Note: Descriptions are shown in the official language in which they were submitted.
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-1- RCA 81,671
LINEARLY POLARIZED GRID REFLECTOR ANTENNA SYSTEM
WITH IMPROVED CROSS-POLARIZATION PERFORMANCE
This invention relates to a linearly poLarized
reflector antenna system and, more particularly, to a
orthogonally polarized dual gridded reflector antenna
system fed by feed horns. Such a dual gridded reflector
system includes a first reflector comprising a first grid
of linear conductors in one orientation to match the
polarization from a first of the feed horns. The first
reflector overlaps a second reflector comprising a second
grid of linear conductors, which are oriented orthogonal
to the one orientation and to match the orthogonal
polarization from a second of the feed horns.
An antenna system of this type, where each of
the first and second feed horns comprises an array of
horns, finds wide use in com~unication satellites for
achieving shaped beams with frequency reuse by orthogonal
linear polarization. It is desirable in such an
application that the antenna be compact and light weighk.
Each of the reflectors is a section of a paraboloid of
revolution with a grid of closely spaced parallel
conductors or elements which are oriented parallel to one
of two orthogonal linear polarizations. The reflectors
are disposed so that their axes are offset from and
parallel to each other. The grid o~ conductors in each of
these sections or parabolic dishes is arranged so the
conductors appear parallel at a distance in the direction
of propagation, in the direction of the parallel axes.
The horns are located at the focal points of the
correspondiny reflectors. An example of such a system is
described in U.S. Patent No. 3,898,667 of Raab.
The reflectors of the prior art antenna system,
as illustrated in the above-identified Raab patent, with
its offset reflector axes and focal points, cause
cross-polarized (unwanted) signals from the parallel
elements in the surace of one reflector to be scattered
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away from khe feed horn/copolariæed (wanted) signal
associated with t.he other beam reflector.
It is desirable to find a low cost, light weight
means for further improving the cross-polarization
rejection of such an antenna system.
In accordance with one embodiment of the present
invention, in a spectrum reuse antenna system comprising:
a first reflector, which contains a first grid of linear
conductors of one orientation, overlapping a second
reflector, which contains a second grid of a second,
orthogonal orientation, and feed horns or arrays of feed
horns with corresponding polarizations at or very near
respective offset focal points of these reflectors, the
improvement for providing improved cross polarization
performance comprises: a gri.d of linear conductors
adjacent the aperture of each of the horns r with the
aperture grid conductors oriented orthogonal to the
conductors on the reflector associated with the particular
feed horn.
DESCRIPTION OF THE DRAWING
FIGURE 1 is a front view of the dual gridded
antenna system for use on a satellite in accordance with
one embodiment of the present invention;
FIGURE 2 is a view of the reflector structure of
Figure 1 taken along lines 2-Z of Figure l;
FIGURE 3a is a view o~ the horizontally
polari2ed array of horns as seen from the reflectors;
FIGURE 3b is a view of the vertically polari~ed
array of horns as seen from the re1ectorsi
FIGURE 4 illustrates isolation contours without
grids over the horns;
FIGURE S illustrates isolation contours with
grids over horns in accordance with the present invention;
FIGURE 6 illustrates the co-polarized measured
patterns for the beams of Figure 4 without the grids in
front of the horn; and
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FIGURE 7 illustrates the co polarized measured
patterns for the beams of Figure 3 with the grids in front
of the horn.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGURES 1 and 2 illustrate an antenna system 11
mounted to a wall of a satellite 10. The antenna system
11 comprises a parabolic reflector 13 comprising a para-
bolic shaped dielectric dish and a grid of horizontally
polarized conduckors 13a with the reflector 13 mounted in
an overlapping manner over a second parabolic reflector 15
comprising a parabolic shaped dielectric dish and a grid
of vertically polarized conductors 15a.
In Figure 1 part of the forward reflector 13 is
cut away to show the rear reflector grid 15. The antenna
system 11 mounted to the ona surface lOa of the main
satellite body lo by mounting posts 17, 18, 19 and 20.
