US3560985A

US3560985A - Compact steerable antenna array

Info

Publication number: US3560985A
Application number: US658426A
Authority: US
Inventors: Zeno G Lyon
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1967-08-04
Filing date: 1967-08-04
Publication date: 1971-02-02
Anticipated expiration: 1988-02-02

Abstract

A COMPACT, STEERABLE, BOARD BANDWIDTH ANTENNA ARRAY WHICH IS CAPABLE OF PROVIDING MULTIPLE INDEPENDENT OUTPUTS WHICH ARE UTILIZABLE BY A PLURALITY OF INDEPENDENT RECEIVING SYSTEMS. THE ARRAY INCLUDES A PLURALITY OF RELATIVELY CLOSELY SPACED (I.E., LESS THAN ONE WAVELENGTH) ANTENNA ELEMENTS WHICH ARE COUPLED TOGETHER INTO GROUPS OF ANTENNA ELEMENTS, EACH GROUP PROVIDING A RADIATION PATTERN WHICH IS SUBSTANTIALLY INVARIANT WITH WAVELENGTH ABOVE A PREDETERMINED VALUE OF WAVELENGTH. THE OUTPUTS OF THESE GROUPS ARE THEN SELECTIVELY COMBINED TO PROVIDE A DIRECTIVE RADIATION PATTERN.

Description

Feb. 2, 1971 z. G. LYON COMPACT STEERABLE ANTENNA ARRAY 5 Sheets-Sheet l Filed Aug. 4. 1967 M v A M L M M m +w 5 W Y M .m E C M a w w. m 3

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2No 6. LYON BY I AGENT Feb. 2, 1971 z. LYON 3560,93

COMPACT STEERABLE ANTENNA ARRAY Filed Aug. 4, 1967 5 Sheets-Sheet 2 :5 n fi' a OUTPUT FROM TWO Elf VENT GROUPS lNVI-NTOR.

ZENU G. LYON Byw 7 AGENT 1971 z. G. LYON 3,560,95

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AGENT Feb. 2, 1971 LYON 3,560,985

COMPACT STEERABLE ANTENNA ARRAY Filed Aug. 1, 1967 5 Sheets-Sheet 5 RAD/A TOR EQUIVALENT C/RCU/T V I e 2 2 Ca 3 R L C 2? OISTR/EUT/O/V AMPL/F/ER SUBTRACTUfi 53 $U87RAU'0 0 DELAY our ur sasrRAcra INVI-JNTOR.

ZENQ 6. A YO/V United States Patent 3,560,985 COMPACT STEERABLE ANTENNA ARRAY Zeno G. Lyon, Scotch Plains, N.J., assignor to InternationalTelephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed Aug. 4, 1967, Ser. No. 658,426 Int. Cl. H01q 21/00; G01s 3/74 US. Cl. 343853 15 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to antenna arrays and more particularly to a compact, steerable, broad bandwidth antenna array which can provide multiple independent outputs which are utilizable by a plurality of independent receiving systems.

Omnidirectional receiving systems are normally relatively simple and inexpensive but provide little or no discrimination against unwanted signals and noise which may arrive on the same frequency with the desired signals. The elimination of undesired signals and noise in the region below approximately 30 mHz. is particularly important because this part of the frequency spectrum is quite limited and is becoming more crowded each year. The noise in the region below 30 mHz. is mainly comprised of man-made noise (from electrical equipment) atmospheric noise (from thunder storms, etc.) and galactic noise. At quiet receiving locations the limiting noise is generally of galactic origin above mHz. From a few hundred kHz. to 10 rnHz., man-made noise is the prime factor at a quiet site and below 100 kHz., the atmospheric noise rises rapidly to become the controlling factor.

Discrimination against these sources of noise, as well as against interfering signals, may be obtained by directional receiving systems. Past attempts to achieve these results include the Rhombic Antenna, which has dimensions of the order of two wavelengths, or greater, and the Wire Grid Lens, which is circular with a diameter of the order of three wavelengths. Both of these antenna arrays require considerable amounts of land. In such a case, there may be no clear economic or performance advantage to the use of a directional receiving antenna over the provision of higher power levels at the transmitting end of the link, since either approach may be capable of providing the desired signal-to-noise ratio S/ N). On the other hand, high directive gain can be obtained from a compact receiving array having elements less than k/ 4 high and an array diameter of less than a Wavelength.

