US3919710A

US3919710A - Turnstile and flared cone UHF antenna

Info

Publication number: US3919710A
Application number: US527790A
Authority: US
Inventors: James C Administrator Fletcher; Donald J Bottoms; Theofanis G Gavrillis
Original assignee: National Aeronautics and Space Administration NASA
Current assignee: National Aeronautics and Space Administration NASA
Priority date: 1974-11-27
Filing date: 1974-11-27
Publication date: 1975-11-11
Anticipated expiration: 1992-11-11

Abstract

An improvement to the turnstile-cup antenna which increases its beamwidth. The improved beamwidth characteristics are attained by tilting the turnstile arms toward slots contained in a truncated cone.

Description

A United States Patent Fletcher et a1.

TURNSTILE AND FLARED CONE UHF ANTENNA Inventors: James C. Fletcher, Administrator of the National Aeronautics and Space Administration, with respect to an invention of Donald J. Bottoms, Denver; Theofanis G. Gavrillis, Littleton, both of Colo.

Filed: Nov. 27, 1974 Appl. No.: 527,790

U.S. Cl. 343/770; 343/797; 343/846 Int. Cl. ..H01Q 13/12; HOlQ 21/24; HOlQ 21/26 Field of Search 343/770, 797, 846

[ Nov. 11, 1975 [56] References Cited UNITED STATES PATENTS 3,618,107 11/1971 Spanos 343/846 3,725,943 3/1973 Spanos 343/797 Primury E.\'uminerEli Lieberman I Attorney, Agent, or FirmHoward J. Osborn; William H. King; John R. Manning [57] ABSTRACT An improvement to the turnstile-cup antenna which increases its beamwidth. The improved beamwidth characteristics are attained by tilting the turnstile arms toward slots contained in a truncated cone.

11 Claims, 3 Drawing Figures US. Patent Nov. 11, 1975 3 G I F TURNSTIILE AND FLARED CONE UHF ANTENNA ORIGIN OF THE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION This invention related generally to antennas and more specifically concerns an improved turnstile and cup antenna for UHF frequencies.

Many antenna applications have particular requirements for broad beamwidth. Broad beamwidth is that property of an antenna which makes it relatively insensitive to changes of orientation or angular variations with respect to the antenna or thepoint of observation. In a conventional turnstile and cup antenna, where a pair of crossarms are supported on a shaft which follows the axis of the cylindrical cup in which it is mounted, the axis of maximum radiation is along the axis of the cup. For such an antenna, tilting the antenna changes the direction of this imaginary axis and sharply reduces the strength of field at the points in space which were on the previous axis. A broad beamwidth antenna minimizes the reduction in field strength which occurs when the axis of the antenna tilts. The ideal broad beamwidth antenna would be one which acts as broad beamwidth wherein the field strength at a remote point is relatively independent of the orientation of the .antenna. Another object of the invention is to accomplish the increase in beamwidth for both linearly and circularly polarized turnstile-cup antennas.

SUMMARY OF THE INVENTION These and other objects of the invention are achieved by the use of a slotted truncated cone rather than the straight-sided cylinder of the conventional turnstilecup antenna. Also the turnstile itself is modified so that the arms of the turnstile are no longer in a plane perpendicular to the axis of the cup, but are angularly tilted back toward the cone.

The antenna feed begins at a conventional radio frequency coaxial connector mounted in the center of and perpendicular to the plane of the base plate of the cone. This coaxial connector protrudes from the base plate in the direction opposite from the structure of the cone. The center conductor of the coaxial connector is continuous through the base plate and to the end of the feed support upon which the antenna arms are mounted. This center conductor is insulated from the feed support, which acts as the outer conductor of the coaxial system, bysuitable radio frequency insulators. When the length of the feed support is short, no sup porting insulator is required. At the end of feed support most remote from the coaxial connector, a conductive tab connects the end of the center connector to the feed support.

2 tenna arms. The four antenna arms are attached at points on the end of the feed support which are 90 apart on the cross-section of the feed support cylinder and oriented so that the conductive tab connecting points bisects the angle between two arms. Two feed slots in the wall of the feed support cylinder are oriented 90 from the conductive tab on the cylindrical cross-section and thus bisect two of the other angles formed by the arms. The ends of the arms are tilted back toward the base of the cone.

The cone of the antenna is formed of conductive m'aterial and is electrically connected to the feed support. The sides ofthe cone are constructed so that if the cone were complete its apex would be approximately in the area of the end of the feed support. The end of the cone is. however. cut off to form a truncated cone and slots are cut in the sides of the cone. These slots are oriented to be exactly alined with the antenna arms when viewed from a point on the cone axis.

