Dual reflector microwave antenna.

The present invention relates generally to microwave antennas and, more particularly, to dual-reflector microwave antennas.

It is a primary object of the present invention to provide an improved dual-reflector microwave antenna which can be efficiently and economically produced at a relatively low cost, and yet, because it is a dual-reflector antenna, permits most of the radio-frequency equipment normally used with such antennas to be located behind the main reflector dish. In this connection, a related object of this invention is to provide such an improved antenna which minimizes blockage of the antenna aperture; facilitates alignment of the antenna; and virtually eliminates the necessity of exposing service and maintenance personnel to any safety hazards due to direct exposure to the electromagnetic energy of signals being transmitted and/or received by the antenna.

It is another object of this invention to provide such an improved dual-reflector microwave antenna that permits the use of small, smooth-walled feed horns.

It is still another object of this invention to provide an improved dual-reflector microwave antenna which can be used for transmission and/or reception of microwave signals, in one or more frequency bands, with or without orthogonally polarized signals in each frequency band.

A further object of this invention is to provide such an improved dual-reflector microwave antenna which provides good performance characteristics which can be tailored to different desired combinations of VSWR, directive gain, and RPE (radiation pattern envelope). A related object is to provide such an antenna which also has a relatively high efficiency.

Yet another object of the present invention is to provide such an improved dual-reflector microwave antenna which is capable of satisfying the latest RPE specifications set by the U.S. Federal Communications Commission for earth station antennas.

In accordance with the present invention, there is provided a microwave antenna which comprises the combination of a paraboloidal main reflector; a subreflector located between the main reflector and the focal point of the main reflector and forming a reflecting surface which is shaped symmetrically about the center thereof, the central portion of the reflecting surface sloping axially away from the main reflector at a rate which reduces as the radius of the subreflector increases, the radially outer portion of the subreflector sloping axially toward the main reflector at a rate which increases as the radius of the subreflector increases; and a feed horn for transmitting microwave signals to, anc'rreceiving microwave signals from the the subreflector, the reflecting surface of the subreflector being positioned and dimensioned to intercept substantially all the energy launched through the feed horn or reflected by the main reflector.

The feed horn is preferably a smooth-walled waveguide having an aperture diameter substantially equal to one wavelength of the midband signals propagated therethrough.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Fig. 1 is a vertical section taken through the middle of a dual-reflector microwave antenna according to the present invention with the spacing between the feed horn and the subreflector, and between the subreflector and the focal point of the main reflector, enlarged for clarity;</li><li>Fig. 2 is an enlarged section of the subreflector portion of the antenna of Fig. 1;</li><li>Fig. 3 is an enlarged section of the feed horn portion of the antenna of Fig. 1; and</li><li>Fig. 4 is a full end elevation of the subreflector portion of the antenna of Fig. 1.</li><li>Figs. 5 and 6 are E-plane and H-plane patterns, respectively, produced by an exemplary antenna embodying the invention, at a frequency of 12.25 GHz;</li><li>Figs. 7 and 8 are E-plane and H-plane patterns, respectively, produced by the same antenna that produced the patterns of Figs. 5 and 6, but at a frequency of 14.5 GHz; and</li><li>Figs. 9 and lO are amplitude and phase patterns produced by a scaled version of an exemplary subreflector, like that illustrated in Figs. 1, 2 and 4, at a scaled frequency of 3.95 GHz.</li></ul>

Turning now to the drawings and referring first to Fig. 1, there is illustrated a dual-reflector antenna comprising a paraboloidal main reflector dish 10, a primary feed horn 11 connected to and supported by a circular waveguide 12 extending along the axis of the dish 10, and a subreflector 13. (The term "feed" as used herein, although having an apparent implication of use in a transmitting mode, will be understood to encompass use in a receiving mode as well, as is conventional in the art.) In the transmitting mode, the feed horn 11 receives microwave signals via the circular waveguide 12 and launches those signals onto the subreflector 13; the subreflector reflects the signals onto the main reflector dish 10, which in turn reflects the signals in a generally planar wave across the face of the paraboloid. In the receiving mode, the paraboloidal main reflector 10 is illuminated by an incoming planar wave and reflects this energy in a spherical wave to illuminate the subreflector 13; the subreflector reflects the incoming energy into the feed horn 11 for transmission to the receiving equipment via the circular waveguide 12.