Patented Feb. 2, 1971 ice BRIEF SUMMARY OF THE INVENTION According to this invention, an antenna array comprises a plurality of antenna elements coupled together to form a plurality of groups of said elements, each group providing a radiation pattern substantially invariant with wavelength above a predetermined value of wavelength. Further provided is apparatus coupled to the groups of antenna elements for combing the output signals from the groups for providing a directive radiation pattern.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a typical circular antenna array and its associated circuitry according to this invention;

FIGS. 2A, 2B and 2C illustrate the receiving radiation patterns of the antenna array of FIG. 1;

FIG. 3 illustrates another typical antenna array according to the invention;

FIGS. 4A and 4B illustrate the receiving radiation pattern resulting from the antenna array of FIG. 3;

FIG. 5 is the equivalent circuit of a monopole antenna element for use with this invention; and

FIG. 6 illustrates another antenna array according to this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, there is illustrated one embodiment of an antenna array according to the invention. This array comprises a plurality of antenna elements 1 through 6 arranged symmetrically about the circumference of a circle having a diameter of 2 k/x where x is the operating wavelength of the system and x is any integer. The spacing S between each adjacent antenna element 1-6 is equal to )\/x. In the center of the circle about which antennas 1-6 are located is mounted another antenna element 7. In this embodiment, each antenna is a vertical monopole above a good ground plane with the height of each monopole being equal to or less than M4 in the frequency band of interest. This circular configuration of monopole antenna elements provides circular symmetry and thus has good flexibility in the selection of the pointing direction of the array. Coupled to each antenna element 1-6 at the bases thereof are distribution amplifiers A1A6, respectively, and at the base of antenna element 7 there are coupled distribution amplifiers A7 and A8. Each of these amplifiers A1-A8 has a bandwidth about the same as that of the entire array, has a low noise figure and provides a moderate gain stabilized by heavy negative feedback. It is pointed out that amplifiers A7 and A8 may be combined as one unit if desired. The

outputs of the distribution amplifiers can be selectively combined to produce a directional radiation pattern. A desirable combining method for pairs of antenna elements is one which produces a cardioid-like pattern. Another combining method will produce a figure-eight pattern, which is also suitable.

A cardioid pattern is desirable since it has a high directivity and the highest front to back ratio of all patterns obtainable from a pair of omnidirectional antenna elements. These desirable qualities can be substantially maintained at all frequencies below f =V /4S, where V is the speed of light and S is the spacing between antenna elements. A further desirable quality of cardioid patterns is that the required phase angle between currents in element pairs is a linear function of frequency. Therefore, the required phase shift can be produced by means of a delay line, and the proper phase relationship between the currents in the element pairs will be maintained at all frequencies at which S permits the basic directive pattern to be formed. The preferred arrangement for this embodiment for generating the basic cardioid radiation patterns is given by two identical antenna elements whose outputs are properly phased and combined. However, it should be noted that the cardioid pattern can also be generated by other methods, e.g. by combination of the outputs of a co-located vertical monopole and loop. The figure-eight radiation pattern previously referred to can be produced by a vertical loop, alone, as well as by pairs of identical vertical elements, as is well known in the art.

The outputs of distribution amplifiers A1-A8 are applied to circuitry for combining pairs of the antenna signals to obtain cardioid radiation patterns at points (A), (B), (C) and (D) in FIG. 1. In this particular example, the antenna elements are coupled together in pairs 1-2, 7-3, 7-6, and 5-4, by means which are well known in the art, each said pair of elements providing a cardioid radiation pattern. This is accomplished by coupling the output of amplifier A1 to the input of a subtractor 12 (such as a balanced hybrid) via a delay element 8. The output from amplifier A2 is coupled directly to the input of subtractor 12. The output of subtractor 12 provides a cardioid radiation pattern as shown in FIG. 2A. The output from amplifier A6 is coupled to subtractor 13 via a delay element 9 and the output from amplifier A7 is applied directly to the other input of subtractor 13. The output of subtractor 13 also provides a cardioid radiation pattern as shown in FIG. 2A. The output of amplifier A8 is coupled to a subtractor 14 via a delay element 10 and the output of amplifier A3 is directly coupled to the other input of subtractor 14. The output of amplifier A5 is coupled to subtractor via a delay element 11 and the output of amplifier A4 is coupled to the other input of subtractor 15. It is again pointed out that outputs (A), (B), (C) and (D) of

subtractors

12, 13, 14 and 15, respectively, each provide the radiation pattern of FIG. 2A.