The result of this structure is an antenna with excellent broad beam characteristics which are believed to bethe result of the conical ground plane effect and parasitic coupling between the antenna arms and the slots in the cone sides.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an end-on view of the preferred embodiment of the invention:

FIG. 2 is a cross-sectional view of section 2-2 of FIG. I;

FIG. 3 is a perspective view of an alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the invention selected for illustration is shown in FIG. 1 as viewed from the open end of the cone on a point on the axis of the cone. FIG. 2 is a cross-sectional view of the same embodiment on the section 22 shown in FIG. I. In both FIGS. 1 and 2 antenna assembly 10 is essentially constructed of truncated cone 12 and turnstile antenna 14. Truncated cone 12 which acts as a ground plane for turnstile antenna l4 is constructed of conventional plated sheet metal as is commonly used in UHF apparatus. Cone base plate 16 forms an angle A with cone sides 18. In the preferred embodiment angle A mea sures For the preferred embodiment cone sides 18 are cut off at a point between three-quarters and one half the height of the complete cone. Petals 20 are formed from the remaining height of sides 18 and are oriented in a plane parallel to cone base plate 16 at 90 spacings. Four slots 21, cut in cone sides 18 are oriented 90 apart and bisect the angles formed by the centers of the petals. Slots 21 run from the upper edge of sides 18 and are limited in their approach'to base plate 16 by considerations of structural stability of cone 12. The exact lengths of slots 21 must be determined experimentally in order to achieve the parasitic coupling desired for beamwidth improvement.

Coaxial connector 22, which is a conventional radio frequency connector suitable for the frequency and power level of operation of antenna 10 is attached to the outer surface of cone 12 at the center point of base plate 16. On the inside of cone 12, at the center of base plate 16 and directly opposite coaxial connector 22 is mounted feed support 24, a hollow cylinder of suitably conductive material for the frequency of operation. Mounted in the center of feed support 24 and coaxial to it is center conductor 26 which is continuous from coaxial connector 22 to far end 25 of feed support 24. Approximate diameters are selected for feed support 24 and center conductor 26 so that the characteristic impedance of the feed support closely matches the feed-point impedance of the turnstile antenna. The characteristic impedance of the radio frequency power source (not shown) which is attached to antenna at coaxial connector 22 must also be considered and matched, if possible. Center conductor 26 passes through cone base plate 16 at hole 28 which is the same diameter as the inside diameter of feed support 24. At far end of feed support 24, center conductor 26 and feed support 24 are joined by conductive tab 30. At far end 25 of feed support 24 a portion of the coaxial section formed by center conductor 26 and feed support 24 contains

slots

32 and 34 whose lengths are one quarter wavelength at the operating frequency. The total length of feed support 24 is adjusted to provide an impedance match and a distance between

antenna arms

36, 38, 40 and 42 and cone slots 21 which is experimentally determined to aid the parasitic coupling be tween them.

Feed slots

32 and 34 are cut into the sides of feed support 24, 180 from each other and 90 from the location of conductive tab 30. These angles are measured around the axis of cone 12 as viewed in FIG. I.

Antenna arms

36, 38, 40 and 42 are mounted on feed support 24 at far end 25 with their far ends tilted back toward cone base 16 at least a visibly perceptible amount.

Arms

36 and 40 are each mounted 45 from conductive tab and 90 from each other. Arm 38 is mounted opposite arm 36, and arm 42 is mounted opposite arm 40. Turnstile antenna 14 is oriented relative to truncated cone 12 so that

antenna arms

36, 38, 40 and 42 are in exact alinement with slots 21 in truncated cone 12.

FIGS. 1 and 2

show arms

40 and 42 of shorter length than

arms

36 and 38. This preferred embodiment can be adjusted so that the current in one pair of diametrically opposed arms is 90 out of phase with the current in the other pair of arms thus yielding a circularly polarized radiation field. Such a field can easily be pictured as an umbrella or dome over the antenna with the support for the umbrella in line with the axis of the antenna cone. Essentially, the antenna operates as if a point source of radiation was located at the end of the feed support. The circular polarization and broad beam characteristics which result from this antenna configuration yield a radiation pattern in which the field strength at a particular point in space is relatively independent of the orientation of the antenna. Thus, field strength is relatively independent of whether the point in space at which field strength is being measured is on the extension of the axis of the antenna or whether it is off the axis of the antenna.

In a particular specific design of the preferred embodiment, the antenna arms are tilted back 30 from the plane perpendicular to the cone axis; the feed support is 8 inches long; the truncated cone length 5.5 inches; the longer antenna arms 7.38 inches and the shorter antenna arms 3.78 inches. This particular design, when operated at 1700 MHz, yielded a radiation distribution in which the voltage ratio of one-half was maintained for the full 180 of space in the direction of the antenna axis. Thus if the antenna were being used for transmitting, a receiver anywhere in the plane perpendicular to the antenna axis which is immediately at 4 the end of the antenna, would still receive half the voltage signal of a receiver an equal distance away but on the axis of the antenna.

