US3718934A

US3718934A - Broadcast antenna assembly

Info

Publication number: US3718934A
Application number: US00118436A
Authority: US
Inventors: R Bogner
Original assignee: Ampex Corp
Current assignee: Ampex Corp
Priority date: 1971-02-24
Filing date: 1971-02-24
Publication date: 1973-02-27
Anticipated expiration: 1990-02-27
Also published as: IT950569B; GB1320294A; ZA718603B; FR2126318A1; DE2208789A1

Abstract

A pair of back-to-back and interlaced antenna arrays, including end-fire parasitic directors, are supported by a common structure and perform almost independently with very little coupling or interaction despite their proximity. Each antenna array may provide an omnidirectional radiation pattern.

Description

United States Patent Bogner 1 Feb. 27, 1973 [54] BROADCAST ANTENNA ASSEMBLY [56] References Cited [72] Inventor: Richard D. Bogner, Roslyn, N.Y. UNITED STATES PATENTS Assigneel Ampex Corporation, Redwood City, 3,122,745 2/1964 Ehrenspeck ..343/819 Calif. 3,124,802 3/1964 Von DallArmi 343/891 X 22 '1 F b. 4 7 1 Fl ed e 2 19 1 Primary Examiner-Herman Karl Saalbach [2]] Appl. No; 118,436 Assistant Examiner-Marvin Nussbaum Attorney-Anderson, Luedeka, Fitch, Even & Tabin Related U.S. Application Data and Robert G Clay [63] Continuationimpart of Ser. No. 661,625, Aug.

18, 1967, Pat. No. 3,587,108. [57] ABSTRACT A pair of back-to-back and interlaced antenna arrays, [52] U.S. Cl. ..343/770, 3344330893132,3344352990; including end fire parasitic directors are supported by [51] Int. CL Jmlq 13/10 Holq 19/10 d 15/14 a common structure and perform almost indepen- [58] Field of 'search mug/797 770 7 89] dently with very little coupling or interaction despite their proximity. Each antenna array may provide an omnidirectional radiation pattern.

13 Claims, 3 Drawing Figures e Ea BROADCAST ANTENNA ASSEMBLY The present invention relates to transmitting antennas employing end-fire parasitic elements, and is a continuation-in-part of Ser. No. 661,625, filed Aug. 18, 1967 now US. Pat. No. 3,587,108, issued June 22, 1971.

The subject matter of the above-cited parent application has particular applicability to broadcasting anten' na design, e.g., for UHF-TV, in the achievement of high gain with an omnidirectional or other desired pattern in the azimuth plane at minimum cost. To achieve high overall gain, it is the usual practice to vertically stack a large number of driven antenna elements, each one defining a bay," to typically provide a vertical array extending for twenty to fifty feet or more for UHF-TV broadcasting (e.g., from 470 MHz. to 890 MHz.). Therefore, the metal tower or mast pole on which the elementary driven antennas are supported will generally be made with a substantially large diameter. That is, the supporting structure will generally have a major transverse dimension greater than one-quarter wavelength, and in practice, often greater than one wavelength, at the transmitted frequency, to provide the strength and rigidity necessary to mechanically support the antenna assembly and maintain stability in the wind and weather conditions to which it may be subjected. Thus, because of the relatively large diameter of the supporting structure, which may be in the form of a conductive pipe or comprised of open-work structural members or girders, a simple, single vertical array of driven antenna elements will not ordinarily provide an omnidirectional or other desired pattern due to the shadowing effects of the supporting structure or tower.

To minimize or avoid such shadowing effects, it has, for example, been a practice to employ dipole or zigzag panels on each of three or four sides of the tower, staggered arrays of many slots around and along the wall of a cylindrical pipe support, or a long helix wrapped around such a cylindrical support. However, the abovecited parent application discloses and claims a very simple and inexpensive antenna assembly structure for readily achieving an omnidirectional or other desired pattern with a single vertical linear array of elementary antennas mounted on a large diameter tower of any conventional construction wherein the assembly comprises the supporting structure, a driven antenna carried by the supporting structure and defining a radiation axis extending therefrom, and at least one end-fire parasitic director carried by the supporting structure. The end-fire parasitic director, in accordance with the preferred embodiment of said application, has a longitudinal axis and comprises a plurality of discrete planar conductive plates disposed in spaced parallel relation along the longitudinal axis and disposed normal thereto. The discrete conductive plates have their respective major dimensions at least a quarterwavelength in the plane of the electric field vector of the transmitted signal, and the director is positioned relative to the driven antenna and supporting structure so as to alter the shape of the antenna pattern produced by the combined effects of the driven antenna and supporting structure, while not intersecting the radiation axis of the driven'antenna, to provide the selected overall antenna pattern for the assembly. One or more of such parasitic directors may be employed, as desired, in combination with the: supporting structure and driven elements to produce any number of antenna pattern shapes, from omnidirectional to highly directional, and these are described in detail in the aforementioned application, the subject matter of which is hereby incorporated into the instant application by reference thereto.