Now the cardioid patterns provided by

subtractors

12, 13, 14, and 15 are combined to provide a more directional, steerable pattern than provided by any one of the pairs of antenna elements alone. The output of subtractor 12 is coupled to one input of subtractor 18 and the output of subtractor 13 is coupled to another input of subtractor 18 via a delay element 16. The radiation pattern provided by so combining the outputs of

subtractors

12 and 13 is a pattern with nulls along lines at 0 and 60 degrees clockwise from the nulls of each original cardioid. This pattern is illustrated in FIG. 2B. In a similar manner the output of subtractor 14 is coupled to one input of a subtractor 19 and the output of subtractor 15 is coupled to the other input of subtractor 19 via a delay element 17. The radiation pattern provided at the output of subtractor 19 is similar to that of subtractor 18 and is shown in FIG. 2B.

Now the outputs of

subtractors

18 and 19 are combined to provide the yet more directional radiation pattern as shown in FIG. 2C. The output of subtractor 18 is coupled to one input of a subtractor 21 and the output of subtractor 19 is coupled to another input of subtractor 21 via a delay element 20. This provides at the output of subtractor 21 the final output from the array which is generally shown in FIG. 2C. The output of subtractor 21 is a radiation pattern with nulls along lines at 0, +60 and 60 degrees from the null of each of the first two combined cardioids. This output is then fed to the receiver circuitry 23 via output lead 22. The receiving circuitry is merely shown as block 23 since such circuitry is well known in the art and a more detailed description thereof is not deemed necessary for a proper understanding of the instant invention. The composite pattern can be readily calculated by one ordinarily skilled in the art by properly multiplying the cardioid-like patterns (i.e., the one aligned at zero degrees, the one aligned at 60 degrees, and"'the' one aligned at +60 degrees).

The resulting radiation patterns for S n/4 differ negligibly from each other. At S= \/4, the calculated side lobes are down 40.6 db and the directive gain is 11.4 db over isotropic with a main beam width of 74 degrees. The directive gain rises to 12.6 db as S is reduced to the side lobes falling from 40.6 db to 53.7 db over the same range. It is therefore seen that nearly uniform pattern performance is obtained over a wide range of frequencies.

Steering of the above described array is accomplished by mechanical rotation or by combining the outputs of the antenna elements 1-7 in different combinations. Many receivers, each having their own combining circuits may be coupled to the outputs of distribution amplifiers A1-A8 and each receiver may sum the antenna outputs differently, independently of the others. Therefore, it is seen that a number of independently shaped and steered directional antenna systems may be provided by the above described, novel antenna array.

The above-described antenna array operates as a superdirective array. This means that a directive radiation pattern is formed by means of a relatively small overall antenna aperture. In the prior art super-directive antenna arrays, the impedance and pattern bandwidths approach zero as the spacing between antenna elements gets closer and closer. As the antenna elements are spaced closer together the impedance bandwidth tends to decrease in the subject array, but the outputs from the antenna elements are combined in the novel manner described herein to provide a wide radiation pattern bandwidth. The theory of super-directive antenna systems is described in Electromagnetic Waves and Radiating Systems, E. C. Jordan, McGraw-Hill, New York, 1950, page 445.

When the outputs of the amplifiers are combined together, both the (desired) signal and the (undesired) received (environmental) noise are reduced. The received noise is reduced more rapidly than the signal, giving rise to a system gain (enhancement of signal-to-noise ratio, 5/ N). The improvement realized is dependent on the ratio of the received noise in the output of subtractor 21 to the total inherent amplifier noise as it appears in the output of subtractor 21. The inherent amplifier noise adds on a power basis, irrespective of the element spacing. The level of the received noise is reduced as the element spacing is reduced. As long as the received noise in the output is substantially higher than the amplifier noise, there is a net improvement in the S/ N. As the received noise approaches the amplifier noise, there is a net loss in S/ N. This governs the minimum spacing between elements, except where S/ N (or gain) can be traded for directivity.

