US 3266044 A
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Aug. 9, 1966 A. D. BRESLER BROAD-BAND ANTENNA FEED Filed April 13, 1964 FIG./
INVENTOR, AARON D. BRESLER.
United States Patent 3,266,044 BROAD-BAND ANTENNA FEED Aaren D. Bresler, Merrick, N.Y., assignor to the United States of America as represented by the Secretary of the Army Filed Apr. 13, 1964, Ser. No. 359,529 15 Claims. (Cl. 343-7925) The present invention relates to a broad-band antenna array and more particularly to a broad-band, four-element antenna feed for a lens or reflector.
In the field of monopulse radar the sum and difference patterns formed by an antenna array are used to determine the angular direction of received pulses. The particular radar is calibrated and a graph is plotted showing the relationship between the angle and the ratio of the amplitudes of the signals received in the sum and the difference channels. In order to develop an array which will give a sum and two difference patterns, i.e., azimuth and elevation difference, an array of four antenna elements and a complex hybrid circuit feeding the antennas are required. Difficulty has been experienced in past systems with radiation from individual antenna elements causing undesirable effects on the other elements of the array and the associated circuitry. The present invention has obviated these diffieulties to some degree by providing a hollow ground plane in the form of a Wedge wherein the associated circuitry may be housed and on which the antenna elements may be mounted. This ground plane will shield the circuitry from the radiation field and will shield from each other the pairs of antenna elements mounted on opposite sides thereof. Also, because of the ground plane, monopole antenna elements may be used and the antenna feed may be relatively broad-band since the need for a balun is eliminated.
It is therefore the object of this invention to provide an improved broad-band, four element antenna feed.
Another object of the invention is to provide a broadband, four element antenna feed which does not require a balun.
Still another object of the invention is to provide a broad-band, four element antenna feed which is relatively free of unwanted coupling effects between antenna elements.
Yet another object of this invention is to provide a broad-band antenna having a ground plane wherein the associated circuitry may be housed to provide shielding from the radiation field.
The exact nature of this invention as well as other ob jects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawing which shows a preferred embodiment of the invention, and wherein- FIG, 1 shows an isometric view of the antenna;
FIG. 2 shows a side view of the antenna; and
FIG. 3 shows a top view of the antenna.
Referring to the drawing wherein the same reference numerals represent like parts throughout, there is shown a hollow metallic wedge having top and bottom walls 11 and 12 respectively, side walls 13 and 14, and rectangular front wall 15. Walls 11 and 12 are angularly disposed with respect to each other. Walls 13 and 14 are also augularly disposed with respect to each other. Wedge 10 is therefore made up of a rectangular wall 15 having two pairs of diverging surfaces extending from the sides thereof to form a truncated pyramid having a vertex axis 16. The wedge 10 is symmetrical about axis 16 which passes through the center of Wall 15 and is perpendicular thereto.
Mounted on wedge 10 are four monopole log-periodic antenna elements 18, 19, and 21. Each antenna element comprises a plurality of spaced radiating arms 22 3,25%,M4 Patented August 9, 1966 mounted on a center conductor 24 and increasing in length from one end of the conductor 24 to the other in a well known fashion to produce a monopole, log-periodic antenna. Antennas 18 and 19 are mounted on wall 11 with arms 22 extending away from and perpendicular to wall 11, while antennas 20 and 21 are mounted on bottom wall 12 with arms 22 extending away from and perpendicular to wall 12. Each antenna element also comprises a plurality of spaced loading arms 23 extending from conductor 24 and parallel to walls 11 and 12. The loading arms 23 are conventional impedance devices. Arms 23 increase in length as do arms 22 and each arm is mounted on conductor 24 at a point between arms 22. These loading arms 23 complement each other on each side and extend away from the arms on its companion antenna. In fact all four elements 18, 19, 20 and 21 are symmetrically mounted about axis 16, and along with the wedge 10 the entire device is substantially a four-way mirror image about two orthogonal planes which intersect along vertex axis 16. One plane may be considered to be parallel to the plane of the paper as viewed in FIG. 2 while the other plane would be parallel to the paper as viewed in FIG. 3.
