Array feed for offset satellite antenna

An offset antenna and feed combination for use in a satellite communication system is disclosed wherein an array of adjacent microwave energy feed elements is disposed within a predetermined shape on the focal surface of the antenna for delivering electromagnetic energy to a remote, correspondingly shaped zone. Electromagnetic energy beams are selectively radiated towards predetermined areas of the overall remote zone by associated cluster groups of feed elements typically arranged as a minimum of one central and six surrounding feed elements. Additionally, each surrounding type of feed element of a cluster may be shared by two or more overlapping clusters, the overlapping clusters generating contiguous beams of electromagnetic energy in different frequency subbands.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to multiple feed microwave antennas, and, more 
particularly, to array feeds for satellite communication system antennas. 
2. Description of the Prior Art 
Inadequate communications capacity is perhaps the major problem facing 
future satellite communication systems. Recently, satellite communication 
system designs have achieved a dramatic increase in the communication 
capacity of the satellite by discarding the single beam satellite antenna 
concept in favor of a multibeam approach. The modern multibeam satellite 
antenna permits many narrow-angle electromagnetic energy beams to be aimed 
at separate ground areas within a predetermined zone on the surface of a 
celestial body from a given aperture. In so doing, a given frequency is 
reused in many parts of the predetermined zone. Although a larger number 
of such beams can be accommodated with but a small increase in the total 
overall weight of the satellite, such a multibeam arrangement requires a 
high gain antenna. In addition, sidelobe radiation associated with each 
beam can cause interference between the beams. One prior art antenna which 
provides sufficient gain and can accommodate many feed elements on its 
focal surface is disclosed in detail in U.S. Pat. No. 3,914,768, which 
issued to the present inventor on Oct. 21, 1975. 
One prior art solution to the sidelobe interference problem is disclosed in 
"Spectral Reuse in 12 GHz Satellite Communication Systems" by D. O. 
Reudink, A. S. Acampora, and Y. S. Yeh, in the Conference Record, 1977 
International Conference on Communications, Volume 3, pp. 37.5-32 to 
37.5-35, June 12 to 15, 1977. This reference teaches the use of plural 
separate narrow beams which are isolated from one another by buffer areas. 
The use of buffer areas, however, severely limits the number of beams 
which can be utilized in a given field of view, and produces areas within 
the predetermined zone on the surface of the celestial body which are not 
serviced by the satellite. Although such a spot beam approach may 
adequately serve nodes of heavy traffic origin, a substantial volume of 
traffic originating in the buffer zone area is left without access to the 
satellite. One prior art solution to the problem of servicing the buffer 
area traffic is to provide terrestrial trunking to and from the closest 
area served by one of the spot beams. The terrestrial backhauling 
approach, however, entails substantial additional costs. A second solution 
is to provide a wide angle area coverage antenna port, in addition to the 
spot beam ports, which will service the buffer areas. This approach 
suffers from three problems; the first being that the power requirements 
of this port alone might exceed the power demand of all other ports 
combined. Secondly, the fact that the wide angle area coverage beam also 
covers areas serviced by the narrow spot beams results in reception 
interference at all of the narrow spot beam receiver ground stations. This 
problem is somewhat alleviated by elaborate wide angle beam shaping 
techniques which selectively cancel the area coverage beam in the areas 
served by the narrow spot beams. By the same token, sidelobe radiation 
from the narrow spot beams interferes with reception at the wide angle 
area coverage ground stations situated in the buffer area. Finally, since 
the narrow angle spot beams do not overlap one another, the associated 
receiving stations on the surface of the celestial body must be situated 
within the relatively small areas illuminated by such beams, thereby 
rendering more difficult the tasks of determining the location of spot 
beam receiver sites and reconfiguring the illumination pattern of the spot 
beams should traffic requirements change or the satellite be reassigned to 
a new orbital position in space. 
Another prior art solution to the problem of providing area coverage using 
plural spot beams is disclosed in "Design Tradeoffs for Multibeam Antennas 
in Communication Satellites," by W. G. Scott, H. S. Luh, and E. W. 