It should be understood that, while this embodiment is related to frequencies below 30 mHz., it may also be used at higher frequencies whenever a sacrifice of efficiency may be tolerated in exchange for directivity.

One factor limiting the width of the band over which the above-described array. can be effectively used is the radiating element itself. Perhaps the most attractive type of radiator for the lower frequencies is the electrically-short vertical monopole. This radiator can be connected directly, without tuning, to the input stage of the respective distribution amplifier A1A8. Its height need only provide a level such that received noise at the output of subtractor 21 overrides the inherent noise of the first stages of the distribution amplifiers appearing at the output of subtractor 21. The element height should not greatly exceed 4 at the highest operating frequency; for example, if it is made equal to 0.075)\ at the lowest operating frequency, it will reach quarter wave resonance in a span of just under two octaves.

Referring now to FIG. 5, a short vertical monopole antenna for use with this invention can be represented as a voltage generator V and a complex source impedance. This source impedance consists of the radiation resistance R,, the antenna loss resistance R and the antenna capacitance C In the frequency range of interest, the antenna reactance X /21rfC is much larger than R R or equivalent noise resistor R however, the amplifier has an output impedance which is much larger than X Thus, essentially, the full value of V is applied to the input of the amplifier. f represents the ratio of sky temperature to room temperature.

The antenna loss resistance R will have little effect on the performance of the system. R is in the order of 1 ohm while amplifier equivalent noise resistor R can be expected to be to 100 times this value. Typical vacuum tubes have R equivalent values between 100 and 200 ohms at room temperature. Thus, V and R can be lumped into the amplifier equivalent noise voltage e and noise resistance R If it were possible to eliminate R and e completely, then R would become the ultimate limit on sensitivity for a given element spacing S and height h.

The signal-to-noise ratio considerations govern the minimum element spacing in wavelengths, which, in turn, determines the useful frequency range. The received noise, which, in this way governs the useful bandwidth, varies with frequency so that it is not possible to generalize about the bandwidth capability in the frequency range below mHz.

The directive gain G of the array may be stated in terms of the improvement provided in signal-to-noise ratio. It is given precisely by the ratio of the S/N for the array to the S/N for a single element. That is,

where:

(S/N) =output signal-to-sky noise ratio for the array (S/N) :output signal-to-sky noise ratio for a single element In this case, G is referred to the gain of a single element. It may be referred to an isotrope by correcting for the gain of the element over isotropic.

It is convenient to rearrange the equation for G as follows:

GD:(SA/SE)/(NA/NE):GC/GN where:

S =Mean square output voltage from the array illuminated by a sinusoidal signal source located for maximum array output.

S =Mean square output voltage from a single element illuminated by the same source that produces S N =[T ;I the mean square sky noise voltage output of the array illuminated by a noise source uniformly distributed over the upper hemisphere.

N lT g l the means square sky noise voltage output of a single element of the array illuminated by the same noise source.

G =N /N =the noise gain.

G =S /S =the combining gain.

It is interesting to note that, in the case of a transmitting 6 array over a perfect ground plane, Equation 1 may be written:

array.

U dSZzIntegral of the power density u.h. over the upper hemisphere.

G =Gain of the element over isotropic.

Therefore, G =U and,

The noise output of the array varies with element spacing and element height. G may be obtained by summing the cross-correlation products from all the array elements.

The combining gain for the subject embodiment is given by which gives 16.7 db for s= \/4. This can be seen approximately by imagining that all three cardiods were oriented n the same direction. The signal would then double in each of the three successive stages of combining to produce 18 db enhancement. However, two cardioids are rotated :60 degrees so that G; is less than 18 at s= \/4. As already explained, the directive gain is given by G Gc/GN In this case, the array gain is taken with respect to a short monopole and it varies from 6.2 db at s/ to 7.8 db as s/)\ 0. Since a short monopole on a perfect ground plane has a gain of 4.76 db over isotropic, the array has a directive gain of 12.6 db over isotropic as s/,\ O; at s/ the reference is a quarter-wave monopole over a ground plane. It has a gain of 5.15 db over isotropic so that the array directive gain is 11.4 db over isotropic at s/)\=%. These gain levels are in good accord with the ratio of 41r steradians to the angular area subtended by the 3 db beamwidths (45 degrees by 74 degrees).