The manner in which the elements 18, 19, 20 and 21 are mounted on wedge 11 is dictated by the requirements one must fulfill to produce the desired sum and difference patterns. It is important in monopulse radar that the crossover level of the sum and difference patterns be frequency independent. This is necessary to maintain the calibration of the device over the entire bandwidth. It is well established that to produce a frequency independent crossover level the phase center should be displaced from the focal axis by an amount directly proportional to the wavelength at the operating frequency. In this connection it has been determined, for example, that to obtain a 3 db crossover level with a paraboloid whose f/D ratio is 0.425, the phase center should be displaced slightly less than 3 from the focal axis.
Therefore, let us first assume that the wedge 10 is mounted such that axis 16 is coincident with the focal axis of a radiation focusing means such as parabolic reflector 30. Also, let it further be assumed that the focal point of the illuminated device 30 is located on the axis 16 at a point 1 somewhere inside the wedge. This will define the location of a focal plane P which passes through point f and is perpendicular to axis 16. The phase center of a monopole log-periodic antenna, fed against a ground plane, will move substantially along conductor 24 a distance directly proportional to the wavelength as the frequency changes. For higher frequencies the shorter arms will radiate while the phase center will be approximately at the base of the radiating arms. At lower frequencies one of the longer arms will radiate depending on the particular frequency.
In order to fulfill the above-mentioned requirements and to obtain the least amount of loss in gain the antenna elements may be mounted on the wedge 10 in the manner shown in the dnawing. The elements 18, 19, 2t) and 21 extend along the wedge 10 passing through plane P at points A, B, C, and D respectively. The particular arm 22 I011 each antenna element which radiates at the midbland frequency of the device should be placed at the points A, B, C, and D. All four antenna elements may then be placed at an angle with respect to the focal axis such that the center axis of conductors 24 all intersect approximately at some point on the axis 16 land are symmetrically mounted labout axis .16. The location of the intersection point and the slope' of Walls 11 and 12 will be dictated by the proportional amount one wishes the phase center to be displaced with respect to the focal axis. It may be desirable in some cases to have the crossover level for the azimuth difference to be of a different value than for the sum difference, in which case the angle between wall 11 and wall 12 would be substantially different than the angle formed between the elements 18 and 19 or elements 20 and 21. However, the crossover level for the two difference patterns will be substantially equal for the device as shown in the drawing.
In operation, the sum pattern of all four antenna elements 18, 19, 26 and 21 is produced by feeding all elements in phase. In this mode the resulting phase center for all four elements will be on axis 16 at a location dependent on the frequency. For the mid-band frequency the phase center for all four elements will be located at focal point For higher frequencies the phase center for all elements will move along axis 16 toward wall and for lower frequencies the phase center will move away from wall 15 along axis 16.
To produce an elevation difference pattern the antenna elements 18 and 19 are fed in phase with each other, and antenna elements 20 and 21 are fed in phase with each other and 180 rout of phase with elements 18 and .19. The phase center of elements 18 and 19 in this case will be approximately on wall 11 and midway between the particular radiating arms 22 on elements 18 and 19. For example, at mid-band the phase center of elements 18 and 19, when fed in phase, will be midway between points A and B. At midband the phase center for elements 20 and 21 will be mid-way between points C and D. It will be obvious that as the frequency changes these phase centers will move along walls 11 and 12. The walls 11 and 12 should be inclined such that the perpendicular distance from this phase center to axis 16 is directly proportional to the wavelength depending on the desired crossover level.
The azimuth difference pattern will be produced when elements 18 and 20 are fed in phase with each other, and elements 19 and 21 are fed in phase with each other and fed 180 rout of phase with elements 18 and 20. In this case the phase center of elements 18 and 20 will be midway between these elements. For the mid-hand frequency the phase center will be midway between points A and C for elements 18 and 29, and midway between points Band D for elements 19 and 21. Therefore, the elements 18, 19, 20 and 21 should be mounted such that the phase centers produced by these difference patterns will move laterally with respect to axis 16 in a manner which is directly proportional to the wavelength depending on the desired crossover level.
The antenna array may be used for both transmitting and receiving. When used as a means for detecting the location of other transmitters the antenna need only be coupled to an appropriate receiver system. In such a system a hybrid coupling means would be connected to the elements 18, 19, 2t) and 21 to provide three channels.
The azimuth difference channel would include phase shifting means which would produce a 180 delay between the signals received by antenna elements 18 and 20 with respect to the signal received by antenna elements 19 and 21 or vice versa. A second channel would be used for elevation difference where a 180 delay would be provided for signals received by elements 18 and 19 with respect to signals received by elements 20 and 21. No delay would be provided for a third channel which would be used for the sum channel.
