection of the relative values of B and A results in a relatively broad null encompassing approximately 100° for the 20 db rejection region, in resultant pattern 33. In other words, at least a 20 db ratio between the maximum amplitude of lobe 34 to the maximum amplitude of lobe 35 subsists for an angle of approximately 50° to either side of Θ=π in the polar diagram of FIG. 2.

The relatively board null of pattern 33 to the rear of the maximum response of lobe substantially prevents multi-path radiation from effecting the image on the screen of television receiver 18, thereby eliminating ghosts from the image on the television receiver screen. As discussed supra, the broad null attained by the present invention is derived by properly selecting the value of resistor 28 in line 24.

In normal operation, dipole 11 and loop 12 are mounted on a common base, not shown, placed on the upper surface of television receiver 18. Main lobe 34 of the resultant pattern is directed toward a television trans3,521,284 mission site originating the signal desired to be detected by placing arms 13 and 14 at right angles to the direction of direct radiation propagation from the site. Thereby, multi-path radiation reaching the array from the rear of the antenna of from the sides thereof is considerably reduced in amplitude relative to the amplitude of directly received radiation to cause substantial elimination of ghost images from the picture of television receiver 18. When it is desired to change the channel being viewed on receiver 18, the base carrying dipole 11 and loop 12 can be rotated, or dipole 11 only can be rotated so that arms 13 and 14 are approximately at right angles to direct propagation from the new transmission site.

The antenna of the present invention acn be madeaesthetically pleasing because of its unobtrusive appearance. In particularly, the total length of dipole element 11 is only slightly in excess of two feet, while the diameter of loop 12 is approximately eighteen inches, if the resonant frequencies thereof are designed as 250 mHz. Since dipole 11 and loop 12 can be placed in horizontal planes only slightly removed from each other because their vertical patterns are decoupled, it is apparent that very little space is required for the antenna of the present invention in comparison with the commonly employed rabbit-ear configuration. By proper aesthetic design, the antenna of the present invention can actually be made to have a pleasing, modernistic appearance.

While the antenna of FIG. 1 can be designed to have a pleasing appearance, it may be unattractive to some individuals and should be located in a fairly readily accessible location so that dipole 11 can be rotated mechanically by hand or a motor, if cost is not an important factor.

To avoid the requirements for an antenna that must be located exteriorly of a television receiver and mechanically rotated, the present invention envisages the embodiment of FIGS. 3-5. The antenna configuration of FIGS. 3-5 is a turnstile antenna including four ports or terminals 41-44 located at 90° with respect to each other from center 45 of the antenna. Respectively connected to each of terminals 41-44 is a separate, radially extending lead 46-49. At the ends of leads 46-49 remote from terminals 41-44 are arcuate leads 51-54, each of which subtends approximately a 45° arc about center 45 and extends in a clockwise direction from the radially extending lead to which it is connected. Connected to each of leads 51-54 is a pair of parallel arcuate leads 55-62. Leads 55-58 each subtend an arc of 90° about center 45, while each of inner leads 59-62 extends arcuately for 45° about center 45 and extends radially inwardly toward one of the terminals 41-44. Connected to the junction of each of the arcuate leads 51-54 and the pair of parallel leads extending therefrom is a ground plane, as indicated by the grounded leads 63. To complete the structure of the turnstile antenna array, leads 64-67 are provided. Leads 64-67 extend radially from an area close to terminals 41-44 to regions proximate arcuate leads 51-54 and are bent in a counterclockwise direction to define arcuate paths, each subtending an angle of approximately 45°.

Thus, the radially extending and arcuate leads connected to each of the terminals 41-44 of the turnstile antenna are positioned in relatively close proximity with a pair of other leads, to provide reactive coupling and establish a turnstile array capable of being excited with loop-like and dipole-like patterns 31 and 32, as well as a resultant pattern 33 as indicated by FIG. 2. As in the embodiment of FIG. 1, the resonant frequency of the array of FIG. 3 is selected to be slightly in excess of the highest frequency in the VHF television band, with a value of approximately 250 mHz. being selected. Thereby, the amplitude versus frequency response of the turnstile is a monotonic function over the entire VHF band being received for both the loop and dipole excitation modes.

Loop mode excitation of the turnstile configuration of FIG. 3 is derived by supplying currents of identical phase to each of terminals 41-44. The in-phase currents supplied to terminals 41-44 establish a loop-like response since the array has a relatively small diameter, between onequarter and one-half a wave length of the designed loop resonant frequency, whereby the in-phase currents supplied to the loop are maintained in phase in the far field. In contrast, dipole mode excitation of the turnstile array of FIG. 3 is obtained by supplying oppositely phased currents to oppositely oriented terminals of the array. For example, energy of phase Φ=0 is supplied to terminals 41 and 42 while energy of phase Φ=π is supplied to terminals 43 and 44. In the alternative, terminals 42 and 44 can remain unexcited while energy of phases Φ=0 and Φ=τΓ are supplied to terminals 41 and 43, respectively. Dipole excitation for the turnstile array occurs in response to terminals 41, 42 being excited with phase Φ=0 and terminals 43, 44 being excited with phase Φ=ττ because such excitation results in current flowing counterclockwise in arcuate conductors 51, 52 and clockwise in arcuate conductors 53, 54. In the far field, these currents produce a radiation pattern equivalent to a dipole having an axis aligned with lines 46 and 48, whereby a radiation pattern including two circles having their diameters aligned with leads 47 and 49 and a pair of nulls extending along leads 46 and 48 is established. To provide isolation for loop and dipole excitation modes of the turnstile of FIG. 3, a relative phase difference of π/2 must exist for the loop and dipole currents.

