Title:
Single frequency, two feed dish antenna having switchable beamwidth
United States Patent 3927408
Abstract:
A switchable beamwidth antenna includes a concave parabolic main reflecting dish which has a central circular region and a surrounding coaxial annular region. A feed means selectively excites only the central region of the main dish via a truncated subreflector for wide beamwidth or substantially the entire main dish for narrow beamwidth. In the embodiment shown, the feed means comprises a truncated concave ellipsoid subreflector and separate feed terminations located at two foci of the ellipsoid. One feed termination directly views all of the main dish while the other feed termination, exciting the main dish via the subreflector, excites only the central region because of the subreflector truncation.
US Patent References:
Multiple reflector multiple frequency band antenna system
Williams et al. - July 1968 - 3394378
PARABOLIC REFLECTOR WITH DUAL CROSS-POLARIZED FEEDS OF DIFFERENT FREQUENCIES
Holtum, Jr. - April 1969 - 3438041
Multiple reflector multiple frequency band antenna system
Williams et al. - July 1968 - 3394378
PARABOLIC REFLECTOR WITH DUAL CROSS-POLARIZED FEEDS OF DIFFERENT FREQUENCIES
Holtum, Jr. - April 1969 - 3438041
Application Number:
05/512825
Publication Date:
12/16/1975
Filing Date:
10/04/1974
View Patent Images:
Export Citation:
Assignee:
The United States of America as represented by the United States Counsel-Code GP (Washington, DC)
Primary Class:
Other Classes:
343/876, 343/837
International Classes:
H01Q3/24; H01Q25/00; H01Q19/12
Field of Search:
343/779,781,837,840
Primary Examiner:
Gensler, Paul L.
Attorney, Agent or Firm:
Kempf, Robert Sandler Ronald Manning John F. F. R.
Claims:
What is claimed is
1. A switchable beamwidth antenna having a wide beamwidth state and a narrow beamwidth state at a single predetermined frequency, said beamwidth state being responsive to a beamwidth selection command source comprising:
2. An electronically selectable beamwidth antenna comprising:
1. A switchable beamwidth antenna having a wide beamwidth state and a narrow beamwidth state at a single predetermined frequency, said beamwidth state being responsive to a beamwidth selection command source comprising:
2. An electronically selectable beamwidth antenna comprising:
Description:
FIELD OF THE INVENTION
The invention relates generally to switchable beamwidth or zoomable antennas and more particularly to a switchable beamwidth antenna employing a common main reflecting dish operable in at least two different beamwidth modes.
ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
The need frequently arises to augment narrow beamwidth (narrow field of view) transmitting and/or receiving antennas with a wide beamwidth "acquisition" mode. Since it is well known that antennas are reciprocal, having the same characteristics for transmitting as for receiving, the meaning of acquisition shall be detailed with respect to a receiving antenna with the understanding that acquisition for narrow beamwidth transmitting antennas and for narrow beamwidth transmitting/receiving antennas such as a radar, is substantially similar.
In the case of a narrow beamwidth (also referred to as high antenna gain) receiving antenna there is great difficulty in pointing the antenna's narrow field of view in the direction of a transmitting station which must be done in order for the antenna to receive. If, at the sacrifice of antenna gain or efficiency, the antenna is initially switched to a wide field of view (wide beamwidth), the antenna is more easily pointed to subtend the transmitting station in the wide field of view. Then, an indication of pointing error may be derived by simultaneous lobing techniques, for example, to more precisely point the antenna. Once the antenna is pointed so that the transmitting station would be in its narrow field of view, "acquisition" is said to have occurred and the antenna may be switched to its narrow beamwidth mode to take advantage of greater antenna gain or efficiency. This narrow beamwidth may then be maintained, subtending the transmitting station (or "tracking") by simultaneous lobing techniques. Similar acquisition may be done to point a radar antenna at a target or a transmitting antenna at a receiving station.