The hybrid circuitry and any other equipment which may be affected by radiation can be conveniently housed in the hollow wedge which, when completely closed on all sides, will act as a shield. The hybrid circuitry may include coaxial lines for coupling the receiver to the antenna. The coaxial line is highly desirable because of its broad band characteristics. A balun would not be necessary to couple these lines to the antenna. The outer conductors of the coaxial lines would be connected directly to the ground plane while the center conductor would pass through the ground plane and be connected directly to 4 the apex of the elements 18, 19, 20 and 21. The ground plane or wedge 10 Will also shield from each other the antenna elements mounted on opposite sides of the wedge.
There will be slight loss in gain due to the defocusing of the antenna when the phase center moves out of the focal plane P. This, however, is not very large and does not create any problem. Of course, in some applications theantenna may be moved axially when the operating frequency is changed to keep the phase center at the focal plane.
It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.
What is claimed is:
1. An antenna comprising two inclined metallic walls, first and second radiation means each mounted on one of said metallic walls, said radiation means being capable of radiating over a broad-band of frequencies, the phase centers of each said radiation means being always in a common plane, said common plane being perpendicular to each of said walls, and an axis being in said common plane bisecting the angle formed by said inclined walls, the displacement of each said phase centers from said axis being directly proportional to the wavelength of the radiation at all frequencies in said broad-band.
2. The antenna according to claim 1 and further including metallic surfaces joining said inclined metallic walls to form a hollow wedge-shaped compartment.
3. The antenna according to claim 1 and wherein said metallic walls are joined by three metallic surfaces to form a hollow truncated pyramid.
4. A broad-band monopulse feed for a radiation focusing means comprising a metallic ground plane in the form of a truncated pyramid having a vertex axis, an array of four broadband monopole antenna elements mounted on said ground plane and being symmetrically arranged about said axis, the displacement of the phase center of each said antenna element from said axis being proportional to the wavelength at all frequencies in said broad-band.
5. The monopulse feed according to claim 4 wherein said vertex axis and the focal axis of said radiation focusing means are substantially coincident.
6. The monopulse feed according to claim 5 and wherein each said radiation element comprises a monopole logperiodic antenna.
7. An antenna array for operation over a broad-band of frequencies comprising a first monopole log-periodic antenna element having a center conductor and a plurality of spaced radiating members extending perpendicular therefrom, and lying in a common plane, said center conductor being mounted at an angle with respect to first and second orthogonal planes, a first metallic ground plane mounted perpendicular to said radiating members and perpendicular to said first orthogonal plane, a second antenna element being mounted on said first ground plane on the opposite side of said first orthogonal plane and forming a mirror image of said first antenna element, and third and fourth antenna elements and a second metallic ground plane mounted on the opposite side of said second orthogonal plane and forming a mirror image of said first and second antenna elements and said first ground plane respectively.
8. The antenna array according to claim 7 and wherein said angles formed by said center conductor and said first and second orthogonal planes are equal.
9. The antenna according to claim 7 and wherein the phase center of each said antenna element is located approximately on said center conductor at a point dependent upon the particular frequency of radiation.
10. The antenna according to claim 9 and wherein the displacements of said phase center of each of said antenna elements from said first and said second orthogonal planes are directly proportional to the Wavelength of the radiation at all frequencies in said broad-band.
11. The antenna according to claim 10 and wherein said displacements of said phase center of each said antenna elements from said first and second orthogonal planes are all equal.
12. The antenna according to claim 10 and further including a radiation focusing means having a focal axis with a focal point located thereon, the axis formed by the intersection of said first and second orthogonal planes being substantially coincident with said focal axis.
13. The antenna according to claim 12 and wherein said phase center of each said antenna element at the rnid-band g frequency and said focal point are all located in a common plane being perpendicular to said first and second orthogonal planes.
14. The antenna according to claim 13 and wherein said first and said second metallic ground planes are joined by metallic surfaces to form a hollow compartment.
15. The antenna according to claim 14 and wherein said metallic ground planes and said metallic surfaces form a hollow truncated pyramid completely closed on all sides.
No references cited.
HERMAN KARL SAALBACH, Primary Examiner.
R. F. HUNT, Assistant Examiner.