US3514781A

US3514781A - Broadband,high gain antenna with relatively constant beamwidth

Info

Publication number: US3514781A
Application number: US688271A
Authority: US
Inventors: Joseph E Ferris; Wiley E Zimmerman
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1967-12-05
Filing date: 1967-12-05
Publication date: 1970-05-26
Anticipated expiration: 1987-05-26

Description

hill/311L511 fil vl-v y Ill-l l-llhlllvic 5&5 39 09 May 26, 1970 F RR s ET AL 3,514,781

BROADBAND, HIGH GAIN ANTENNA WITH RELATIVELY CONSTANT BEAMWIDTH Filed Dec. 5, 1967 82 0000 o gggszoo I N VE N TORS,

JOSEPH E. FERR/S 8 WILEY E. Z/MMERMAN.

A TTORNE Y8.

3,514,781 Patented May 26, 1970 United States Patent Oflice 3,514,781 BROADBAND, HIGH GAINANTENNA WITH RELATIVELY CONSTANT BEAMWIDTH Joseph E. Ferris, Saline, and Wiley E. Zimmerman, Ann Arbor, Mich., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Dec. 5, 1967, Ser. No. 688,271

Int. Cl. H01q 15/02, 15/16, 19/12 US. Cl. 343840 Claims ABSTRACT OF THE DISCLOSURE In certain applications, for example in direction finding equipment, it is necessary to have an antenna with a high gain over a wide frequency range and at the same time a beamwidth which does not vary appreciably with frequency. It has hitherto been impossible to attain all of these desiderata with a single antenna and in such applications it has been necessary to use several antennas, each designed to cover a portion of the desired wide frequency range or to use a single antenna with reduced gain. The requirement of high gain, of the order of 20 db with respect to an isotropic source, dictates the use of a parabolic reflector. The beamwidths of parabolic antennas, however, vary with frequency, the beamwidth in any given direction being directly proportional to the number of wavelengths across the parabolic reflector in the given direction. Thus, for a given solid parabolic reflector, a doubling of the applied frequency will reduce the beamwidth of 50% assuming that the aperture distribution remains the same. Stated another? way, if it is desired to maintain a constant beamwidth, the size of the parabolic reflector must decrease as the applied frequency goes up.

-In accordance with the present invention, the effective reflecting area of a parabolic reflector is caused to vary inversely with frequency. This is accomplished by providing the reflector with a plurality of concentric circular arrays of holes, the size of the holes of the innermost array being the smallest, with the hole size progressively increasing toward the edges of the reflector. In addition to the more or less symmetrical arrangement of holes in these circular arrays, other randomly arranged holes are placed between and among the holes of the arrays. These randomly arranged holes improve the antenna performance by breaking up the currents which would otherwise flow in circular paths on the parabolic surface between the arrays of holes. The geometry of the arrays of holes is such that at the highest frequency of interest only the hole-free area of the parabola within the innermost hole array is effective in reflecting and focusing the energy from the feed. At the lowest frequency of interest the diameters of the largest of the holes is smaller than the wavelength and hence the reflectivity of the entire parabolic surface is high at such wavelength. At intermediate frequencies, the wavelengths are such that one or more of the outer arrays of large holes will be substantially transparent or non-reflective and one or more of the inner arrays of smaller holes will be reflective and hence the effective reflecting area of the reflector is made frequency dependent in the desired manner.

It is thus an object of this invention to provide an improved parabolic antenna of wide bandwidth and of reduoed beamwidth variation with frequency.

Another object of this invention is to provide a parabolic reflector, the effective reflecting area of which is large at a given frequency and decreases at frequencies higher than said'given frequency.

These and other objects and advantages of the invention will become apparent from the following detailed description and drawing, the sole figure of which is an illustrative antenna embodying the novel concepts of the present invention.

The drawing is a pictorial view looking into the aperture of an antenna comprising a parabolic reflector 7 and an offset feed 3.,The invention is illustrated in connection with an offset feed antenna however the practice of the invention is not limited thereto. Such antennas have advantages well known in the prior art, for example, the offset feed is not in the path of the reflected energy so that there is no aperture blocking by the feed and no reradiation into the feed, which causes standing waves therein. The illustrated antenna was designed for operation over the frequency range of 1 to 10 gigahertz (1000 to 10,000 megacycles). In order to accommodate this frequency range, it was necessary to use a wideband feed. The illustrated wideband feed 3 comprises a ridged horn which is energized or excited by a coaxial line (not shown) which is applied to the connector 5 in the throat of the ridged horn. The position of the ridge within the horn is indicated at 5. Such wideband feeds per se are known and no further discussion thereof is necessary. The feed is located at the focus of the parabolic reflector, in accordance with the usual practice. The reflector 7 in the areas between the holes therein is of reflective material, for example, it may be formed of a solid metal of good reflective characteristics, or formed of fiberglass with metallic paint on the surface thereof, and the holes then drilled to form the perforated surface shown. The point 21 on the surface of the reflector is the center of six concentric arrays of holes, these arrays being numbered 19, 17, 15, 13, 11 and 9 from the innermost to the outermost. As can be seen, the holes of each array are the same size, but the hole size increases with the diameter of the arrays. The feed is arranged so that the center of its radiation pattern strikes the point 21. The diameter of the innermost array is chosen so that the hole-free area within this innermost array has the proper dimensions to achieve the desired beamwidth at the highest frequency of operation. Thus the hole-free area should have a diameter of several times the wavelength at the highest operating frequency. The hole size of the inner array 19 is made approximately equal to the wavelength at the highest operating frequency, and hence this array and all of the other arrays will be substantially transparent or non-reflective at the shortest wavelength. Thus at the highest frequency the effective size of the reflector is equal to the area within the innermost array. The hole diameter of the outermost array is made approximately one half of the wavelength of the lowest frequency of operation and thus the entire reflector has substantial reflectivity at this frequency. As the frequency increases the apparent or effective reflecting area goes down, for example, as the wavelength approaches the hole diameter of the outermost array 9, most of the energy directed to this region will pass through the large holes and will not be reflected or focused in the direction of the antenna aperture. At a still higher frequency, both of the outer arrays 9 and 11 will be transparent to the radiation from the feed 5 and the effective reflecting area will be still further reduced.