As is conventional in dual-reflector antennas, the subreflector 13 is located between the main reflector dish lO and the focal point F of the paraboloidal surface of the main reflector. To support the subreflector 13 in this desired position, the subreflector is mounted on the large end of a dielectric (e.g., fiberglass) cone 14 fastened at its smaller end to a hub 15 fitted within a standard mounting ring 16 for the main reflector dish 10. The fiberglass cone 14 is relatively thin and introduces only a negligible amount of VSWR and pattern degradation into the antenna system. Alternatively, the subreflector can be supported by a tripod or a quadpod arrangement (each pod running from about the main reflector edge to the rear of the subreflector). The subreflector 13 is positioned and dimensioned to intercept substantially all the energy launched through the'feed horn 11 in the transmitting mode, and substantially all the incoming energy reflected by the main reflector 10 in the receiving mode, while at the same time minimizing blockage of the aperture of the main reflector 10. (In the present example the ratio of subreflector to main reflector diameter, for example, is about 0.05). As in most dual reflector antennas, the subreflector preferably intercepts at least 98% of the energy from the feed horn and, to achieve this result, the subreflector has a diameter of about nine wavelengths at the midband frequency and is positioned very close to the feed horn.

The feed horn 11 is preferably a smooth-walled circular waveguide having an inside diameter equal to approximately one wavelength of the midband signals propagated therethrough. Such a feed horn launches signals onto the subreflector with substantially equal E and H plane patterns, and is extremely economical to manufacture. The axial length of the horn 11 is not critical since it is simply a continuation of the circular waveguide 12. The horn 11 and the waveguide 12 have the same inside diameter and are connected by a pair of coupling flanges 17 and 18 fastened together by a plurality of screws 19.

To suppress radiation (in the direction of the main dish) from the external surface of the horn 11, the mouth or free end of the horn is surrounded by a quarter-wave choke 20 comprising a short conductive cylinder 21, concentric with the horn 11, and a shorting ring 22. The inner surface of the cylinder 21 is spaced away from the outer surface of the horn 11 along a length of the horn about equal to a quarter wavelength from the end of the horn, and then the cylinder 21 is shorted to the horn 11 by the ring 22 to form a quarter-wave coaxial choke which suppresses current flow in the outer surface of the horn;,.

If desired, a flared feed horn maybe used in place of the straight horn in the illustrative embodiment, or a tapered waveguide section can be used between a straight horn of one diameter and a straight supporting waveguide of a different diameter.

In accordance with one important aspect of the present invention, the subreflector 13 forms a reflecting surface 30 which is shaped symmetrically about the center thereof, the central portion of the reflecting surface 3o sloping axially away from the main reflector 10 at a rate which reduces as the radius of the subreflector increases, and the radially outer portion of the reflecting surface 30 sloping axially toward the main reflector 10 at a rate which increases as the radius of the subreflector increases. Thus, in the illustrative embodiment the subreflector surface 30 slopes away from the main reflector 10 between the center of the subreflector and a radius rl, and then slopes back toward the main reflector 10 from the radius rl out to the outer periphery of the subreflector. More specifically, from the center of the subreflector out to the radius rl, the subreflector surface slopes away from the main reflector at a rate which decreases as the radius increases; then from the radius rl out to the periphery of the subreflector, the reflecting surface slopes toward the main reflector at a rate which increases as the radius increases. The end result is a subreflector surface with a concave radial cross-section.

As can be seen in Fig. 1, the center of the subreflector is preferably located closer to the plane a of the aperture of the main reflector 10 than is the outer periphery of the subreflector. That is, there is an axial offset x between the center and the outer periphery of the subreflector 13. The combination of this axial offset and the concave radial-eross-sectiqnal configuration of the subreflector results in substantially equal ray paths between the feed horn 11 and the main reflector 10 (via the subreflector 13) across the unblocked portion of the aperture of the main reflector, and minimal energy loss in the blocked portionsof the apertures of both the main reflector and the subreflector. More particularly, the subreflector 13 reflects most of the energy from the feed horn 11 in a spherical wavefront within the annular sector s between the small central blockage caused by the feed horn 11 (or the subreflector 13, whichever is larger) and the periphery of the main reflector 10; and, similarly, the main reflector 10 reflects most of the energy from the subreflector in a planar wavefront within the annular region m between the outer edges of the subreflector 13 and the main reflector 10, i.e., outside the blockage of the subreflector 13. Thus, the blockages produced by the feed horn 11 and the subreflector 13 are small to start with (typically less than 5% of the total area of the main reflector aperture at frequencies of 12 to 14 GHz), and the configuration of the subreflector reduces the effect of those blockages even further.