Referring to FIG. 3, there is illustrated another embodiment of an antenna array according to this invention. In this array, the cardioid radiation patterns provided by each pair of antennas are set for maxima at 0, and 90. This configuration gives somewhat more directivity than that obtained with the preceding array. Antenna elements 30-35 are mounted as shown in FIG. 4 spaced a distance S apart, S being approximately equal to or less than M4. It is recognized that this spacing between adacent antenna elements may be different for different pairs but for convenience of analysis they are made equal in this embodiment. The antenna elements 30-35 comprise vertical monopoles which are loaded with resistances to given the required element bandwidth.

Antenna element 30 is coupled to one input of subtractor 24 via a delay element 38. Antenna element 31 is coupled to the other input of subtractor 24 via power splitter 37. The radiation pattern at the output of subtractor 24 is a cardioid pattern as illustrated generally in FIG. 2A. Antenna element 31 is further coupled to one input of subtractor 25 via power splitter 37 and delay element 39. Antenna element 32 is coupled directly to the other input of subtractor 25. Again, the radiation pattern at the output of subtractor 25 is generally shown in FIG. 2A. Antenna element 33 is coupled to one input of subtractor 28 via delay element 43 and antenna element 34 is coupled to the other input of subtractor 28 via power splitter 36. The output radiation pattern of subtractor 28 is generally shown in FIG. 2A. Antenna element 34 is further coupled to one input of subtractor 29 via power splitter 36 and delay element 44. Directly coupled to the other input of subtractor 29 is antenna element 35, the output radiation pattern of subtractor 29 being substantially shown in FIG. 2A.

The radiation patterns at the outputs of

subtractors

24 and 25 are combined in order to provide a more directive radiation pattern. The output of subtractor 24 is directly coupled to one input of subtractor 26 and the output of subtractor 25 is coupled to the other input of subtractor 26 via a delay element 40. The output of subtractor 28 is coupled directly to one input of subtractor 23 and the output of subtractor 29 is coupled to the other input of subtractor 23 via delay element 42. The more directive radiation patterns appearing at the outputs of

subtractors

26 and 23 is generally shown in FIG. 4A.

The outputs of

subtractors

23 and 26 are now combined to provide yet a more directive radiation pattern. The output of subtractor 26 is coupled to one input of subtractor 27 and the output of subtractor 23 is coupled to the other input of subtractor 27 via delay element 41. The radiation pattern generally illustrated in FIG. 4B represents the output of subtractor 27, this output being fed to the receiver 45. The receiver is of a type well known in the art and a further description thereof is deemed unnecessary for a proper understanding of the instant invention.

It is noted that the above described rectangular array is somewhat more compact than the previously described circular array of FIG. 1 and provides a more directive radiation pattern than the array of FIG. 1. This embodiment may be utilized without providing amplifiers at the base of each antenna element, as shown for convenience, in FIG. 3; but if the array is to provide simultaneous beam formation in opposite directions, amplifiers could be added for isolation purposes. Also, if there is a source of noise after the array, it may be desirable to utilize an amplifier associated with each antenna element or at the output of subtractor 27. Note, that this array of FIG. 3 does not have the multi-use directional flexibility of the array of FIG. 1 which has circular symmetry.

The two embodiments described above depend on the generation of cardioid radiation patterns by pairs of antenna elements, but it is pointed out that the invention can also make use of figure-eight radiation patterns in combination with cardioid radiation patterns. The basic principles of my invention are not restricted to the use of either type of radiation pattern and it should be clear that other radiation patterns having the appropriate characteristics may also be used.

Referring to FIG. 6, there is shown another embodiment of my invention which utilizes vertical loop antenna elements providing figure-eight radiation patterns. The outputs from the antennas are combined in pairs, each pair providing a figure-eight radiation pattern multiplied by a cardioid radiation pattern. Mounted on a support surface are vertical

loop antenna elements

51, 52, 53 and 54. Each of the antenna elements 51-54, respectively, are mounted equidistant about the circumference of a circle of the diameter D, D being equal to or less than one half wavelength. The output of antenna element 52 is coupled to one input of subtractor 56 via a delay element and the output from antenna element 51 is coupled directly to the other input of subtractor 56. The output of loop antenna 53 is coupled to one input of subtractor 57 via a delay element 58 and the output from loop antenna element 54 is coupled directly to the other input of subtractor 57. The output from subtractor 57 is coupled to one input of subtractor 59' via a delay element 50 and the output of subtractor 56 is coupled directly to the other input of subtractor 59. The output lead 61 from subtractor 59 provides the output of this system.

