US 3611393 A
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United States Patent SERVODRIVER lnventor Lynden Underwood Kibler Middletown, NJ.
Appl. No. 32,072
Filed Apr. 27, 1970 Patented Oct. 5, I971 Assignee Bell Telephone Laboratories, Incorporated Murray Hill, NJ.
PARABOLIC TRIPOD FEED SUPPORT FOR PARABOLIC DISH ANTENNA 5 Claims, 5 Drawing Figs.
[1.8. CI 343/761, 343/781, 343/839, 343/840, 343/885 Int. Cl l-I0lq 3/12, l-lOlq l9/ l2 Field of Search 343/703,
BALANCER AND  References Cited UNITED STATES PATENTS 3,164,835 l/l965 Alsberg 343/78l 3,209,36l 9/ l965 Webb 343/782 Primary Examiner Eli Lieberman Attorneys-R. J. Guenther and E. W. Adams, Jr.
ABSTRACT: A tripod feed support structure for the front feedhom of parabolic reflector antennas has curved members as the support structure which correspond to a paraboloid .of revolution. The curved members reflect incident radio wave energy into a focal point having, for example. absorbent material. The structure substantially eliminates the residual effects of aperture blockage caused by the feed support structure.
TRANSMITTER SERVOMECHAN ISM SERVOMECHANISM PATENTED OCT 5 l9?! SHEET 1 BF 3 lNl/ENTOR U. K/BLER AT TOENE V PARABOLIC TRIPOD FEED SUPPORT FOR PARABOLIC DISH ANTENNA BACKGROUND OFTHE INVENTION This invention relates to parabolic dish antennas and, more particularly, to parabolic antennas utilizing a front feedhom which requires a support structure to, position the feedhom at the focal point of the parabolic reflector.
Parabolic reflector antennas offer many desirable advantages and are probably the most popular antennas used in microwave applications requiring high gain, high directivity, low spillover, and high-noise immunity. A parabolic reflector has the property of convening circular wave fronts which originate at the focal point into radio wave energy having plane wave fronts. The plane wave front property of radio wave energy propagating from the antenna is responsible for the highly directional characteristic of the parabolic reflector antenna. This conversion is reversibleso that the antenna is equally suitable for the reception. or the transmission of microwave radio signals.
In parabolic antennas, thereis a direct correlation between antenna performance and the size of the reflector relative to the wavelength of the operating frequency. The reflector requires unifonn illumination of the reflector surfaceto optimize antenna performance. Unfortunately, uniform illumination of large reflectors presents problems, depending upon the particular illumination method employed. The most common illumination method is to use a. front feedhom which inherently propagates spherical wave fronts which are converted after reflection fromthe parabolic surface into a beam of radio wave energy, possessing plane wave fronts. If a symmetrical feedhorn is used possessing equal boundary conditions for the electric and magnetic fields, the parabolic reflector antenna is symmetrical and can be used for circular polarization applications. One disadvantage that has plagued this illumination method is that the blockage of the main beam-radiation pattern by the supportrstructure required to position the feedhorn rigidly at the focal point of the parabolic reflector degrades antenna performance. This intercepted radio wave energy scatters and interacts with the main beam of plane wave front radio wave energy, and produces spillover and the generation of side lobe patterns which reduce the efficiency. directivity, and noise immunity; of the antenna performance.
An'alternate illumination method, has been to use a combination of a rear feedhom with variously positioned Cassegrainian subreflectors which reflect radio wave energy back to illuminate the main parabolic reflector. In one variation of this alternative, blockage by the support structure is complete- Iy' eliminated because the feedhom and subreflector are supported and fed by a waveguide structure. The waveguide structure extends from the center of the main parabolic reflector along the central axis of the parabolic reflector to the focal point of the reflector where the subreflector is locateda short distance from the opening of the feedhom. However, in large high-precision parabolic antennasthe illumination pattern suffers significantly, resulting in degradation of the performance. In another variation of this alternative, a rear feedhom is located at the center of the parabolic reflector and illuminates a subreflector displaced a distance from the horn along the center axis of the parabolic reflector. The subreflector reflects the feedhorn radio wave energy and illuminates the main parabolic reflector which beams the radio wave energy into space. As in the front feedhorn illumination method, the displaced subreflector requires a support structure which obstructs and scatters the radio wave energy of the main beam. This illumination method reduces the effective reflector surface area and is expensive and difficult, to construct, andv requires elaborate mechanical design for large antenna systems.
In radio astronomy work, which requires the ultimate in the aforementioned antenna characteristics, the conventional front feedhom illuminator is often used in conjunction with a very large parabolic reflector. The parabolic reflector-is made large so as to increase directivity and, correspondingly, dwarf the effects of aperture blockage by the feedhorn support structure. Although the problem is mitigated to a significant degree, the problem still exists and hampers measurements requiring close tolerances and precision.
