Shaped dual reflector antenna system for generating a plurality of beam coverages

A dual-reflector antenna system (40) is provided for generating a shaped main beam radiation pattern (20) and at least one additional secondary spot beam radiation pattern (30,32). The antenna system (40) includes a main shaped reflector (10) having a shaped reflective surface (11) operatively coupled to a subreflector (12) for communicating therewith. A main feed horn (14) communicates directly with the subreflector (12) so as to reflect first energy to and from the main reflector (10) within a shaped beam radiation pattern (20). In a preferred embodiment, the subreflector (12) has an ellipsoidal reflective surface (13) which communicates directly with the main reflector (10) via an inverted reflective path (17) which has a converging focal point (18). One or more auxiliary feed horns (24,26) are operatively coupled directly to the main reflector so as to directly communicate therewith and reflect second energy within one or more additional radiation patterns (30,32). The first and second feed horns (24,26) are preferably located separate from the reflective path (17) so as to avoid interference therewith. In an alternate embodiment, a subreflector (12') with a hyperboloidal reflective surface (13') may be used.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates generally to antenna reflector systems and, more 
particularly, to a system and method for generating a plurality of beam 
coverages with a dual-reflector antenna system. 
2. Discussion 
Communication satellites and the like commonly employ antenna reflector 
systems for focusing or reflecting signals within beam radiation patterns. 
Shaped reflectors have been provided, which in combination with a single 
feed, have been employed to cover a selected shaped beam radiation 
pattern. For instance, a shaped reflector may be deployed in space to 
provide adequate coverage throughout a geographic area such as the 
mainland portion of the United States. However, typical requirements 
imposed for communication satellites and the like have generally required 
coverage of the mainland portion of the United States as well as coverage 
of remote locations such as Hawaii, Puerto Rico and Alaska, for example. 
Separate antenna reflector systems have been employed to separately 
generate each beam pattern coverage. Such systems generally require 
separate dual-reflector systems for each of the feed horns utilized 
therewith. This generally results in unnecessary complexity and weight 
which are undesirable for space-related applications and the like. It is 
conceivable that a dual-gridded shaped reflector could be used to provide 
multiple beam coverage to a limited extent. However, dual-gridded shaped 
reflectors must conform with dual linear polarization specifications. In 
addition, the dual gridded reflector requires polarization grids which 
generally results in rather extensive manufacturing requirements and high 
costs amongst other disadvantages. 
More recently, a conventional dual-reflector antenna system exists for 
providing a main beam coverage in addition to a secondary spot beam 
coverage. The conventional dual-reflector antenna system generally 
includes a subreflector that is positioned to communicate with a main 
reflector. While transmitting, the subreflector is illuminated with a 
primary energy signal generated by a first feed horn. The primary energy 
signal is reflected off the subreflector and the main reflector to produce 
a first or main beam coverage. In addition, the conventional 
dual-reflector configuration usually employs a second feed horn which is 
generally positioned beside the main feed horn. The second feed horn 
likewise illuminates the subreflector with a second energy signal which in 
turn is reflected from the main reflector to produce a second or secondary 
spot beam coverage. 
While the conventional dual-reflector configuration may serve well for some 
applications, there are limitations which generally make it unfeasible for 
space related applications and the like. For instance, the secondary spot 
beam may be required to cover a much smaller geographic area than the main 
beam coverage. Due to the difference in the sizes of the main beam and the 
spot beam coverage, the second feed horn must generally be defocused to 
get a good performance over the geographic area covered by the spot beam. 
This generally requires that one of the feed horns be positioned behind 
the other feed horn, thereby causing a partial blockage of one of the beam 
paths. In doing so, one of the feed horns is usually positioned within the 
beam coverage of the other feed horn. As a consequence, the partial 
blockage exhibited by the conventional dual-reflector configuration 
degrades the overall performance of the reflector antenna system. 
It is therefore desirable to provide for an enhanced dual-reflector antenna 
system which more effectively generates a secondary spot beam coverage in 
addition to a main beam coverage. It is further desirable to provide for 
such a reflector antenna system which generates a plurality of secondary 
spot beam coverages. In particular, it is desirable to provide for such a 
multi-beam shaped dual-reflector configuration which does not suffer from 
feed horn blockage or interference such as that which exists with the 
aforementioned conventional approach. In addition, it is desirable to 
provide for a more highly integrated low cost dual-reflector antenna 
configuration which may be easily manufactured. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention, a dual-reflector 
antenna system is provided for generating a shaped main beam radiation 
pattern and at least one secondary spot beam radiation pattern. The 
antenna system includes a main shaped reflector operatively coupled to a 
subreflector for communicating therewith. A main feed horn communicates 
directly with the subreflector so as to reflect energy to and from the 
main reflector within a main shaped beam radiation pattern. One or more 
auxiliary feed horns are provided which directly communicate with the main 
reflector so as to reflect energy within one or more secondary radiation 
beam patterns.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to FIG. 1, a side view of a conventional offset fed shaped 
gregorian dual-reflector antenna system is illustrated therein. The 
antenna system is shown in accordance with one example for providing a 
shaped beam radiation coverage 20 over a geographic area such as the 
United States mainland 22. In doing so, the antenna system may be located 
on a satellite or other spacecraft which provides a field of view of the 
desired geographic area. 