The present invention stem in part from the recognition that if a second antenna, or antenna array, is mounted on the supporting structure of the antenna assembly of said parent application in generally back-toback relation, such as with the two arrays vertically staggered, the two antenna arrays perform almost indepcndently, i.e., with very little coupling or interaction, despite their close proximity. lt has been found that two separate antenna arrays with their respectively associated end-fire directors constructed in the manner taught by said parent application may be mounted on a single supporting structure to provide respective selected antenna patterns which are partially or completely overlapping in azimuth without significant mutual coupling between the driven antenna elements of the two arrays.

Accordingly, it is an object of the present invention to provide an improved broadcast antenna assembly having two independently operable antenna arrays, e.g., for two different television channels or for video and audio transmission of a single channel, on a single supporting structure or mast and having overlapping antenna patterns. When one antenna array is used for the video broadcast signal of a television channel and the other is used for the audio signal, the necessity for an expensive and critical diplexer, conventionally required to combine the video and audio signals prior to their being fed to a single antenna, is eliminated. Also, since the two antennas provided by the present antenna assembly can each carry both video and audio signals, in an emergency, a complete back-up antenna is provided at all times, with no increase in size, height, or wind load over one antenna. Additionally, since the audio bandwidth is relatively narrow, a simple series feed system can be used for the audio antenna array, leaving the inside of the mast or supporting structure relatively free for the parallel feed system used with the higher power video antenna.

It is another object of the present invention to provide such an improved antenna assembly having two antenna arrays on a common supporting structure wherein each array produces an omnidirectional antenna pattern without significant mutual coupling therebetween, and it is a further object to provide such an improved antenna assembly which can be constructed at relatively low cost. Both antennas of the single antenna assembly can be made omnidirectional in the horizontal or azimuth plane, or directional, within certain limits, since the driven antenna elements face in generally opposite directions.

These and other objects and advantages of the present invention are more particularly set forth in the following detailed description and in the accompanying drawing, of which:

FIG. 1 is a perspective view showing a portion of a section of an antenna assembly in accordance with an embodiment of the present invention;

FIG. 2 is a side view in elevation of the antenna assembly of FIG. 2; and

FIG. 3 is a sectional view of the antenna assembly of FIG. 2 taken along line 3-3 and showing in plan view the general anteii'na patterns produced by the antenna assembly.

In general, referring to FIGS. 1 through 3, the antenna assembly in accordance with the present embodiment of the invention is for transmitting signals of given wavelengths fed thereto from a suitable transmitter or transmitters. The assembly generally comprises an elongated conductive supporting structure, illustrated as a vertical tubular mast section l0,'having a major transverse dimension d at least a quarter-wavelength at the transmitted signal frequencies and at least one bay, indicated generally as 12, including two driven

antennas

14a and 14b carried by the supporting structure at perimetrically spaced positions and defining respective radiation axes 16a and 16b extending therefrom. The radiation axis of each driven antenna element is considered herein to be the axis of symmetry of the beam radiated from that element in the direction away from the radiating side of the element and usually is the direction of maximum radiation. Transmission lines, illustrated as

coaxial cables

17a and 17b, are provided for feeding the respective electrical signals to each of the driven

antennas

14a and 14b, as well' as to corresponding other driven antennas in each of the bays vertically stacked along the mast. At least one end-fire parasitic director, such as 18a, is associated with one of the driven antenna elements, such as 14a, and has a longitudinal axis indicated as 20. The end-fire director comprises a plurality of discrete planar conductive plates 22 disposed in spaced parallel relation along the longitudinal axis 20 and normal thereto. The discrete conductive plates 22 have their respective major dimensions at least a quarter-wavelength in the plane of the electric field vector of the transmitted signal from the associated driven antenna element. The end-fire parasitic director 18a, together with the director 18a in the illustrated embodiment, are so positioned relative to their associated driven antenna 14a and the supporting structure 10 as to alter the shape of the antenna patte m produced by the combined effects thereof, while not intersecting the radiation axis 16a of the associated driven antenna, to provide a selected antenna pattern 24 which overlaps the antenna pattern 26 of the other driven antenna 14b without creating significant mutual coupling between the two driven antennas.