Conventional support structures in parabolic antennas utilize straight struts, which may have a specific cross section shape to reduce the effects of spillover and aperture blockage. Other approaches have been implemented, using dielectric support members instead of metallic support members but these are limited since they still obstruct radio wave energy to a limited extent and do not have the high tensile strength necessary, to support the feedhorn rigidly in large antennas.
SUMMARY OF THE INVENTION The present invention is a front feedhom support structure designed to eliminate the scattering effects caused by the blockage of radio wave energy by the feedhom support structure located in the main, beam of the antenna radiation pattern. The feedhom support structure has curved supporting struts shaped to conform to a paraboloid of revolution. This configuration does not remove the obstruction, but redirects the obstructed energy into a predetennined focal point different from the focal point of the main reflector. In one embodiment of the present invention, radio wave absorbent materialis placed to dissipate the unwanted energy completely, thus reducing the interaction of unwanted radio wave energy with the main beam of the radio wave energy.
In an alternative embodiment of the invention, radio wave detectors are placed at the second focal point to detect the magnitude or the phase of the energy to achieve a balanced condition indicating proper alignment of the feedhom for substantially ideal'ante nna performance. Radio wave energy diffraction by the edges of the feedhorn support structure members has been proven to be a second order effect in relation to the scattering of the intercepted radio wave energy. Thus, the structure substantially eliminates the deleterious effect produced by the obstruction and scattering caused by the location of the feedhorn support structure in the main beam of radio wave energy radiating from the antenna.
It is a feature of the present invention that the support structure for the antenna feedhom focuses energy incident thereon toward a single, point or area where it may be absorbed or otherwise prevented from interfering with the principal radiation of the antenna.
It is another feature-of one embodiment of the present invention thatthe focused energy from the energy incident upon the support structure is used to determine proper alignment of the antenna feedhom in order to obtain optimum antenna performance.
These andother features of the present invention will be readily apparent form the following detailed description. taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a perspective view of the overall parabolic as sembly antenna in accordance with the invention;
FIG. 2 is a ray diagram useful in explaining the operation of one embodiment of the invention;
FIGS. 3A and 3B are the side and the enlarged cross section views, respectively, of the parabolic strut; and
FIG. 4 is a ray diagram usefulin explaining the operation of an alternate embodiment of the invention.
DETAILED DESCRIPTION FIG. I shows a parabolic antenna 11 comprising a rotatable stand 13, a parabolic reflector 12, a feedhorn illuminator l6. parabolic struts I7 forming a tripod support for the feedhorn illuminator l6, and a receiver or a transmitter 14, depending upon the particular mode of operation of the invention.
FIG. 2 is a ray diagram of the antenna of FIG. 1 comprising the feedhorn illuminator 16, the parabolic reflector 12, a paraboloid of revolution surface 24, and radio wave absorbent material 29 on the feedhorn illuminator 16 corresponding to the focal region of the paraboloid of revolution 24.
The operation of the parabolic antenna will now be explained by reference to FIG. 2. The feedhorn illuminator 16 is shown emitting two illumination rays, the illumination ray 26 and the illumination ray 21, which originate at the focal point of the parabolic reflector 12 which corresponds to the location of the feedhorn illuminator 16. The illumination ray 21 represents a typical energy path of a radio wave energy transmitted by the parabolic antenna. To be more specific, illumination ray 21 reflects from the parabolic reflector 12 to result in transmitted ray 25. This path of the illumination ray 2! and the transmitted ray 25 represents the normal propagation path of the major portion of the radio wave energy emitted by a parabolic antenna which is illuminated by a front feedhorn located at its focal point.
The illumination ray 26 originating at the focal point of the parabolic reflector 12 is reflected by the parabolic reflector 12 and results in reflected ray 27. The reflected ray 27 represents radio wave energy which is obstructed by the paraboloid of revolution surface 24. A redirected ray 28 is shown reflecting from the paraboloid of revolution surface 24 which originates from the reflected ray 27. The redirected ray 28 is shown impinging on the radio wave absorbent material 29. In the absence of the absorbent material and the paraboloid of revolution, this radio wave energy would normally be scattered, thus creating undesirable side lobes in the conventional parabolic reflector antenna configuration. In accordance with the present invention, however, this obstructed wave energy is directed by the paraboloid 24 toward a second focal point removed from the first focal point of the parabolic reflector 12. The radio wave absorbent material 29 is located in a region in the path of the radio wave energy which is obstructed by the feed support structure and redirected to the second focal point. In this location, the radio wave absorbent material is slightly displaced from the second focal point todissipate the unwanted energy and remove the undesirable characteristics produced by this energy.
