US 3733609 A
A reflector antenna has a reflector with an offset feed. The reflector is partially encompassed by a shroud which may be reflecting, absorbing, or both, to energy radiated by the feed. The shroud is effective to block energy radiated by the feed that would otherwise spill over the edge of the reflector, only along a portion of the edge less than its entirety.
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Description (OCR text may contain errors)
United States Patent [191 Bartlett  SHROUDED OFFSET PARABOLIC REFLECTOR ANTENNA  Inventor: Homer E. Bartlett, Melbourne, Fla.
 Assignee: Radiation Incorporation, Melbourne, Fla.
 Filed: July 2, 1971  Appl. No.: 159,352 I  US. Cl ..343/782, 343/840  Int. Cl. ..H0lq 19/14  Field of Search ..343/786, 837, 840, 343/782 [5 6] References Cited UNITED STATES PATENTS 3,156,917 11/1964 Parmeiggiani ..343/840 1 May 15,1973
3,550,142 12/1970 Dawson ..343/840 3,646,565 2/1972 Robinson et al ..343/84O 3,510,873 5/1970 Trevisan ..343/786 FOREIGN PATENTS OR APPLICATIONS 861,719 1/1953 Germany ..343/840 1,048,298 1/1959 Germany ..343/840 Primary Examiner-Eli Lieberman Attomey- Donald R. Greene 57 ABSTRACT A reflector antenna has a reflector with an offset feed. The reflector is partially encompassed by a shroud which may be reflecting, absorbing, or both, to energy radiated by the feed. The shroud is effective to block energy radiated by the feed that would otherwise spill over the edge of the reflector, only along a portion of the edge less than its entirety.
11 Claims, 2 Drawing Figures Patented May 15, 1973 4 3,733,609
\NvENToR H. E. BARTLETT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to reflector antennas, and more particularly to a reflector antenna in which the reflector is partially shrouded relative to an offset feed.
2. Prior Art In the past it has been common to employ metal tunnels on parabolic antennas to eliminate interference from sources at relatively large angles with respect to the antenna boresight. This enhances interference from other locations via reflections from the metal tunnel. For this reason microwave absorber must be used inside the tunnel. The effectiveness of this is limited by the quality of available microwave absorbers.
An absorber'lined tunnel cannot be used for noise temperature reduction. Although the tunnel shields against some of the noise radiated from the ground, the absorber in the tunnel generates as much noise as the ground does. A tunnel without absorber can be used to lower the antenna noise temperature for boresight angles near zenith, but is ineffective for boresight angles near the horizon. For most applications the antenna noise temperature for boresight angles near the horizon is the most important case.
An additional source of wide-angle interference in conventional reflector antennas with tunnels is the result of reflections from the feed system and feedsupport mechanism.
SUMMARY OF THE INVENTION The present invention provides an economical reflector antenna design capable of achieving great discrimination between a target (such as a remote receiving station, an artificial satellite, or the like) and sources or objects tending to produce electromagnetic or radio frequency interference with the target. In this context, target refers to an active source of RF signal as well as a passive station for receiving only. The present invention can also be used for noise temperature reduction at low (near horizon) elevation angles.
Briefly, according to a preferred embodiment of the invention, an offset-fed parabolic reflector antenna is provided with a partial shroud about the reflector. The
shroud is constructed and arranged such that spillover radiation is restricted to only one side or sector of the antenna, that side or sector being away from the direction of sources of interference and/or from sources productive of antenna noise temperature. Furthermore, the shroud is constructed to absorb or to reflect energy radiating from the feed, or to provide partial absorption and partial reflection, depending upon the particular application of the antenna. Accordingly, very little wide angle radiation occurs on the side or sector of the antenna opposite that at which the spillover radiation is permitted (this low wide-angle radiation side hereinafter being called the favored side), either because of absorption or reflection, or both, of what would otherwise be spillover.
In the preferred embodiment, the shroud has the form of a hollow cylinder truncated along a plane at a skew angle relative to the, plane containing the boundary of the reflector.
BRIEF DESCRIPTION OF DRAWINGS Other objects, features, and advantages of the invention will become apparent from a consideration of the detailed description of the preferred embodiment, with reference to the accompanying figures of drawing, in
FIG. 2 is a fragmentary side view of the embodiment of FIG. 1.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, the reflector antenna comprises a parabolicreflector 10, an offset (relative to the reflector axis) feed such as a horn 12, a shroud 15, partially encircling the reflecting surface of reflector 10, and conventional support and feed mechanisms for the antenna.