Likewise, a further endfire parasitic director 18b, with the director 18b in the illustrated embodiment, are associated with the other of the two driven antennas 14b, and these directors also have the longitudinal axes and structure which were recited in connection with the director 18a. The directors 18b and 18b are also positioned in a manner so as not to intersect the radiation axis 16b of their associated driven antenna 14b and provide a further selected antenna pattern 26 as illustrated in FIG. 3.

The driven antenna elements corresponding to 14a and 14b in each bay 12-1, l22, 12-3, 12-5, etc. (as shown in FIG. 1) form the two generally opposed arrays A and B, respectively, and each driven antenna element has associated therewith two of the end-fire parasitic directors, as previously described, positioned on generally opposite sides of the respectively associated driven antenna. In the illustrated embodiment the end-fire parasitic directors are so arranged as to provide respective overlapping omnidirectional antenna patterns of the same general type, as illustrated in FIG. 3, and each pair of directors associated with a respective driven antenna have their longitudinal axes 20 oriented to intersect, if extended, on the same general side as the radiation axis from their associated driven antenna. The longitudinal axis 20 of each parasitic director preferably lies in a plane which is approximately normal to the longitudinal axis of the supporting structure 10, and such planes associated with the directors of each bay should intersect the supporting structure preferably within a half-wavelength of the center of their respectively associated driven antenna, measured along the longitudinal axis of the supporting structure. Desirably, they should intersect the supporting structure at the center of each driven antenna as shown in the illustrated embodiment. 1

More particularly, referring to FIG. 1, the tubular mast 10 provides support for the antenna assembly, and may be typically formed from standard pipe of hot-dip galvanized structural steel to retard corrosion and provide strength at low cost. The individual driven antenna elements of the illustrated assembly are of the slot type, such as 28, shown open in bay 12-2, which may be cut into, or mounted on, the mast. Behind each of the slots of the assembly of FIG. 1 is a cavity 30 of the type commonly employed in radiating antennas to form and impedance-match the slots, and the cavities may be merely formed as sheet metal structures. A dielectric cover 32 may be placed over each of the slots as a radome,

and may be formed, for example, of glassfiber cloth impregnated with a synthetic resin. Each slot of each respective antenna array (i.e., arrays A and B) is fed a portion of the transmitter power for that respective array through the respective

transmission line networks

17a and 17b which may be located within the tubular mast l0 and connected in a known manner to each slot for excitation thereof.

Mounted onto the cavities 30, and thus onto the tubular mast, on either side of each slot antenna are flanges or brackets 34 which support the end-fire parasitic directors 18. The end-fire directors include metal support rods 36 aligned with the longitudinal director axis, and attached to the brackets 34 so that they extend generally outward from the tubular mast 10 on opposite sides of the slots. Then, mounted on each of the support rods 36 are the planar conductive members or plates 22, illustrated in the form of conductive disks, spaced axially along the rod and disposed normal thereto.

At the bottom end of the tubular mast 10 is a flange 38 which may be fastened to another such flange to interconnect two adjacent mast sections for extending the length of an antenna section, or which may be fastened to a tower or other base structure, such as by means of suitable bolts or other fasteners through the flange holes shown in FIG. 1. Suitable coaxial connectors of conventional type may also be provided for interconnecting the various sections of the coaxialtransmission lines 18a and 1811 as required.

The illustrated end-fire directorsare formed by socalled plate on-rod" or disk-on-rod" elements which act as parasitic directors for the present antenna assembly. There is no connection, except a structural one, between the elementary slot antennas which receive transmitter power and transmit electromagnetic radiation and the end-fire directors. That is, the end-fire directors act on the radiating electromagnetic fields, but are not conductively connected to the driven antenna elements. Thus, the brackets 34 could be mounted directly intothe tubular mast and need not necessarily be mounted onto the ends of the cavity mounting plates. Moreover, it is not, in general, necessary for the directors to be located directly adjacent to the center of their associated elementary antennas in each bay 12 of the antenna assembly. However, as hereinbefore indicated, the axis of each director should preferably lie in a plane which is about normal to the axis of the tubular mast and which intersects the tubular mast within a half-wavelength of the center of the respective elementary driven antenna. Also, more than one level or set of end-fire directors may be associated with each elementary antenna, and various exemplary arrangements are shown and described in the aforementioned parent application which may be employed in connection with the present invention; also, other arrangements as well may be employed based on the general teachings thereof. By directing the particular angle (relative to the radiation axis of the associated driven antenna element), location, and design of the directors, the horizontal or azimuth plane of radiation can be readily controlled to provide the desired antenna patterns.