The acquisition problem is particularly acute for narrow beamwidth antennas having large main reflector dishes of the type considered by the National Aeronautics and Space Administration for Tracking and Data Relay Satellites to relay to earth the data collected from orbiting earth observation satellites. These antennas, operating at 15 Gigahertz, would have a main dish on the order of 12.5 feet in diameter with a consequent narrow beamwidth of only 0.3 degrees. Initial pointing of the narrow beamwidth antenna of the Data Relay Satellite toward an Earth Observation Satellite would be quite difficult to achieve because of significant relative motion between these satellites. Thus, a means for increasing the beamwidth of the antenna to effect acquisition is required.
Numerous techniques were considered and found to be unsatisfactory.
In one technique either a feed or a subreflector is axially shifted in position to defocus the antenna. This technique is not acceptable because, though the beamwidth is generally widened, the antenna pattern amplitude and phase characteristics are distorted. In another technique, a polarization sensitive grating is placed in front of the main dish to serve as a smaller main dish for a wide beamwidth mode. This grating, though smaller than the dish, is sufficiently forward to intercept all radiation coming from a feed. The grating passes, for example, vertically polarized radiation to the main dish, producing a narrow beamwidth but reflects horizontal polarization producing wider beamwidth. Thus, beamwidth can be switched by switching feed polarization. This technique suffers from restrictions on feed polarization; in particular, it does not permit the use of circular polarization which has both horizontal and vertical polarization components.
Another technique for increasing the beamwidth of the antenna is to change the frequency of operation. Since beamwidth is inversely proportional to the area of the main dish measured in wavelengths, the beamwidth may be decreased by decreasing frequency (increasing wavelength). This is an undesirable antenna system complication for satellite users which may also necessitate additional antenna feeds and consequent increased blockage of the satellite main dish, which causes a decreased antenna gain or efficiency of the antenna. Moreover, it is desirable to interface with existing user single frequency equipment.
Still another technique for increasing beamwidth is to provide two feeds at one feed point with one feed exciting the entire main reflector for narrow beamwidth and a second feed exciting a smaller region of the main reflector for wide beamwidth. There are many difficulties with this approach the chief one being that if the second feed is to be sufficiently directive to excite only a portion of the main reflector, it would have to be geometrically large; such a large feed would significantly increase blockage of the main dish reflector decreasing antenna gain or efficiency. Furthermore, there are obvious difficulties in placing two feeds at the same point; the second feed must be displaced from the antenna axis if the first feed is located on the axis. It is desirable to have the capability of positioning the feed or feeds in both wide and narrow beamwidth modes on the axis of the antenna.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a new and improved switchable beamwidth antenna in which beamwidth switching is independent of feed polarization, feed directivity, or feed frequency.
It is a further object of the present invention to provide a new and improved switchable beamwidth antenna employing a main reflecting dish and feed therefor with a narrow beamwidth mode and a wide beamwidth acquisition mode wherein the feed or feeds in both modes can excite the main reflecting dish from the antenna axis.
It is yet a further object of the present invention to provide a new and improved switchable beamwidth antenna allowing flexibility in feed design, for example, permitting the use of electronically or mechanically scanned feed arrays for scanning the viewing direction of the antenna.
SUMMARY OF THE INVENTION
The present invention includes a switchable beamwidth antenna having a main reflecting concave parabolic dish and feed means effectively at a focal point on the dish boresight axis. Since the beamwidth of an antenna of this type is inversely proportional to the main dish area, the beamwidth can be increased by operatively using only a portion of the main dish. For wide beamwidth operation, the feed means excites only a central circular region of the main dish via a truncated subreflector while for the narrow beamwidth mode the entire dish is excited by the feed.
The invention has one main embodiment.