Matthews in Conference Record, 1976 International Conference on 
Communications, Vol. 1, pp. 4.1-4.6, June 14-16, 1976. This reference 
teaches the use of spot beams, each of which is launched by an associated 
cluster of plural feed elements. Feed elements may be shared by 
overlapping clusters, the respective clusters producing beams which are 
independent of each other because of diverse polarization direction. Thus, 
the shared feed elements are of a dual polarized type, and capable of 
down-link transmission throughout the entire electromagnetic energy 
frequency spectrum of the down-link frequency allocations. 
SUMMARY OF THE INVENTION 
In accordance with an illustrative embodiment of the invention, the 
foregoing and other problems of the prior art are solved by providing a 
communication satellite with a high gain multibeam antenna having a focal 
surface with many feed elements arranged in clusters disposed therein, 
wherein overlapping clusters share feed elements and launch respective 
beams of electromagnetic energy in plural frequency subbands and at one or 
both orthogonal polarizations to provide discrimination therebetween. 
Reflectors of the antenna direct the beams to a predetermined zone on the 
surface of the celestial body. 
It is an aspect of the present invention to provide a feed array wherein 
the number of feed elements and their configuration in each of the cluster 
groups are advantageously adjustable to achieve desirable beam shapes. A 
typical beam is launched by a cluster generally containing a minimum of 
seven feed elements arranged as one central element and six surrounding 
elements. Sidelobe radiation is reduced in such cluster groups by exciting 
the surrounding feed elements at a lower power level than the central feed 
element. Some of the feed elements are shared between overlapping cluster 
groups which generate contiguous beams, and independence of the beams is 
maintained by energizing overlapping clusters and their shared elements at 
diverse frequencies, times or polarization direction. 
It is an aspect of an embodiment of the present invention that high 
communication traffic density areas on the celestial body are served by 
stationary spot beams typically comprising seven or more grouped feed 
elements. Areas of lower traffic density are served by area coverage spot 
beams which are each generated by separate, overlapping clusters of feed 
elements illustratively containing seven feed elements. Although the 
stationary spot beams and the area coverage spot beams may each be 
generated by groups or clusters each containing seven feed elements, the 
area coverage spot beams have a larger "footprint" on the surface of the 
celestial body as a result of their contour being defined illustratively, 
at -7 dB, as opposed to -3 dB for the stationary spot beams. In addition, 
the area coverage spot beams can be selectively switched on and off on a 
time division basis, the maximum number of such area coverage spot beams 
which are simultaneously activated being limited by the available power in 
the satellite. 
Other and further aspects of the present invention will become apparent 
during the course of the following description and by reference to the 
accompanying drawings.

DETAILED DESCRIPTION 
FIG. 1 is a side view which schematically illustrates the essential 
elements of a prior art Cassegranian antenna desired to yield efficient 
multiple beam operation. The antenna is of an offset Cassegranian design, 
thereby avoiding aperture blockage which is common in symmetrical 
Cassegranian antennas. The conventional reflective and focal surfaces are 
positioned asymmetrically with respect to antenna aperture axis 10. This 
antenna design is characterized by disposing a main reflector 11 entirely 
on one side of an imaginary plane, shown in this side view as dashed line 
14. A subreflector 12 and a focal surface 13 are disposed on the other 
side of plane 14 from main reflector 11. A plurality of feed horns 15 are 
disposed on the focal surface, which feedhorns launch electromagnetic 
energy toward subreflector 12. The energy is reflected by subreflector 12 
to main reflector 11 and subsequently to a distant target area, not shown 
in the figure, within the viewing area of the offset Cassegranian antenna. 
It is to be understood that reference herein to a Cassegranian antenna is 
for merely illustrative purposes. Other types of antennas, including 
configurations which are not offset, can be employed in the practice of 
the invention. 
It is elemental to persons skilled in the antenna art that an antenna 
capable of supporting a large number of multiple beams must have a large 
effective focal length (MF) to diameter (D) ratio. A large ratio (i.e. 