Suitable plates 22 for the end-fire directors 18 have a major dimension preferably between a quarterwavelength and a half-wavelength at the transmitter frequency, and more preferably, about one-third wavelength. The plate major dimension is considered that dimension in the plane of the electric field or E vector, being the horizontal diameter of the disk in the presently illustrated embodiment. For a slot antenna or launcher, for example, in a vertical array, the plane of the major E vector is the horizontal plane. For a dipole, it would be the vertical plane, and for both used together, it would be both planes. A suitable range of interplate spacing is between one-eighth and one-half wavelength at the transmitter signal frequency. However, while these spacing dimensions are preferred, closer spacings are permissible. Also, it may be desirable in some applications to make the major dimension of the discrete metallic members or plates nonuniform, with the plates farther from the mast larger than plates close to the mast on any one support rod.

The respective elementary antennas of each separate array, A and B, are preferably spaced apart a distance between 0.9 and 1.0 wavelength along the mast, and the distance between the successive antenna elements of the alternate arrays along the mast is about a halfwavelength in the illustrated assembly. The transmission lines are connected to each driven slot, as indicated, such that each is fed a portion of the respective transmitter power of proper relative phase and amplitude. Since the entire antenna assembly can be built in sections, the power divider system forthe antenna assembly may, for example, be placed just below the antenna assembly, with separate lines feeding all antenna elements above it. On the other hand, the divider may be placed between the sections with the separate lines feeding the antenna elements above and below the divider, and the divider may be located entirely within the antenna supporting mast, itself.

Referring now to FIG. 3, there is shown a transverse sectional view of the antenna assembly of FIGS. 1 and 2 taken over a typical bay l2 and showing the horizontal or azimuth plane radiation pattern produced by this system, plotted from the center of the mast 10. The end-

fire directors

18a and 18a associated with the slot 14a, and the end-fire directors 18b and 18b associated with the slot 14b, are each attached to the mast 10 by means of a flange bracket 34, as previously described, and extend outward from the mast at about on either side of their associated slot element 14. Likewise, the structures of the end-fire directorsand driven elements in each of the other bays of the illustrated antenna assembly are identical to those described above. The resulting

antenna patterns

24 and 26 respectively produced by these A and B arrays are omnidirectional within about 1' 1.5 db over the entire 360 azimuth. It is understood, of course, that although an elementary slot antenna element alone may be omnidirectional without the use of any director, the resulting radiation pattern of the slot and mast, or tower,

combined, has a substantial null resulting from the shadow effects" of the large supporting structure. The directors, shaped and angled as herein shown and described, however, fill the null areas around the tower and provide a full omnidirectional radiation pattern, as shown, where the pattern from each of the arrays on diametrically opposite sides of the mast are substantially overlapped. The presence of the parasitic end-fire directors results in the general increase or reinforcement of the antenna radiation intensity or power in the axis direction of each director. Thus, by changing the position, angle and design of the directors, it is possible to direct the radiation patterns to cover only certain areas with lobes, while having large nulls of various depth, as desired, in other areas or angular sectors, as generally described in the aforementioned parent application. But this is limited to some extent by the spacing around the mast perimeter between the corresponding antenna elements of the two arrays in any bay, these elements being back-to-back in the illustrated assembly.

Although the antenna assembly in accordance with the illustrated embodiment of the invention employs slots as the driven antenna element, or launcher, it is of course understood that any elementary type of antenna could be used, such as, but not limited to, a dipole, vee, loop, or short helix. Similarly, aithough the end-fire directors are illustrated as being of the disk-on-rod" type, any plate, mesh or rod member structure may be employed transversely on the support rod or, for example, within a non-conductive tube, to construct a suitable director. The term plate" is thus used herein in an electrical or electromagnetic wave sense, and includes mechanical structures that affect the wave in the general manner of a plate but which may actually have the mechanical or physical form of a mesh or screen, wires or rods, or other conductive elements whose overall effect is that of a generally planar configuration transverse to the longitudinal axis of the director. Examples of some of these various forms are described in greater detail in the aforementioned parent application.