In this embodiment, electrical beam switching is attained by providing a Gregorian configuration wherein a truncated concave ellipsoid subreflector has a major axis located on the boresight axis of the parabolic dish. The ellipsoid subreflector has two foci in front of the subreflector, whereby the subreflector focus nearest the subreflector is coincident with the focus of the main dish. A separate feed is provided at each subreflector focus and may be selectively activated to provide beam switching. Another method of beam switching or zooming using a single feed is disclosed in U.S. Pat. No. 3,866,233. A first feed, located at the main dish focal point, faces the main dish and excites its entire surface. The second feed, located at the focus of the ellipsoid furthest from the subreflector, faces the subreflector and excites the main dish via the subreflector. The ellipsoid has the characteristic that a real image of the second feed is formed at its nearer focus whereby, to the main dish, the second feed also appears to be at the focus of the main dish. Because of the effective truncation or reduction in size of the ellipsoid subreflector, the second feed excites only a central region of the main dish via the subreflector.
The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially where taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic drawing in cross-section of the embodiment of the switchable beamwidth antenna having a main dish and a truncated subreflector, wherein the antenna is shown in the narrow beamwidth mode;
FIG. 2 is a schematic illustration of the embodiment of FIG. 1 in the wide beamwidth mode;
FIG. 3 is a design graph indicating the antenna beamwidth for the embodiment of the switchable beamwidth antenna in the wide beamwidth mode of FIG. 2, versus the subreflector diameter (or degree of truncation); and
FIG. 4 is a schematic drawing in front view of a multi-frequency feed.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 and 2 are cross-sectional illustrations of an embodiment 49 of the switchable beamwidth antenna of the invention wherein two feeds 51 and 53 and a truncated or reduced size concave ellipsoid subreflector 55, located on the antenna boresight axis 20, cooperate with the main reflecting concave paraboloidal dish 15 in a Gregorian configuration. Feed 53 is located at the main dish focal point 27 facing the main dish. In FIG. 1, source 21 supplies the feed 53 with radio frequency or microwave radiation via conduit 23 and microwave switch 57 in response to a narrow beamwidth command issued to switch 57. Feed 53 directly illuminates the main dish 15 with a cone of radiation having the included angle θ 2 , bounded by rays 59a and 61a, which optimally just illuminates the entire main dish. Upon reflection of this cone by main dish 15, a collimated output beam, bounded by rays 59b and 61b, is produced having an initial large radius, equal to the dish radius, and a corresponding narrow beamwidth given by diffraction theory.
In FIG. 2, feed 51, facing the ellipsoidal subreflector 55, illuminates the main dish via the subreflector to produce a wide beamwidth. The ellipsoidal subreflector has a major axis that is coincident with boresight axis 20, and on which lie a near focus 63 and a far focus 65 that are positioned between the concave sides of the subreflector and main dish 15. The subreflector 55 is positioned with its near focus 63 coincident with the main dish focal point 27, and the feed 51 is positioned at the subreflector far focus 65. Microwave source 21 supplies energy to feed 51 via conduit 23 and switch 57 in response to a wide beamwidth command issued to the switch 57. The feed 51 illuminates the subreflector with a cone of radiation having a small included angle, θ 5 , which optimally just subtends the subreflector. This cone of radiation is bounded by rays 67a and 69a . Due to the concavity of the subreflector, the radiation reflected therefrom, defined by rays 67b and 69b, goes through a focus at the subreflector near focal point 63. Therefore, a cone of radiation with included angle θ 4 aimed at the main reflector appears to be initiated from the focal point 63, providing that the feed 53 at focus 63 does not excessively obstruct the passage of rays through point 63. Either mechanical or electrical means of achieving this are admissible. In particular, it was found experimentally that a 4-wavelength displacement of the feed toward the ellipsoidal subreflector was adequate. This cone of radiation strikes the dish 15 at a reduced radius. A reduced radius collimated beam bounded by rays 69c and 67c is produced because the subreflector is truncated or of too small a size for the feed to illuminate the outer annular region 18 of the main dish. To illuminate the entire main reflector from feed 51 via subreflector 55, the subreflector would have to be larger by an annular region 71, shown in FIG. 1. Thus, in this embodiment, beamwidth switching is accomplished by electrically switching microwave excitation from source 21 between feed 53 for wide beamwidth and feed 51 for narrow beamwidth.