MF/D.gtoreq.3) is achieved by an offset Cassegranian antenna of the type 
shown in FIG. 1. In one embodiment of the invention where the feedhorns 
are operated at a frequency of 12 GHz, the antenna parameters shown in 
FIG. 1 can comprise the following illustrative dimensions: 
Table of Antenna Dimensions 
F=D=426.72 cm. 
f'=336.88 
f=89.84 
h=133.28 
H=346.64 
S=497.12 
s=109.47 
s'=356.52 
R=132.08 
From the above table of illustrative antenna dimensions it can be seen that 
the focal length F of the main reflector 11 is equal to the aperture 
diameter D, approximately 4.27 meters. The magnification of subreflector 
12 is defined by the formula M=f'/f, which in this embodiment equals 3.75. 
From these basic choices, MF/D=3.75 and the parameters S, s, s' can be 
derived using the basic Cassegranian equations which are found in 
"Microwave Antennas Derived from the Cassegranian Telescope", by Peter W. 
Hannon, IRE Transactions on Antennas and Propagation, 9, No. 2 (March 
1961), pages 140-153. 
FIG. 2 illustrates the back view of an exemplary configuration of the feed 
element array disposed on focal surface 13 shown in FIG. 1. In FIG. 2, 
each plus sign (+) identifies one feed element. The exemplary array 
contains a plurality of feed elements in a configuration 196.7 centimeters 
wide by 104.8 centimeters tall. Other embodiments may contain different 
numbers of feed elements in different array configurations. The diameters 
of the feed beams may be calculated using, for example, the Gaussian-beam 
equations found in an article entitled "Laser Beams and Resonators", by H. 
Kogelnik and T. Li, Applied Optics, Vol. 5, No. 10 (October 1966), pages 
1550-1567. For a seven-element cluster producing one beam, it has been 
calculated that the -3 dB contour at the focal surface has a diameter of 
10.0 centimeters, or approximately 4 wavelengths at 12 GHz. Applying 
standard Gaussian-beam equations found in the hereinabove mentioned 
Kogelnik and Li reference, it is found that the seven-element feed cluster 
is bounded by the -20.5 dB contour at the waist of the feed beam, which 
contour has a diameter of 26.2 cm. Since the seven-element clusters are 
each essentially three feed elements in diameter, the contour diameter is 
divided by three thereby determining that the center-to-center spacing of 
the individual elements in the seven-element feed clusters is 8.73 
centimeters. Applying methods of mathematical analysis known to persons 
skilled in the art, it is determined that all parts of the contiguous 
United States can be covered using 218 feed elements in the exemplary feed 
array as shown in FIG. 2. The overall configuration of the array resembles 
the shape of the predetermined zone on the surface of the celestial body, 
which in the exemplary configuration of FIG. 2, is the United States. As a 
result of image inversion, feed element 30 near the upper left-hand corner 
of the exemplary array illuminates an area in the southeastern United 
States, and feed element 31 at the bottom of FIG. 2, illuminates an area 
in the northern United States. Typical seven feed element groups (32 and 
33) for radiating fixed high communication traffic spot beams are shown 
for Los Angeles and San Francisco, respectively. A 12 element group 34 is 
shown for covering New York, Boston and Albany, and a 10 element group 35 
is shown for covering Chicago and Detroit. It can therefore be seen that 
individual high traffic city coverage spot beams are formed from 
advantageously configured numbers of feed elements. 