In specific constructions of an antenna assembly in accordance with the principles of the present invention, it has been found that the mutual or crosscoupling between two slot-type antenna arrays disposed on opposite sides of the mast, or back-toback," and in what may be termed an interlaced arrangement, as in the illustrated embodiment, varied from a maximum of 30.5 db to a minimum of 38 db, with a mean value of about 34 db, measured at the bottom feed points of the antenna assembly with all of the slot elements fed in parallel. The antenna assembly structure included end-fire parasitic directors of the plate-on-rod" type having two square plates on each support rod and comprised two directors per bay on opposite sides of each driven slot element in the general manner of the illustrated embodiment. Such an antenna assembly having four bays resulted in crosscoupling between the back-to-back antenna arrays of 34 db, while the same general antenna assembly having only two bays resulted in cross-coupling of 38 db. Thus, although the cross-coupling between the interlaced separate antenna arrays on the mast appears to increase somewhat with the number of bays for a small number of bays, it is believed that the cross-coupling should stay below some satisfactory maximum even for a large number of bays. For example, the crosscoupling should still be about 30 db or so even when 20 bays are employed.

It was also found that the departure from omnidirectionality for each of the arrays individually (i.e., without the other array) was i 1 db, and this changed to only i 1.5 db when both arrays were utilized.

These results were obtained using a signal frequency of 659 MHz. (channel 45), and all of the driven slot antenna elements and directors were supported on a conductive mast pipe having an outside diameter of 10 5'4 inches.

Thus, there has been described an antenna assembly having two separate antenna arrays mounted on a common large supporting structure and capable of providing omnidirectional radiation patterns with substantially independent performance from each array and with very little coupling or interaction therebetween, despite the close proximity of the arrays and the overlapping nature or characteristic of their respective patterns. As previously indicated, the two antenna arrays may be used to transmit two different television channel signals from the same mast or to provide video and audio signal transmission without the use of any diplexer. In the latter case, since the audio bandwidth is very narrow, a simple series feed system may be used for one antenna array, leaving the inside of the pipe substantially free for the parallel fed higher power video array.

Hence, it can be seen that an antenna assembly constructed in accordance with the principles of the present invention will provide a substantial reduction in cost for broadcast antennas, particularly for television applications, without any sacrifice in performance. Additionally, it provides increased versatility of utilization in the design of a broadcast system.

Although a preferred embodiment of the present invention has been illustrated and described, various modifications thereof will be apparent to those skilled in the art, and although such modifications may result in structures which are mechanically different, they may embody the principles of the invention. Accordingly, the scope of the invention should be defined only by the appended claims and equivalents thereof.

Various features of the invention are set forth in the following claims.

What is claimed is:

1. An antenna assembly for transmitting signals of given wavelengths, comprising an elongated conductive supporting structure having a major transverse dimension at least a quarter-wavelength of the transmitted signals, at least one bay including two driven antennas carried by said supporting structure at perimetrically spaced positions and defining respective radiation axes extending therefrom, means for feeding respective electrical signals to said driven antennas, and at least one end-fire parasitic director associated with one of said driven antennas and having a longitudinal axis, said director comprising a plurality of discrete planar conductive plates disposed in spaced parallel relation along said longitudinal axis and being normal thereto, said discrete conductive plates having their respective major dimensions at least a quarterwavelength in the plane of the electric field vector of the transmitted signal from said associated driven antenna, and said end-fire parasitic director being so positioned relative to said associated driven antenna and supporting structure as to alter the shape of the antenna pattern produced by the combined effects of said associated driven antenna and supporting structure, while not intersecting the radiation axis of said associated driven antenna, to provide a selected antenna pattern overlapping the antenna pattern of the other driven antenna without significant coupling between the driven antennas.

2. The antenna assembly of claim 1 comprising at least one further end-fire parasitic director associated with the other of said driven antennas and having a longitudinal axis, said further director comprising a plurality of discrete planar conductive plates disposed in spaced parallel relation along said longitudinal axis and being normal thereto, said discrete conductive plates having their respective major dimensions at least a quarter-wavelength in the plane of the electric field vector of the transmitted signal from said other driven antenna, and said further end-fire parasitic director being so positioned relative to said other driven antenna and supporting structure as to alter the shape of the antenna pattern produced by the combined effects of said other driven antenna and supporting structure, while not intersecting the radiation axis of said other driven antenna, to provide a further selected antenna pattern.