Practical dimensions for the inventive embodiment have been determined in conjunction with a five foot focal length, 12.5 foot diameter main parabolic dish 15 for 15 Gigahertz (0.0656 foot wavelength) operation. The narrow beamwidth of such a dish without subreflector truncation is approximately 0.30°.
For this embodiment, the near focus 63 of ellipsoid subreflector 55 is 1.25 feet in front of the subreflector, and its further focus 65 is 5.00 feet in front of the subreflector. If the subreflector were not truncated, it would be 2.8 feet in diameter. Truncation of the feed to approximately two feet in diameter yields a 50% increase in beamwidth.
FIG. 3 is a design curve for the embodiment with the beamwidth in degrees as ordinate and the diameter of the subreflector 55 as abscissa. As can be seen, a smooth curve is obtained which can yield a multiplication of the beamwidth by a factor up to three as the subreflector is truncated. This Gregorian embodiment 49 is particularly attractive because of the instantaneous and reliable nature of the electronic switching of microwave switch 57.
As mentioned earlier, geometric optic principles are approximate in nature and diffraction theory must be resorted to in order to fully describe the invention. The numerical embodiments of the invention were verified using a computer simulation which accounted for the diffraction effects between the subreflector and the main dish and for the diffraction effects at the main dish. The subreflectors were divided up into plural square regions, each having sides of 0.3 wavelengths, and the main dish was also divided into plural square regions having sides of 2 wavelengths. The square regions were considered to be coherence areas. The radiation from a feed with assumed radiation patterns was collected in each square region on the subreflector and a resultant source obtained for each square. The radiation from each of these sources was collected in each square of the main reflector and a second set of sources on the main reflector was thereby obtained. The pattern of far field radiation from these second sources was then calculated to obtain the far field pattern of the antenna with various sized subreflectors. These patterns verified that the design curve of FIG. 3 is indeed a smooth curve.
FIG. 4 shows a nested multifrequency feed 111 adapted to be used with the invention. Four large S band horns 113 abut to define a square four feed array centered about axis 20. Nested at the interior corners of the horns 113 are four smaller X-band horns 115 which abut to form a smaller square array centered about axis 20. The antenna of the invention is adaptable to multifrequency operation by using such a nested feed. Also, the square four feed array is adaptable to either amplitude or phase simultaneous lobing or monopulse techniques for determining antenna pointing errors. For simultaneous lobing, it is important that the phase and amplitude of signals received at each of the four horns be equal when the transmitter of the signal is located on axis 20. It is further important that these parameters smoothly vary as the transmitting source departs from the antenna axis by an angle. Computer simulation has verified that these parameters vary smoothly for source angles ranging from zero to well in excess of half the beamwidth.
Having described an embodiment of my invention it is clear that numerous modifications of this embodiment are possible within the invention's spirit and scope. Of particular import is the fact that antennas are reciprocal devices useful for transmitting, receiving, or both. It is intended that the invention not be limited except by the following claims which are generic to transmitting and/or receiving antennas.
The invention relates generally to switchable beamwidth or zoomable antennas and more particularly to a switchable beamwidth antenna employing a common main reflecting dish operable in at least two different beamwidth modes.
ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
The need frequently arises to augment narrow beamwidth (narrow field of view) transmitting and/or receiving antennas with a wide beamwidth "acquisition" mode. Since it is well known that antennas are reciprocal, having the same characteristics for transmitting as for receiving, the meaning of acquisition shall be detailed with respect to a receiving antenna with the understanding that acquisition for narrow beamwidth transmitting antennas and for narrow beamwidth transmitting/receiving antennas such as a radar, is substantially similar.