In accordance with the principles of the present invention, the supporting struts 17 of FIG. 1 are shaped to conform to the paraboloid of revolution 24 of FIG. 2. FIGS. 3A and 3B are a side view and a cross section, respectively, of one of the support struts 17 showing the shape thereof. With support members 17 thus shaped, energy impinging thereon from reflector 12 will follow the path of ray 28 in FIG. 2 and be absorbed by the material 29, thereby preventing that energy from interfering with the antenna reflection pattern to create, for example, unwanted side lobes. The support struts 17 are preferably constructed of a metallic material which has properties compatible with the structural requirements of the support members 17.
FIG. 4 depicts an alternative embodiment of the invention comprising a parabolic reflector 32, a front feedhorn illuminator 36, parabolic struts 37 of the configuration shown in FIGS. 3A and 38, a plurality of pyramidal horns 34, radio wave detectors 33 and servomechanisms 31. The front feedhorn illuminator 36 is located at the focal point of the parabolic reflector 32 and emits radio wave energy generated by the transmitter'40 to illuminate the parabolic reflector 32. The path of radio wave energy which is obstructed by the parabolic struts 37 is illustrated by rays 38. These rays 38 follow a path corresponding to the obstructed radio wave energy which is illustrated in FIG. 2. In this embodiment of the invention, the obstructed radio wave energy enters the pyramidal horns 34 and is detected by the radio wave detectors 33 located at the throats of the pyramidal horns 34. Another pyramidal horn 34 (not shown in FIG. 4) is located behind the front feedhorn illuminator 36 to receive the radio wave energy obstructed by the rear parabolic strut 37. The output of the three radio wave detectors 33 is compared in a balancer and servodriver 3( The balancer and servodriver compares the output of the three radio wave detectors 33 and correspondingly adjusts each of the servomechanisms 31 to insure the proper illumination of the parabolic reflector 32. The three radio wave detectors 33 can be designed to have an output which will produce an equal amplitude or a net phase difference corresponding to the proper alignment of the feedhorn illuminator 36.
Any change in the alignment of the feedhorn illuminator 36 due to gravity, wind, or other prevailing factors will cause a change in the amplitude or the phase balance between the radio wave detectors 33. This change is detected by the balancer and servodriver 30 and used to control the servomechanisms 31 for correcting the alignment error. Each of the three servomechanisms 31 effectively changes the length of one of the parabolic struts 37 to maintain the correct alignment of the feedhorn illuminator 36. In the conventional parabolic reflector antenna, the changing of the angle of elevation in the earths gravitational field changes the loading on the support members for the front feedhorn, thus producing an error in alignment of the front feedhorn with respect to the parabolic reflector. Hence, the alternative illustrative embodiment of the present invention overcomes this inherent problem of the conventional parabolic reflector antennas. Although the tripod arrangement is used in this illustrative embodiment, other arrangements employing different numbers of parabolic struts may be conveniently used.
In all cases, it is to be understood that the foregoing arrangements are merely illustrative of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.
What is claimed is:
1. An antenna system for radio wave energy comprising a parabolic reflector having a first focal point,
a feed member located at said first focal point for directing radio wave energy toward said reflector to illuminate said reflector,
a plurality of curved support members for supporting said feed member, said support members being shaped to conform to a paraboloid of revolution having a second focal region on said feed member removed from said first focal point so that radio wave energy incident upon said members is reflected toward said feed member at said second focal region, and
radio energy absorbent material located at said second focal region to eliminate substantially the residual effects of aperture blockage by said support members by absorbing the energy incident at said second focal point.
2. The antenna system of claim 1 wherein said plurality of curved support members comprises three metallic support members forming a tripod feed support structure for said feed member.
3. An antenna system for radio wave energy comprising a parabolic reflector having a first focal point, a feed member located at said first focal point for directing wave energy toward said reflector to illuminate said reflector. a plurality of curved support members for supporting said feed member, each of said support members being shaped to conform to a paraboloid of revolution having a second focal region on said feed member removed from said first focal point such that radio wave energy incident upon each support member is reflected toward said feed member at said region, a plurality of radio wave'detectors located to receive radio wave energy reflected by said support members, and means responsive to the output of said detectors for maintaining correct alignment of said feed member.
4. The antenna system of claim 3 wherein said responsive means comprises a means for comparing the signals from said detectors to control servomechanisms located at the base of each separate member to vary the effective length of each support member whereby the proper alignment of said feed member is maintained respective to said parabolic reflector.
5. The antenna system of claim 4 wherein said plurality of curved support members comprises three metallic support members each having one of said detectors located in said focal region.