An antenna comprising only the systematic, concentric arrays of

holes

19, 17, 15, 13, 11 and 9 was built and tested. The radiation pattern tests on this antenna were unsatisfactory in that high side lobes were produced and the 3 main lobe was poorly shaped. It was concluded that this poor radiation pattern was caused by currents flowing in the circular metallic areas between the circular hole arrays. In order to minimize these loop currents, small, randomly spaced holes were placed on the reflector surface between the concentric arrays and outside of the outermost array. The effect of these randomly spaced holes was to cause the reflector surface currents to add in random fashion in the far field of the antenna. The random distribution of the currents tended to smooth out the undesired lobing structures noted above. The randomly located holes are indicated at 25 and 27in the drawing. Within the concentric arrays of holes the random holes 27 are made smaller in size than the holes of the innermost array 19. Outside of the outer array 9, the random hole size can be made somewhat larger, as indicated by the holes 25. Some of the circular holes in the drawing appear to be elliptical because of the curvature of the reflector. While all of the holes in the illustrative embodiment are circular, circular holes are not necessary to the practice of the invention.

In an antenna constructed according to the present invention, the parabolic focal length F was 13 inches and the aperture dimension D was 48 inches, yielding an F to D ratio of .27. The innermost array 19 was composed of holes 1 inch in diameter, the next array 17 had holes of 1.4 inches diameter; array 15, 1.95 inches in diameter; array 13, 2.7 inches diameter; array 11, 3.8 inches diameter; and outermost array 9 had holes 5.3 inches in diameter. The diameter of the hole-free area around the point 21 was approximately 6 inches and the diameter of the circle connected the centers of the holes of the outer array 9 was approximately 40 inches. This antenna exhibited a beamwidth variation of 2 /2 to 1 over the frequency range of 1 to 10 gigahertz. While this is not a constant beamwidth, it is much better than the variation which would be obtained in the absence of a reflector perforated in this way. A solid reflector antenna similar to that in the drawing was found to have a beamwidth variation of more than 4 to 1 over the same frequency range.

Parabolic reflectors have been constructed in the past either with holes or of metallic mesh, to reduce both the weight and the wind resistance thereof, however in these prior art reflectors the holes or mesh openings have been purposely made small enough relative to the operating wavelengths, so that the entire structure has high reflectivity over the entire frequency range of interest.

While the invention has been described in connection with an illustrative embodiment, obvious modifications thereof are possible without departing from the inventive concepts disclosed herein, accordingly, the invention should be limited only by the scope of the appended claims.

What is claimed is: 1. .A. broadband, high gain antenna. with a relatively constant beamwidth comprising; a parabolic reflector, a broadbanded feed at the focus of said reflector, the surface of said reflector being perforated with a plurality of concentric circular arrays of holes, the size of the holes of the innermost array being the smallest, the size of the holes of each of the other arrays progressively increasing toward the edge of said reflector, said feed having its radiation pattern directed at the center of said arrays, said reflector further comprising a plurality of randomly spaced holes located between and outside of said circular arrays of holes.

2. The antenna of claim 1 wherein the diameter of the holes of said innermost array is approximately equal to the shortest wavelength to be radiated by said antenna and diameter of the holes of the outermost array is approximately one-half of the longest wavelength to be radiated by said antenna, the diameters of the holes of the intermediate arrays being between those of said inner and outer arrays.

3. The antenna of claim 2 in which said broadbanded horn comprises a ridged horn excited by a coaxial line.

4. A broadband, high gain antenna with a relatively constant beamwidth comprising, a parabolic reflector, a broadbanded feed at the focus of said reflector, the surface of said reflector being perforated with a plurality of concentric circular arrays of holes, the diameter of the holes of the innermost array being approximately one inch, the diameter of the holes of the outermost array being approximately 5.3 inches, the diameter of the circle connecting the centers of the holes of said outermost array being approximately 40 inches, the hole diameters of the intermediate arrays being intermediate those of said inner and outer arrays, a hole-free area of approxi mately 6 inches diameter within said innermost array, said reflector being further perforated with a plurality of randomly spaced holes between said circular arrays of holes, and means to apply a microwave signal having a frequency in the range of 1 to 10 gigahertz to said broadbanded feed.

5. The antenna of claim 5 wherein said feed is offset from the aperture of said antenna.

References Cited UNITED STATES PATENTS 2,636,125 4/ 1953 Southworth 343-909 X 2,985,880 5/1961 McMillan 343910 HERMAN KARL SAALBACH, Primary Examiner T. J. VEZEAU, Assistant Examiner US. Cl. X.R. 343-909, 91 2