Areas having low communication traffic volume requirements in the 
predetermined zone, such as the central portion in FIG. 2, can be covered 
by a plurality of spot beams produced by a plurality of feed clusters, 
each cluster being comprised of seven feed elements as will be more 
clearly understood in the discussion relating to FIG. 3. However, as 
indicated above, the coverage zones of such area coverage spot beams are 
defined by -7 dB rather than -3 dB contours, as is the case with the high 
traffic city coverage spot beams. In accordance with the present 
invention, the high traffic city spot beams may utilize the full frequency 
spectrum and may be continuously energized. The low traffic area coverage 
spot beams, on the other hand, utilize, for example, one-quarter of the 
available bandwidth each, so as to permit low traffic area coverage spot 
beams of the same frequency to be more widely separated than high traffic 
city coverage beams. The frequency spectrum distribution among the low 
traffic area coverage spot beam is illustrated in the central portion of 
FIG. 2 wherein nine center elements 41-49 of nine feed clusters for nine 
respective area coverage spot beams are shown identified by dashed 
circles. The full frequency spectrum is divided into four subbands 
denominated f.sub.1, f.sub.2, f.sub.3, and f.sub.4. Center feed element 45 
in FIG. 2 launches electromagnetic energy within the frequency subband 
f.sub.1 as indicated in the dashed circle. The remaining eight dashed 
circles each launch electromagnetic energy in respective frequency 
subbands identified therein. Although only nine center feed elements are 
shown in FIG. 2, it is to be understood that many more such center feed 
elements may be distributed throughout the feed element array. Each such 
feed element will launch energy in a respective one of the four frequency 
subbands, which frequency subbands are distributed throughout the array in 
such a manner as to prevent adjacent feed element clusters from operating 
in the same frequency subbands. For example, center feed element 41 which 
operated in frequency subband f.sub.4 is separated from nearby center feed 
elements 43 and 47, which also operate in frequency subband f.sub.4 by 
center feed elements 42 and 44 which operate in frequency subbands f.sub.2 
and f.sub.3, respectively. 
FIG. 3 illustrates the configuration of plural seven-element feed clusters 
operating at frequencies f.sub.1, f.sub.2, f.sub.3, and f.sub.4, for 
contiguous low traffic area coverage spot beams. The center feed elements 
of the shown clusters are identified with the symbols which correspond to 
those used to identify the center feed elements in the central portion of 
FIG. 2. In FIG. 3, three clusters 50, 60, and 70, are each shown 
surrounded by a heavy curving line. Feed element 42 is the center element 
of cluster 50, feed element 45 is the center element of cluster 60, and 
feed element 48 is the center element of cluster 70. In addition, feed 
element 44 is the center element of the cluster of feed elements 
identified by horizontal stripes, and feed element 46 is the center 
element of the cluster identified by vertical stripes. The stripes are 
included to facilitate identification of the clusters of feed elements 
associated with center feed elements 44 and 46, and are not representative 
of direction of polarization. 
Feed element cluster 60 comprises center feed element 45 operating in 
frequency subband f.sub.1 and surrounding feed elements 61 through 66. 
Feed element 63 is shown to be shared between cluster 60 and cluster 70 
wherein center feed element 48 operates in frequency subband f.sub.2. 
Accordingly, feed element 63 is arranged to enable transmission of 
electromagnetic energy in both frequency subbands f.sub.1 and f.sub.2. In 
similar fashion, feed element 66, which is shared by clusters 60 and 50, 
is arranged to enable transmission of electromagnetic energy in both 
frequency subbands f.sub.1 and f.sub.2. Feed elements 61 and 62 are shared 
by cluster 60 and respective ones of the horizontally and vertically 
striped clusters. Each of these feed elements, therefore, launches 
electromagnetic energy in both frequency subbands f.sub.1 and f.sub.3. In 
this embodiment of the invention, none of the shared feed elements 
operates in more than two frequency subbands. Persons skilled in the art, 
however, can easily devise arrangements wherein each of the feed elements 
within a cluster is shared by more than two clusters, and consequently 
operates in more than two frequency subbands, without departing from the 
spirit and scope of the invention. 
In accordance with the present invention, all low traffic area coverage 
spot beams might advantageously launch electromagnetic energy of the same 
polarization direction irrespective of the frequency subband. Such 
polarization would advantageously be orthogonal to the polarization of 
high traffic city coverage spot beams. With such arrangements, low traffic 
area coverage spot beams are isolated from one another by frequency 
diversification, while low traffic area coverage spot beams are isolated 
from adjacent high traffic city coverage spot beams by polarization 
diversification. It is to be further noted that interbeam interference 
which would result from sidelobe radiation is reduced in such arrangement 
by energizing the center fee elements of each spot beam to a higher power 
intensity than the surrounding feed elements within the cluster. 
The hereinabove described exemplary embodiment is illustrative of the 
application of the principles of the invention. It is to be understood 
that, in light of this teaching, numerous other arrangements may be 
devised by persons skilled in the art without departing from the spirit 
and scope of the invention.