In a constructed embodiment, reflector 10 was 6 feet in diameter and was illuminated by a mechanically conical scanning 13 inch diameter multi-choked horn 12 operating at a frequency of about 2 GHz. The multichoked horn has a radiation pattern which drops off very rapidly, thereby reducing wide-angle radiation attributable to spillover. Moreover, the horn is offset so that it produces no blockage of radiation reflected from the reflector, thereby eliminating blockage contributions to the wide-angle radiation pattern.
According to the principal feature of the invention,
horn 12 is positioned in its offset relationship to reflector 10 such that at the side or sector of the reflector closest to the horn the spillover radiation from the horn is readily reflected and/or absorbed by a shroud 15 of relatively small dimensions. The shroud constitutes a right circular cylinder of diameter equal to the diameter of reflector 10, one end of which encircles the periphery of the reflector (and is fastened or supported thereat) and the other end of which has been out along a plane through the cylinder and at an angle to its axis such that there is little material at the narrowest portion 17 of the shroud wall. In practice, the shroud is conveniently produced by cutting an appropriate pattern from a thin sheet of metal such as aluminum (the thickness being unimportant except for considerations of assembly ease), followed by development in the manner indicated above. The length of the pattern may be slightly larger than is' necessary to construct a shroud of the desired diameter, to permit the ends of the pattern to overlap at the narrow portion 17 where they may be riveted or otherwise fastened together.
As shown in FIGS. 1 and 2, it is preferable that the mouth of horn 12 project into the region encompassed by shroud 15, and a suitable cutout 20 (FIG. 1) may be provided in the angled end of the shroud for that purpose, but it is also desirable that no portion of the lip 22 of the horn extend into the boundary of the reflector along a line parallel to the boresight (FIG. 2). While these geometric considerations are not critical to the invention, they do ensure the effective capture of virtually all of what would otherwise be spillover radiation at the favored side of the antenna, with little or no aperture blockage by the horn.
In operation of the reflector antenna, and with reference now to FIG. 2, radiation emanating from the phase center 25 of horn 12 illuminates the concave refleeting surface of reflector 10. That energy radiated beyond the boundary of the reflector is ordinarily lost as spillover radiation. The presence of shroud 15, however, causes some of this otherwise spillover radiation, depicted by ray 26, to be reflected onto reflector and then toward the opposite side of the boresight. Since the effectiveness of the shroud for this purpose diminishes with decreasing width of its wall, the amount of spillover radiation increases from a minimum (virtually zero) at point A to a relative maximum at point B. The energy incident directly upon the reflector from the horn is collimated in the direction of the boresight for illumination of the target. The spillover of the feed radiation past point B, depicted by ray 27, is not reflected to the favored side of the antenna as is the case with conventional reflector antennas with tunnels.
It will be appreciated, then, that very low wide-angle radiation occurs along the favored side of the antenna, i.e., in a sector generally extending in either direction along the periphery of the reflector from point A to a region about midway to point B. In some instances, it may be desirable to employ an energy absorbing material along the inside surface of shroud or to assemble the shroud from commercially available energy absorbing material, such as Emerson & Cuming AN 75, which is absorbent to energy at microwave frequencies. This produces a small, but sometimes significant, improvement in discrimination against sources of electromagnetic interference.
The antenna is simply positioned such that the direction to the source of interference is on the favored side of the radiation pattern of the antenna, to enable absorption of interfering energy from that source incident on the reflector. Although some spillover from the horn is absorbed using this embodiment, a considerable amount is still reflected from the absorbing material because of the high angle of incidence of that radiation (e.g., angle a in FIG. 2) and the imperfect characteristic of all commercially available absorbing material.
Embodiments employing no absorber, i.e., a substantially purely reflective shroud, are particularly advantageous for reducing antenna noise temperature at low elevation angles. This is achieved by positioning the antenna with the favored side toward the earth, so that virtually all of the spillover radiation is directed toward the cool sky. The antenna noise temperature improvement is typically greater than 3 db. In contrast, an absorber shroud has a temperature comparable to that of the earth because of the absorbed spillover radiation, and therefore would not provide this advantage.