In the case of a narrow beamwidth (also referred to as high antenna gain) receiving antenna there is great difficulty in pointing the antenna's narrow field of view in the direction of a transmitting station which must be done in order for the antenna to receive. If, at the sacrifice of antenna gain or efficiency, the antenna is initially switched to a wide field of view (wide beamwidth), the antenna is more easily pointed to subtend the transmitting station in the wide field of view. Then, an indication of pointing error may be derived by simultaneous lobing techniques, for example, to more precisely point the antenna. Once the antenna is pointed so that the transmitting station would be in its narrow field of view, "acquisition" is said to have occurred and the antenna may be switched to its narrow beamwidth mode to take advantage of greater antenna gain or efficiency. This narrow beamwidth may then be maintained, subtending the transmitting station (or "tracking") by simultaneous lobing techniques. Similar acquisition may be done to point a radar antenna at a target or a transmitting antenna at a receiving station.
The acquisition problem is particularly acute for narrow beamwidth antennas having large main reflector dishes of the type considered by the National Aeronautics and Space Administration for Tracking and Data Relay Satellites to relay to earth the data collected from orbiting earth observation satellites. These antennas, operating at 15 Gigahertz, would have a main dish on the order of 12.5 feet in diameter with a consequent narrow beamwidth of only 0.3 degrees. Initial pointing of the narrow beamwidth antenna of the Data Relay Satellite toward an Earth Observation Satellite would be quite difficult to achieve because of significant relative motion between these satellites. Thus, a means for increasing the beamwidth of the antenna to effect acquisition is required.
Numerous techniques were considered and found to be unsatisfactory.
In one technique either a feed or a subreflector is axially shifted in position to defocus the antenna. This technique is not acceptable because, though the beamwidth is generally widened, the antenna pattern amplitude and phase characteristics are distorted. In another technique, a polarization sensitive grating is placed in front of the main dish to serve as a smaller main dish for a wide beamwidth mode. This grating, though smaller than the dish, is sufficiently forward to intercept all radiation coming from a feed. The grating passes, for example, vertically polarized radiation to the main dish, producing a narrow beamwidth but reflects horizontal polarization producing wider beamwidth. Thus, beamwidth can be switched by switching feed polarization. This technique suffers from restrictions on feed polarization; in particular, it does not permit the use of circular polarization which has both horizontal and vertical polarization components.
Another technique for increasing the beamwidth of the antenna is to change the frequency of operation. Since beamwidth is inversely proportional to the area of the main dish measured in wavelengths, the beamwidth may be decreased by decreasing frequency (increasing wavelength). This is an undesirable antenna system complication for satellite users which may also necessitate additional antenna feeds and consequent increased blockage of the satellite main dish, which causes a decreased antenna gain or efficiency of the antenna. Moreover, it is desirable to interface with existing user single frequency equipment.
Still another technique for increasing beamwidth is to provide two feeds at one feed point with one feed exciting the entire main reflector for narrow beamwidth and a second feed exciting a smaller region of the main reflector for wide beamwidth. There are many difficulties with this approach the chief one being that if the second feed is to be sufficiently directive to excite only a portion of the main reflector, it would have to be geometrically large; such a large feed would significantly increase blockage of the main dish reflector decreasing antenna gain or efficiency. Furthermore, there are obvious difficulties in placing two feeds at the same point; the second feed must be displaced from the antenna axis if the first feed is located on the axis. It is desirable to have the capability of positioning the feed or feeds in both wide and narrow beamwidth modes on the axis of the antenna.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a new and improved switchable beamwidth antenna in which beamwidth switching is independent of feed polarization, feed directivity, or feed frequency.
It is a further object of the present invention to provide a new and improved switchable beamwidth antenna employing a main reflecting dish and feed therefor with a narrow beamwidth mode and a wide beamwidth acquisition mode wherein the feed or feeds in both modes can excite the main reflecting dish from the antenna axis.
It is yet a further object of the present invention to provide a new and improved switchable beamwidth antenna allowing flexibility in feed design, for example, permitting the use of electronically or mechanically scanned feed arrays for scanning the viewing direction of the antenna.