3. The antenna assembly of claim 2 wherein each of said driven antennas has associated therewith at least two of said end-fire parasitic directors positioned on generally opposite sides oftheir respectively associated driven antenna.

4. The antenna assembly of claim 3 wherein said parasitic directors associated with each of said driven antennas are so arranged to provide respective overlapping omnidirectional antenna patterns, while said assembly permits substantially independent operation of said driven antennas.

5. The antenna assembly of claim 1 wherein said driven antennas are perimetrically spaced at diametrically opposed positions on said supporting structure.

6. The antenna assembly of claim 2 wherein said two directors associated with each driven antenna have their respective extended longitudinal axes oriented to intersect in the general direction of the radiation axis of their respectively associated driven antenna.

7. The antenna assembly of claim 2 wherein the longitudinal axis of each of said parasitic directors lies in a plane which is approximately normal to the longitudinal axis of said supporting structure, said plane intersecting said supporting structure within a halfwavelength of the center of the respectively associated driven antenna, measured along the longitudinal axis of said supporting structure.

8. The antenna assembly of claim 7 wherein said plane intersects said supporting structure at the about the center of the respectively associated driven antenna.

9. The antenna assembly of claim 7 wherein the longitudinal axes of said directors extend generally from said supporting structure at angles of about 120 to said respectively associated driven antenna on each side thereof.

10. The antenna assembly of claim 1 comprising a plurality of said bays spaced along the longitudinal axis of said elongated supporting structure and at least one of said end-fire parasitic directors associated with each bay.

11. The antenna assembly of claim 10 wherein the driven antennas of each bay are on generally opposite sides of said supporting structure and staggered along the longitudinal axis of said structure relative to each other.

12. The antenna assembly of claim 11 wherein the driven antennas in each bay on one side of said supporting structure are spaced approximately a wavelength apart along the longitudinal axis of said structure.

13. The antenna assembly of claim 2 wherein said elongated supporting structure has a vertical longitudinal axis, and said driven antennas are on generally opposite sides of said structure and staggered relative to each other, said assembly comprising two said endfire parasitic directors associated with each of said driven antennas and oriented in generally opposite directions to provide two omnidirectional radiation patterns about said supporting structure.

Claims

1. An antenna assembly for transmitting signals of given wavelengths, comprising an elongated conductive supporting structure having a major transverse dimension at least a quarterwavelength of the transmitted signals, at least one bay including two driven antennas carried by said supporting structure at perimetrically spaced positions and defining respective radiation axes extending therefrom, means for feeding respective electrical signals to said driven antennas, and at least one end-fire parasitic director associated with one of said driven antennas and having a longitudinal axis, said director comprising a plurality of discrete planar conductive plates disposed in spaced parallel relation along said longitudinal axis and being normal thereto, said discrete conductive plates having their respective major dimensions at least a quarter-wavelength in the plane of the electric field vector of the transmitted signal from said associated driven antenna, and said end-fire parasitic director being so positioned relative to said associated driven antenna and supporting structure as to alter the shape of the antenna pattern produced by the combined effects of said associated driven antenna and supporting structure, while not intersecting the radiation axis of said associated driven antenna, to provide a selected antenna pattern overlapping the antenna pattern of the other driven antenna without significant coupling between the driven antennas.

3. The antenna assembly of claim 2 wherein each of said driven antennas has associated therewith at least two of said end-fire parasitic directors positioned on generally opposite sides of their respEctively associated driven antenna.

7. The antenna assembly of claim 2 wherein the longitudinal axis of each of said parasitic directors lies in a plane which is approximately normal to the longitudinal axis of said supporting structure, said plane intersecting said supporting structure within a half-wavelength of the center of the respectively associated driven antenna, measured along the longitudinal axis of said supporting structure.

9. The antenna assembly of claim 7 wherein the longitudinal axes of said directors extend generally from said supporting structure at angles of about 120* to said respectively associated driven antenna on each side thereof.

13. The antenna assembly of claim 2 wherein said elongated supporting structure has a vertical longitudinal axis, and said driven antennas are on generally opposite sides of said structure and staggered relative to each other, said assembly comprising two said end-fire parasitic directors associated with each of said driven antennas and oriented in generally opposite directions to provide two omnidirectional radiation patterns about said supporting structure.