It is understood that the form of the invention herein shown is merely a preferred embodiment. Various changes may be made in the shape, size, or arrangement of parts; equivalent'means may be substituted for those illustrated and described and certain features may be used independently from other features without departing from the spirit and scope of the invention. For example, as shown in FIG. 3, the petals may be removed from the top of cone sides 18 with only a slight sacrifice of broad beam characteristics. Also,

antenna arms

36, 38, 40 and 42 may be made the same length to yield linear polarization with broad beamwidth characteristics. Such polarization gives radiation essentially in a single plane but the broad beamwidth remains in that the antenna is relatively independent of orientation in that one plane.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. A broad beamwidth antenna for use at UHF comprising:

truncated cone means which is comprised of a cone base plate and cone sides which are cut off before reaching the cone apex, with equally spaced slots cut into said sides;

feed support means coaxial with the axis of said truncated cone and attached to the inside of said truncated cone means at a hole in the center of said cone base plate, extending beyond the cut-off sides of said cone sides, and containing feed slots in the feed support means sides;

center conductor means coaxial with and mounted within said feed support means and extending from the hole in the cone base plate to the opposite end of the feed support means, forming a coaxial conductor with the feed support means;

conductive tab means joining said center conductor means and said feed support means at the end remote from the cone base plate of said truncated cone;

connector means attached to said center conductor means and said cone base plate; and

a turnstile means comprising at least two antenna arms means radially attached to said feed support means at the end remote from the cone base plate, equidistant from each other, whose sides are tilted back toward the cone base plate a visually perceptible amount, and each of whose axis is alined with the center line of one of the slots cut into the cone sides.

2. A broad beamwidth antenna for use at UI-IF as in claim 1 wherein said turnstile means comprises four amrs all of equal length thereby yielding a linearly polarized antenna field.

3. A broad beamwidth antenna for use at UHF as in claim 1 wherein said turnstile means comprises two pair of arms, each pair comprising two arms mounted 5. A broad beamwidth antenna for use at UHF as in claim 1 wherein the truncated cone means comprises a cone base plate and cone sides which form an angle of 6. A broad beamwidth antenna for use at UHF as in claim 1 wherein the truncated cone means is of such a size that the uncut cone length is approximately equal to the length of the feed support.

7. A broad beamwidth antenna for use at UHF as in claim 1 wherein the truncated cone means is cut off at between threequarters and one-half the uncut cone height.

8. A broad beamwidth antenna for use at UHF as in claim 1 wherein petals are added to the cone sides,

conductor means are coaxial cylinders.

cated between the slots cut in the sides, and oriented in a plane parallel to the cone base plate.

9. A broad beamwidth antenna for use at UHF as in claim 1 wherein the feed support means and the center 10. A broad beamwidth antenna for use at UHF as in claim 1 wherein the slots cut in the cone sides are all of equal length and said length is chosen to optimise parasitic coupling between the slots cut in the cone sides and the antenna arms means.

11. A broad beamwidth antenna for use at UHF as in claim 1 wherein there are four equidistant slots in the cone sides and wherein the turnstile means comprises four antenna arms means.

Claims

1. A broad beamwidth antenna for use at UHF comprising: truncated cone means which is comprised of a cone base plate and cone sides which are cut off before reaching the cone apex, with equally spaced slots cut into said sides; feed support means coaxial with the axis of said truncated cone and attached to the inside of said truncated cone means at a hole in the center of said cone base plate, extending beyond the cut-off sides of said cone sides, and containing feed slots in the feed support means sides; center conductor means coaxial with and mounted within said feed support means and extending from the hole in the cone base plate to the opposite end of the feed support means, forming a coaxial conductor with the feed support means; conductive tab means joining said center conductor means and said feed support means at the end remote from the cone base plate of said truncated cone; connector means attached to said center conductor means and said cone base plate; and a turnstile means comprising at least two antenna arms means radially attached to said feed support means at the end remote from the cone base plate, equidistant from each other, whose sides are tilted back toward the cone base plate a visually perceptible amount, and each of whose axis is alined with the center line of one of the slots cut into the cone sides.

2. A broad beamwidth antenna for use at UHF as in claim 1 wherein said turnstile means comprises four amrs all of equal length thereby yielding a linearly polarized antenna field.

3. A broad beamwidth antenna for use at UHF as in claim 1 wherein said turnstile means comprises two pair of arms, each pair comprising two arms mounted diametrically opposite, with the two pairs of arms being different length, thereby yielding currents 90* out of phase in the two pairs of arms and yielding a circularly polarized antenna field.

4. A broad beamwidth antenna for use at UHF as in claim 1 wherein the antenna arms means are tilted back 30* from the plane which is perpendicular to the axis of the truncated cone.

5. A broad beamwidth antenna for use at UHF as in claim 1 wherein the truncated cone means comprises a cone base plate and cone sides which form an angle of 45*.

6. A broad beamwidth antenna for use at UHF as in claim 1 wherein the truncated cone means is of such a size that the uncut cone length is approximately equal to the length of the feed support.

8. A broad beamwidth antenna for use at UHF as in claim 1 wherein petals are added to the cone sides, located between the slots cut in the sides, and oriented in a plane parallel to the cone base plate.

9. A broad beamwidth antenna for use at UHF as in claim 1 wherein the feed support means and the center conductor means are coaxial cylinders.

10. A broad beamwidth antenna for use at UHF as in claim 1 wherein the slots cut in the cone sides are all of equal length and said length is chosen to optimise parasitic coupling between the slots cut in the cone sides and the antenna arms means.