SUMMARY OF THE INVENTION
The present invention includes a switchable beamwidth antenna having a main reflecting concave parabolic dish and feed means effectively at a focal point on the dish boresight axis. Since the beamwidth of an antenna of this type is inversely proportional to the main dish area, the beamwidth can be increased by operatively using only a portion of the main dish. For wide beamwidth operation, the feed means excites only a central circular region of the main dish via a truncated subreflector while for the narrow beamwidth mode the entire dish is excited by the feed.
The invention has one main embodiment.
In this embodiment, electrical beam switching is attained by providing a Gregorian configuration wherein a truncated concave ellipsoid subreflector has a major axis located on the boresight axis of the parabolic dish. The ellipsoid subreflector has two foci in front of the subreflector, whereby the subreflector focus nearest the subreflector is coincident with the focus of the main dish. A separate feed is provided at each subreflector focus and may be selectively activated to provide beam switching. Another method of beam switching or zooming using a single feed is disclosed in U.S. Pat. No. 3,866,233. A first feed, located at the main dish focal point, faces the main dish and excites its entire surface. The second feed, located at the focus of the ellipsoid furthest from the subreflector, faces the subreflector and excites the main dish via the subreflector. The ellipsoid has the characteristic that a real image of the second feed is formed at its nearer focus whereby, to the main dish, the second feed also appears to be at the focus of the main dish. Because of the effective truncation or reduction in size of the ellipsoid subreflector, the second feed excites only a central region of the main dish via the subreflector.
The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially where taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic drawing in cross-section of the embodiment of the switchable beamwidth antenna having a main dish and a truncated subreflector, wherein the antenna is shown in the narrow beamwidth mode;
FIG. 2 is a schematic illustration of the embodiment of FIG. 1 in the wide beamwidth mode;
FIG. 3 is a design graph indicating the antenna beamwidth for the embodiment of the switchable beamwidth antenna in the wide beamwidth mode of FIG. 2, versus the subreflector diameter (or degree of truncation); and
FIG. 4 is a schematic drawing in front view of a multi-frequency feed.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 and 2 are cross-sectional illustrations of an embodiment 49 of the switchable beamwidth antenna of the invention wherein two feeds 51 and 53 and a truncated or reduced size concave ellipsoid subreflector 55, located on the antenna boresight axis 20, cooperate with the main reflecting concave paraboloidal dish 15 in a Gregorian configuration. Feed 53 is located at the main dish focal point 27 facing the main dish. In FIG. 1, source 21 supplies the feed 53 with radio frequency or microwave radiation via conduit 23 and microwave switch 57 in response to a narrow beamwidth command issued to switch 57. Feed 53 directly illuminates the main dish 15 with a cone of radiation having the included angle θ 2 , bounded by rays 59a and 61a, which optimally just illuminates the entire main dish. Upon reflection of this cone by main dish 15, a collimated output beam, bounded by rays 59b and 61b, is produced having an initial large radius, equal to the dish radius, and a corresponding narrow beamwidth given by diffraction theory.