The dual-reflector antenna system includes a shaped main reflector 10 
operatively coupled to an offset fed subreflector 12. The main reflector 
10 has a shaped reflective surface 11 which generates phase error 
throughout the reflective surface of the main reflector 10 so as to 
provide a selected shaped beam radiation pattern 20. In the conventional 
Gregorian system, the subreflector 12 has an ellipsoidal reflective 
surface 13 which communicates directly with the shaped reflective surface 
11 via an inverted beam pattern 17 which has a converging focal point 18 
therebetween. A main feed horn 14 is operatively coupled to the 
ellipsoidal reflective surface 13 of subreflector 12 for communicating 
directly therewith. 
The dual-reflector antenna system operates to transmit and/or receive 
energy within the shaped beam radiation pattern coverage 20. While 
transmitting, the main feed horn 14 directly illuminates the subreflector 
12 which in turn reflects the energy and illuminates the shaped reflective 
surface 11 of main reflector 10. The main reflector 10 in turn reflects 
the energy within the shaped beam radiating pattern coverage 20. While 
receiving, the main shaped reflector 10 is illuminated with radiating 
energy received from the shaped beam radiation pattern coverage 20. The 
shaped reflector 10 in turn reflects and focuses the received energy so as 
to illuminate the ellipsoidal reflective surface 13 of subreflector 12. 
The focused energy is then received by the main feed horn 14 in the 
vicinity of a beam focal point 16. 
With particular reference to FIGS. 2 and 3, a shaped dual-reflector antenna 
system 40 is shown for providing a plurality of beam radiation patterns 
20, 30 and 32 in accordance with a preferred embodiment of the present 
invention. In doing so, the preferred embodiment employs a dual-reflector 
antenna system such as the one shown and described above in accordance 
with FIG. 1 for providing a main shaped beam radiation pattern coverage 
20. According to the present invention, the shaped dual-reflector antenna 
system 40 further includes the addition of one or more auxiliary feed 
horns such as auxiliary feed horns 24 and 26. The auxiliary feed horns 24 
and 26 are appropriately located so as to directly illuminate the shaped 
reflective surface 11 of shaped main reflector 10. That is, the auxiliary 
feed horns 24 and 26 are operatively coupled directly to the shaped 
reflective surface 11 without the use of subreflector 12. As shown in FIG. 
2, auxiliary feed horns 24 and 26 are located in the vicinity of an 
effective focal plane 28 and are preferably located separate from the 
inverted beam pattern 17. As a result, the auxiliary feed horns 24 and 26 
do not interfere with the radiating energy which passes between the main 
reflector 10 and subreflector 12 via inverted beam pattern 17. While the 
preferred embodiment is described herein in connection with two auxiliary 
feed horns 24 and 26, any number of auxiliary feed horns may be employed 
in accordance with the present invention. 
In operation, auxiliary feed horn 24 illuminates the reflective surface 11 
of the main shaped reflector 10 so as to transmit and/or receive radiating 
energy within a first secondary spot beam radiation pattern coverage 30. 
Beam radiation pattern coverage 30 may, for instance, be employed to cover 
a geographic area such as Alaska 34. The second auxiliary feed horn 26 
likewise directly illuminates the shaped reflective surface 11 of main 
reflector 10 so as to transmit and/or receive radiating energy within a 
second secondary spot beam radiation pattern coverage 32. Beam radiation 
pattern coverage 32 may, for instance, cover a geographic area such as 
Hawaii 36. 
While the main shaped beam radiation pattern coverage 20 and first and 
second secondary spot beam radiation pattern coverages 30 and 32 are shown 
separate from one another in a particular embodiment in FIG. 3, the beam 
pattern coverages 20, 30, and 32 may be provided for in a number of sizes 
and locations to achieve the desired beam pattern coverages. For instance, 
feed horns 14, 24 and 26 may be axially moved along each respective 
associated beam axis so as to focus or defocus the size of the respective 
beam pattern coverage associated therewith. In addition, the auxiliary 
feed horns 24 and 26 may be moved along the effective focal plane 28 so as 
to change the location of the spot beam radiation pattern coverages 30 and 
32. That is, feed horns 24 and 26 may be positioned further away from 
inverted beam pattern 17 along effective focal plane 28 for purposes of 
providing beam pattern coverages 30 and 32 which are further displaced 
from beam pattern 20. 
While the preferred embodiment has been described in connection with a 
gregorian dual-reflector, it is conceivable that other subreflector shapes 
such as a hyperboloidal subreflector with a hyperbolic shape may be 
employed in place of the elliptical shape without departing from the 
spirit of this invention. In accordance with an alternate embodiment of 
the present invention, a cassegrain dual-reflector antenna system 40' 
which employs a hyperboloidal subreflector 12' with a hyperbolic 
reflective surface 13' is shown in FIG. 4. 
The Cassegrain dual-reflector antenna system 40' may provide for a more 
compact system since the main reflector 10 and hyperboloidal subreflector 
12' may be positioned closer to one another. However, a hyperbolic 
subreflector surface 13' generally has a more limited desirability in that 
a hyperbolic reflective surface 13' will not provide an effective 
converging focal point such as focal point 18. As a result, the auxiliary 
feed horns 24 and 26 generally will have to be located outside the beam 
pattern 17 in order to prevent any interference therewith. Thus, while the 
cassegrain system may provide a similar performance, such an arrangement 
may result in more limited operating capabilities. 
In view of the foregoing, it can be appreciated that the present invention 
enables the user to achieve an enhanced dual-reflector antenna system 40 
for generating one or more secondary beam coverages in addition to a main 
shaped beam radiation pattern. Thus, while this invention has been 
disclosed herein in combination with a particular example thereof, no 
limitation is intended thereby except as defined in the following claims. 
This is because a skilled practitioner will recognize that other 
modifications can be made without departing from the spirit of this 
invention after studying the specification and drawings.