Each of the antenna elements 5l54 provide a figureeight radiation pattern, the outputs thereof being combined in pairs. The output of each pair provides a figureeight radiation pattern multiplied by a cardioid radiation pattern directed 45 degrees clockwise from the direction of the final overall pattern. The resultant pattern obtained from combining the radiation patterns which appear at the outputs of

subtractors

56 and 57, is a figure eight radiation pattern multiplied by one cardioid pattern directed 45 degrees clockwise and by another cardioid pat tern directed 45 degrees counter-clockwise with respect to the direction of the final overall pattern. The final radiation pattern at the output of subtractor 59 is similar to the radiation pattern shown in FIG. 2C except that an additional lobe is now present which is pointed in the opposite direction to the direction of the main lobe. This third undesired lobe is of low enough amplitude so as not to degrade the directivity of the system.

It is pointed out that this system is much less complex than the previously described systems in that only four antenna elements, three subtractors and three delay elements are required. However, for applications where the gain need not be as great as that provided by the embodiments of FIGS. 1 and 3, this embodiment may prove quite satisfactory.

While the subject invention is most useful for receiving purposes, as described hereinabove, it is pointed out that it may also be used in transmitting applications whenever a reduction in either the bandwidth or efiiciency, or both, could be tolerated. For example, the following modes of operation are possible:

(1) As described previously but with the elements loaded resistively or otherwise broadbanded, and with armplifiers supplying power to the antenna elements. Broad bandwidth will result, but the gain will be reduced and will be a function of frequency.

(2) As described previously, but with the amplifiers supplying power to matched antenna elements. Narrow bandwidth will result, but the gain will be higher and will also be a function of frequency.

In the above two transmitting modes, the pattern is dependent on the shape of the cardioid and/or figureeight radiation patterns which themselves are substantially independent of frequency and will remain unaffected by the feed arrangement, provided that the transmission lines, delay lines, amplifiers, and subtractors provide the proper impedance matches.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the accompanying claims.

I claim:

1. An antenna array comprising:

a plurality of antenna elements;

first means coupling said antenna elements into a plurality of groups of said elements, each said group providing a radiation pattern substantially invariant with wavelength above a predetermined value of wavelength, and the spacing between adjacent elements of each said group and said first means for coupling said antenna elements into said groups are such that each said group provides a cardioid radiation pattern; and

means coupled to said groups of said elements for combining the output signals from said groups to provide a directive radiation pattern.

2. An antenna array according to claim 1 wherein said first coupling means includes a plurality of individual coupling means, each said individual coupling means being associated with only one of said groups of antenna elements.

3. An antenna array according to claim 1 wherein adjacent antenna elements of said array have a predetermined spacing therebetween.

4. An antenna array according to claim 3 wherein adjacent antenna elements of each said group are spaced less than one wavelength apart.

5. An antenna array according to claim 2 wherein each said individual coupling means includes:

a delay element coupled to one antenna element of one of said groups; and

a subtractor element, one input thereof being coupled to another antenna element of said one group and the other input thereof being coupled to the output of said delay element.

6. An antenna array according to claim 2 wherein said combining means includes a plurality of individual combining means, each said individual combining means being associated with two of said individual coupling means.

7. An antenna array according to claim 6 wherein each said individual coupling means; and

a delay element coupled to the output of one of said individual coupling means; and a subtractor element, one input thereof being coupled to the output of another of said individual coupling means and the other input thereof being coupled to the output of said delay element. 8. An antenna array according to claim 1 further comprising:

second means coupling said antenna elements into a second plurality of groups of said elements, each of said second plurality of groups providing a radiation pattern substantially invariant with wavelength above a predetermined value; and second means coupled to said second plurality of groups of antenna elements for combining the output signals from said second plurality of groups to provide a second directive radiation pattern which points in a direction different from that of the first directive radiation pattern, 9. An antenna array according to claim 1 wherein: said plurality of antenna elements includes seven antenna elements, six of said elements being spaced substantially equidistant about the periphery of a circle having a predetermined diameter and the seventh one of said elements being located substantially at the center of said circle; said first coupling means includes:

third means coupling the first and second of said antenna elements together to provide a radiation patter-n substantially invariant with wavelength above a predetermined value; fourth means coupling the third and seventh of said antenna elements together to provide said substantially invariant radiation pattern; fifth means coupling the fourth and fifth of said antenna elements together to provide said substantially invariant radiation pattern; and sixth means coupling sixth and seventh of said antenna elements together to provide said substantially invariant radiation pattern; said combining means includes:

third means for combining the outputs of said third and sixth coupling means; fourth means for combining the outputs of said fourth and fifth coupling means; and fifth means for combining the outputs of said third and fourth combining means, the output of said fifth combining means providing said directive radiation pattern. 10. An antenna array according to claim 9 wherein each of said third through sixth coupling means includes:

a first delay element coupled to one antenna element of each said group; and a first subtractor element, one input thereof being coupled to the output of said first delay element and the other input thereof being coupled to the other antenna element of said group; and

wherein each of said third, fourth and fifth combining means includes:

a second delay element coupled to the output of one of said coupling means; and a second subtractor element, one input thereof being coupled to the output of said second delay element and the other input thereof being coupled to the output of another of said coupling means. 11. In an antenna array according to claim 1 wherein: said plurality of antenna elements includes six antenna elements, three of said elements being spaced along a first line and the other three of said elements being spaced along a second line substantially parallel to said first line; said first coupling means includes:

seventh means coupling the first and second of said antenna elements together to provide a radiation pattern substantially invariant with wavelength above a predetermined value; eighth means coupling the second and third of said antenna elements together to provide said substantially invariant radiation pattern; ninth means coupling the fourth and fifth of said antenna elements together to provide said substantially invariant radiation pattern; and tenth means coupling the fifth and sixth of said antenna elements together to provide said substantially invariant radiation pattern; said combining means includes:

sixth means for combining the outputs of said seventh and eighth coupling means; seventh means for combining the outputs of said ninth and tenth coupling means; and eighth means for combining the outputs of said sixth and seventh combining means, the output of said eighth combining means providing said directive radiation pattern.

12. An antenna array according to claim 11 wherein each of said seventh through tenth coupling means includes:

a first delay element coupled to one antenna element of each said group; and

a first subtractor element, one input thereof being coupled to the output of said first delay element and the other input thereof being coupled to the other antenna element of said group; and wherein each of said sixth, seventh and eighth combining means includes:

a second delay element coupled to the output of one of said coupling means; and

a second subtractor element, one input thereof being coupled to the output of said second delay element and the other input thereof being coupled to the output of another of said coupling means.

13. An antenna array comprising:

a plurality of antenna elements, each said antenna element providing a figure-eight radiation pattern; first means coupling said antenna elements into a plurality of groups of said elements, each said group providing a radiation pattern substantially invariant with wavelength above a predetermined value of wavelength; said first coupling means further coupling said antenna elements in pairs, the output radiation pattern from each said pair being a figure-eight radiation pattern multiplied by a cardioid radiation pattern; and

14. An antenna array according to claim 13 wherein each said antenna element is a vertical loop antenna.

15. An antenna array according to claim 13 wherein:

said plurality of antenna elements includes four antenna elements, each said antenna element providing a figure-eight radiation pattern;

1 l 1 2 said first coupling means includes: another subtractor element to'the other input of a subtractor element having one input thereof said third subtractor, the output of said third coupled to the output of one of said antenna elesubtractor providing said directive radiation patments; tern. a delay element coupling the output of a second 5 References Cited antenna element to the second input of said sub- UNITED STATES PATENTS tractor element; another subtractor element having one input thereof coupled to a third of said antenna elements; another delay element coupling the output of the fourth antenna element to the other input of 2,444,425 7/1948 Busignies 343--100.6UX 3,255,450 6/1966 Butler 343100(.6)

10 RODNEY D. BENNETT, 111., Primary Examiner said another subtractor; and R. E. BERGER, Assistant Examiner said combining means includes:

a third subtractor, one input thereof being coupled US. Cl. X.R.

to the output of said subtractor; and 15 343--100, 854

a third delay element coupling the output of said