An important distinction between the loop and dipole antenna of FIG. 1 and the turnstile configuration of FIGS. 3-5 is that the turnstile loop and dipole mode excitations have similar frequency versus amplitude responses that are monotonic functions of frequency to the fourth power below the resonant frequency of the turnstile array. Thereby, the loop and dipole mode excitation applied to terminals 41-44 is through a network having similar, broad band frequency versus amplitude responses for both excitation modes. Of course, the important similarity between the antenna arrays of FIGS. 1 and 3 is that the loop and dipole mode excitations thereof result in substantially identical patterns 33, having a relatively wide null to the rear of the main lobe 34 and virtually no side lobes.

To excite the turnstile antenna of FIG. 3 in the manner indicated, the coupling network of FIG. 4 includes, broadly, a switch 71 excited by hybrid 72, which in turn is fed from quadrature coupler 73 that is connected to the antenna terminals 17 of television receiver 18. To achieve similar frequency versus amplitude responses between terminals 17 and the turnstile array for both the dipole and loop excitation modes throughout the VHF television band, hybrid 72 and quadrature coupler 73 are of the wide band type, having constant frequency response throughout the VHF television spectrum.

Switch 71 includes four arcuate stator contacts 74-77, respectively connected to terminals 41-44 of the turnstile array. Selectively bridging only a pair of arcuate contacts 74-77 are ganged arcuate rotor contacts 78 and 79. Rotor contacts 78 and 79 are rotatable to four separate, predetermined arcuate positions, whereby: terminals 41 and 42 are connected together through rotor contact 78 while terminals 43 and 44 are connected together through rotor contact 79; terminals 42 and 43 are connected together through rotor contact 78 while terminals 41 and 44 are connected together through rotor contact 79; terminals 43 and 44 are connected together through rotor contact 78 while terminals 41 and 42 are connected together through rotor contact 79; and terminals 41 and 44 are connected together through rotor contact 78 while terminals 42 and 43 are connected together through rotor contact 79.

Rotor contacts 78 and 79 are connected through suitable brushes, not shown, via lines 81 and 82 to arms and 84 of hybrid 72. Hybrid 72 is excited with energy in relative quadrature phase relationship at arms 85 and

3,521,284 __ with energy represented as a and jjl-a², respectively, where: a is an attenuation factor introduced by quadrature coupler 73, and j=V— 1.

Energy fed to arm 86 is attenuated by resistor 87, whereby the resultant turnstile pattern has a shape indicated by cardioid-like pattern 33, FIG. 2. Because of resistor 87, radiation derived from arms 83 and 84 of hybrid 72 is respectively represented as:

a+jWl-fl² and a-jbyjl-a²

The values of a and b, as well as the radiation efficiency of the turnstile array for the loop and dipole modes, enable the values of A and B for patterns 31 and 32 to be derived for the embodiment of FIGS. 3-5 with the same criteria as for the embodiment of FIG. 1.

Excitation for arms 85 and 86 of hybrid 72 is from quadrature output ports 88 and 89 of wide band quadrature coupler 73. Input port 91 of coupler 73 is connected to one of the antenna terminals 17 of television receiver 18, the other terminal of which is grounded, hence connected to a ground plane conductor (not shown) for each of the leads and coupling elements of the FIG. 4 network. Connected to the remaining terminal 92 of quadrature coupler 73 is matching resistor 94.

In normal usage in conjunction with a television receiver, the turnstile antenna of FIG. 3 is mounted in a horizontal plane slightly beneath the upper surface of a television receiver having a plastic casing 95, as illustrated in FIG. 5. Casing 95 includes a hinged door 96 on its upper surface 97, whereby access may be had to knob 98 for rotating ganged rotor contacts 78 and 79. Rotation of knob 98 establishes the four different connections of terminals 41-44 to the coupler network of FIG. 4. As knob 98 is rotated to one of the four positions thereof, the main lobe 34 of pattern 33 is rotated 90°.

With rotors 78 and 79 in the position illustrated by FIG. 4 and assuming the loop and dipole excitation currents are additive as derived from arm 83 of hybrid 72 and subtractive as derived from arm 84 of the hybrid, the resultant pattern main lobe is pointed in a direction to the right of the array and aligned with lead 47 and terminal 42, while a relatively broad null exists to the left of the array and has its center aligned with lead 49 and terminal 44. If knob 98 is rotated, whereby the contacts 78 and 79 are shifted clockwise 90°, the main lobe of the pattern is shifted clockwise 90°. By shifting the main lobe 90° in steps, the main lobe can be pointed approximately at all VHF television transmission sites in the region and substantial elimination of ghosts is attained because of the relatively broad null to the rear of the main lobe.

While we have described and illustrated several specifiic embodiments of our invention, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, other elements than loops and dipoles or turnstiles can be utilized for the antenna of the present invention as long as excitation for the omnidirectional pattern is 90° out of phase with excitation creating the dipole-like pattern. Exemplary of other elements which may be utilized are conical antennas, discones, and slots. It is also to be understood that the resultant pattern of the turnstile configuration of FIGS. 3-5 can be changed electrically by means other than the switch specifically illustrated. For example, phase shifting arrays can be employed, although they are generally thought to be excessively expensive for commercial adaptation.