In FIG. 2, feed 51, facing the ellipsoidal subreflector 55, illuminates the main dish via the subreflector to produce a wide beamwidth. The ellipsoidal subreflector has a major axis that is coincident with boresight axis 20, and on which lie a near focus 63 and a far focus 65 that are positioned between the concave sides of the subreflector and main dish 15. The subreflector 55 is positioned with its near focus 63 coincident with the main dish focal point 27, and the feed 51 is positioned at the subreflector far focus 65. Microwave source 21 supplies energy to feed 51 via conduit 23 and switch 57 in response to a wide beamwidth command issued to the switch 57. The feed 51 illuminates the subreflector with a cone of radiation having a small included angle, θ 5 , which optimally just subtends the subreflector. This cone of radiation is bounded by rays 67a and 69a . Due to the concavity of the subreflector, the radiation reflected therefrom, defined by rays 67b and 69b, goes through a focus at the subreflector near focal point 63. Therefore, a cone of radiation with included angle θ 4 aimed at the main reflector appears to be initiated from the focal point 63, providing that the feed 53 at focus 63 does not excessively obstruct the passage of rays through point 63. Either mechanical or electrical means of achieving this are admissible. In particular, it was found experimentally that a 4-wavelength displacement of the feed toward the ellipsoidal subreflector was adequate. This cone of radiation strikes the dish 15 at a reduced radius. A reduced radius collimated beam bounded by rays 69c and 67c is produced because the subreflector is truncated or of too small a size for the feed to illuminate the outer annular region 18 of the main dish. To illuminate the entire main reflector from feed 51 via subreflector 55, the subreflector would have to be larger by an annular region 71, shown in FIG. 1. Thus, in this embodiment, beamwidth switching is accomplished by electrically switching microwave excitation from source 21 between feed 53 for wide beamwidth and feed 51 for narrow beamwidth.
Practical dimensions for the inventive embodiment have been determined in conjunction with a five foot focal length, 12.5 foot diameter main parabolic dish 15 for 15 Gigahertz (0.0656 foot wavelength) operation. The narrow beamwidth of such a dish without subreflector truncation is approximately 0.30°.
For this embodiment, the near focus 63 of ellipsoid subreflector 55 is 1.25 feet in front of the subreflector, and its further focus 65 is 5.00 feet in front of the subreflector. If the subreflector were not truncated, it would be 2.8 feet in diameter. Truncation of the feed to approximately two feet in diameter yields a 50% increase in beamwidth.
FIG. 3 is a design curve for the embodiment with the beamwidth in degrees as ordinate and the diameter of the subreflector 55 as abscissa. As can be seen, a smooth curve is obtained which can yield a multiplication of the beamwidth by a factor up to three as the subreflector is truncated. This Gregorian embodiment 49 is particularly attractive because of the instantaneous and reliable nature of the electronic switching of microwave switch 57.
As mentioned earlier, geometric optic principles are approximate in nature and diffraction theory must be resorted to in order to fully describe the invention. The numerical embodiments of the invention were verified using a computer simulation which accounted for the diffraction effects between the subreflector and the main dish and for the diffraction effects at the main dish. The subreflectors were divided up into plural square regions, each having sides of 0.3 wavelengths, and the main dish was also divided into plural square regions having sides of 2 wavelengths. The square regions were considered to be coherence areas. The radiation from a feed with assumed radiation patterns was collected in each square region on the subreflector and a resultant source obtained for each square. The radiation from each of these sources was collected in each square of the main reflector and a second set of sources on the main reflector was thereby obtained. The pattern of far field radiation from these second sources was then calculated to obtain the far field pattern of the antenna with various sized subreflectors. These patterns verified that the design curve of FIG. 3 is indeed a smooth curve.
FIG. 4 shows a nested multifrequency feed 111 adapted to be used with the invention. Four large S band horns 113 abut to define a square four feed array centered about axis 20. Nested at the interior corners of the horns 113 are four smaller X-band horns 115 which abut to form a smaller square array centered about axis 20. The antenna of the invention is adaptable to multifrequency operation by using such a nested feed. Also, the square four feed array is adaptable to either amplitude or phase simultaneous lobing or monopulse techniques for determining antenna pointing errors. For simultaneous lobing, it is important that the phase and amplitude of signals received at each of the four horns be equal when the transmitter of the signal is located on axis 20. It is further important that these parameters smoothly vary as the transmitting source departs from the antenna axis by an angle. Computer simulation has verified that these parameters vary smoothly for source angles ranging from zero to well in excess of half the beamwidth.
Having described an embodiment of my invention it is clear that numerous modifications of this embodiment are possible within the invention's spirit and scope. Of particular import is the fact that antennas are reciprocal devices useful for transmitting, receiving, or both. It is intended that the invention not be limited except by the following claims which are generic to transmitting and/or receiving antennas.