The constructed embodiment referred to above, which had absorber in the shroud, exhibited a wide angle radiation level more than 50 db down from the peak of the main beam of the antenna, at angles greater than on the favored side. This is 19 db below isotropic level, i.e., the level of radiation of an isotropic radiator with the same power input as that of the constructed embodiment. In contrast, a conventional reflector antenna typically displays wide angle radiation levels of 0 db below isotropic level. The wide-angle radiation of an antenna embodiment using a reflective shroud was about 4 db higher than that of the absorber shroud embodiment for wide angle radiation on the favored side at angles greater than 25.
The invention is useful in such diverse applications as satellite communications with small ground station antennas. particularl with advancing numbers of orbiting satellites and increased satellite power; and surface communication links wherever reflection or RF interference problems are present.
What is claimed is:
1. A reflector antenna, comprising:
a feed; 1
an offset reflector for said feed; an
shroud means partially encompassing said reflector for preventing spillover radiation from said feed along only a limited sector of the boundary of said reflector less than the entire boundary thereof, said shroud means comprises a hollow cylinder truncated at one end spaced from said reflector and along a plane at a skew angle relative to the plane containing the boundary of said reflector and secured adjacent its other said end to the periphery of said reflector such that the walls of said cylinder extend continuously from said reflector to said open end.
2. The reflector antenna of claim 1, wherein:
said shroud means is at least partly absorbent to radiation emanating from said feed.
3. The reflector antenna of claim 1, wherein:
said shroud means is at least partly reflective to radiation emanating from said feed.
4. The reflector antenna of claim 1, wherein:
said cylinder is composed of sheet metal.
5. The reflector antenna of claim 4, wherein:
said cylinder is lined along at least part of its interior surface with material absorbent to radiation emanating from said feed.
6. The reflector antenna of claim 1, wherein:
said cylinder is composed of sheet material absorbent to radiation emanating from said feed.
7. A reflector antenna, comprising:
a feed line for said horn;
a parabolic reflector positioned for illumination by energy radiated from said horn and having an axis tilted relative to the axis of said horn, for highly directive reflection of said energy without aperture blockage by said horn; and
shroud means partly encompassing said reflector along the periphery thereof to block energy radiated from said horn that would otherwise spill over the edge of said reflector, only along a portion of the edge less than the entirety thereof, said shroud means comprises a sheet of material forming a cylinder having a free end cut off along a plane at a skew angle to the encompassing said periphery such that the walls of said cylinder extend continuously from said reflector to said free end.
8. The reflector antenna of claim 7, wherein:
the interior surface of said cylinder is reflective to energy radiated from said horn.
9. The reflector antenna of claim 7, wherein:
the interior of said cylinder is absorbent to energy radiated from said horn.
10. A reflector antenna comprising:
a parabolic reflector positioned for illumination by energy radiated from said feed and having an axis tilted relative to the axis of said feed, for highly directive reflection of said energy without aperture blockage by said horn; and
shroud means partly encompassing said reflector along the periphery thereof to block energy radiated from said feed that would otherwise spill over the edge of said reflector, only along a portion of the edge less than the entirety thereof, said shroud means comprising a substantially continuous wall which is circular in cross section with one end thereof having a diameter essentially corresponding with that of said reflector and positioned adjacent thereto so as to essentially encompass the periphery of said reflector and having an open end spaced from said reflector, said open end being truncated along a plane at a skew angle relative to the plane containing the periphery of said reflector so that the length of said wall from said one end to said open end uniformly increases in both a clockwise direction and a counterclockwise direction from a first peripheral open end edge point spaced proximate to said reflector to substantially a second peripheral open 'end edge point, located from said first point, and which is spaced substantially away from said reflector relative to that of said first point;
said feed being positioned relative to said shroud means so as to be located proximate to said second peripheral open end edge point so that spillover radiation from said feed in a direction away from said reflector axis is blocked by the elongated inner wall proximate to said second edge point of said shroud means whereas at least a portion of the spillover radiation in the direction of said reflector axis is not blocked by the shorter inner wall proximate to the first edge point of said shroud means.
11. A reflector antenna as set forth-in claim 10, wherein said continuous wall is of cylindrical configuration.