Moreover, this result is achieved while also achieving a relatively uniform distribution of electromagnetic field (in both amplitude and phase) across the unblocked portion of the aperture.

The antenna described is not only relatively simple and inexpensive to manufacture, but it also provides excellent performance characteristics. This antenna can be tailored to provide different RPE's, gains, and VSWR's for different applications. Any one of these performance characteristics can be optimised, with only a relatively small downgrading of the other characteristics. In general, the trade-off in performance is between gain and VSWR or RPE.

"Figs. 5 through 8 are far field patterns produced by an antenna like that illustrated in Figs. 1-4 but with a 3-inch tapered waveguide section inserted between a 3-inch horn 11 of WC940 waveguide and a 4.5 foot length of WC680 waveguide. The mouth of the feed horn was positioned one inch from the center of the subreflector, and the center of the front of the subreflector was positioned 0.79 inch in front of the focal point of the main reflector. This antenna had a main reflector with a 15 foot diameter and a subreflector with an 8.86-inch diameter supported on struts rather than a dielectric cone. Figs. 5 and 6 are E and H plane patterns taken at 12.25 GHz, and Figs. 7 and 8 are E and H plane patterns taken at 14.5 GHz. In each of these figures the expanded scale is for the expanded pattern, and the compressed scale is for the compressed pattern. The broken-line curves superimposed on Figs. 5-8 represent the U.S. Federal Communications Commission specification (dB_i(θ) = 32-25 log 9) for the maximum level of side lobes between 1° and 7°; from 7° out, no more than 10% of the side lobes can exceed the specified level. The antenna tested satisfied all these criteria at both 12.25 and 14.5 GHz, and also exhibited a narrow main beam. The first side lobes were relatively high, which is characteristic of many dual-reflector antennas; if desired, the first side lobes can be reduced by using a slightly different subreflector shape, although this will reduce the gain of the antenna.

The directive gains of the antenna (including RMS surface degradations of both the main reflector and the subreflector) that produced the patterns of Figs. 5-8 were as follows:

<tables id="tabl0001" num="0001"><img file="EP0102846A1_D0001.tif" /></tables>

Return loss measurements indicated that the antenna had a VSWR in the range of 1.25 to 1.30 at both frequencies.

In another example an antenna like the one described above but with a 30 inch subreflector spaced 2.67 inches in front of the focal point of the main reflector, and with a flared feed horn with a 1.75 inch aperture, was tested at various frequencies ranging from 3.7 GHz to 4.4 GHz. The results are set forth in Table I.

The same subreflector, mounted on a fiberglass cone rather than struts was also tested at 3.95 GHz and produced the amplitude and phase patterns shown in Figs. 9 and lO. These are E-plane patterns; the H-plane patterns (not shown here) would be expected to be similar, particularly in view of the other test results described above. As can be seen in Figs. 9 and 10, the subreflector blockage extended out to about 16° from the center of the pattern, and the edge of the main reflector was located at about 74°. Both the amplitude pattern and the phase pattern were measured on a constant radius relative to the focal point of the paraboloid of the main reflector. As can be seen in Fig. 9, the amplitude increased rapidly near the outer edge of the subreflector blockage, and then continued to increase at a slower rate across the major portion of the unblocked segment of the aperture, i.e., between the edge of the subreflector blockage and the periphery of the main reflector. At the periphery of the main reflector, the amplitude dropped off rapidly. This behaviour ensures a uniform wave over the unblocked main reflector aperture region m.

The phase pattern remained relatively constant across the unblocked portion of the aperture, as can be seen in Fig. 10.

Scaled versions of the above antennas can be made to operate at other frequencies with comparable results. Also, shrouds may be added to such antennas to reduce the main relfector spillover and improve the RPE in this region.

While the invention has been described in connection with certain preferred embodiments, it will be understood that it is not intended to limit the invention to those particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. <tables id="tabl0002" num="0002"><img file="EP0102846A1_D0002.tif" /></tables>