These posts mount the rear parabolic reflector 15 to the
sur~ace lOa of the main satellite body ~0. a forward or
outboard parabolic reflector 13 is mounted to the
reflector 15, for example, by a peripheral dielectric ring
16 which extends between the reflectors 13 and 15. These
reflectors may be further mounted to each other via
stiffening ribs. A typical example of such stiffening
ribs is described in Canadian Patent No. 1,245,759, issued
November 2~, 1988 to PareXh and entitled "~ual Gridded
Reflector Structure".
The reflectors 13 and 15 are sections of a
paraboloid of re~olution where the vertex of the
reflectors lies near the bottom edge as illustrated in
FIGURE 2. The vertex for the parabolic reflec~or 13 is
located at V1 and the vertex for the second raflector 15
is located at V2. The two pa~abolic reflectors are
mounted offset from each other such as shown by the vertex
points V1 and V2, and the focal axes are slightly o~fset
and parallel to each other. This is in correspondenca
with the above-identified Raab patent, so that the
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cross-polarized fields generated by the parallel elements
on a surface of each reflector from one or more feed horns
of an array associated with that reflector are scattered
away from the copolarized beam of the other reflector.
The antenna system includes beam-shaping feed
horn arrays 21 and 23 as illustrated in Figures 3a and 3b
for respective polarizations. The offset between the two
reflectors is sufficient to assure enough space around
each of the focus points to accomodate both horn arrays.
The horizontally polarized horn array 21 is
located at the focus Fl of the parabolic reflector 13,
which includes the horizontal grid conductors 13a. See
Figures 2 and 3a. The array 21 is generally centered with
respect to the focus point Fl as illustrated in Figure 3a.
The vertically polarized array of horns 23 is located at
focus point F2 for the reflector 15 that includes the
vertical grid conductor 15a. The array 23 is generally
center~d with respect to focus point F2 as illustrated in
Figure 3b. Each array of horns is directed to point to
the center of a respective reflector, in order to fully
illuminate the reflector primarily at its center.
Each of the horizontally polarized feed horns of
array 21 is adapted for radiating and receiving
horizontally polarized RF signals. Similarly, each of the
vertically polarized feed horns of array 23 is adapted for
radiating and receiving vertically polarized RF signals.
Figures 3a and 3b also show the input waveguides at the
throats of the horns. The input waveguides 22 of horns in
array 21 are narrower in width than in height so as to
propagate signals with the E fields perpendicular to the
vertical broad surfaces and henca excite horizontally
polarized signal waves. The input waveguides 24 of horns
in array 23 are broader in width than in height so as to
excite vertically polarized signal waves. The arrays 21
and 23 are mounted to the satellite body 10 by support arm
30 in Figure 2.
~ ccording to the teachings of the present
invention, a grid 21a of vertical conductors is placed
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across the aperture of the array of horizontally polarized
horns 21. Thus, grid 21a is adjacent each horn in array
21. See Figures 2 and 3. This grid 21a of vertical
conductors (shown in Figure 3a) may be provided by a sheet
of dielectric such as mylar or other dielectric material
with shallow depth conductors printed or otherwise formed
thereon. These conductors can be, for example,
0.0003-inch thick copper foil. The sheet may be mounted
to a frame 21b, which is fixed to the horn support arm 30
via flange 31 such that the sheet is held flat against the
horns. See Figure 2.
The spacing between the conductors on the grid
21a and the si~e of the conductors is like that of the
grids 13a and 15a on the reflectors 13 and 15. This is
arranged so as to couple signals between the horn and
reflector 13 with low attenuation and yet provide optimum
rejection of vertically polarized signals from the grid of
conductor 15. The width of the conductors is, for
example, 0.003 inch and the center to center spacing of
the conductor is 0.02 inches for one tested embodiment at
frequenci~s of 11 and 14 GHz. The geometry of the grids
(spacing and width) is selected to provide the best
comprise between the lowest transmission loss with the
copolarized component and the highest reflectiv.ity for the
cross-polarized component.
Across the aperture of the vertically polariæed
horn array 23 is a grid 23a o horizontally orientated
conductors that are of the same spacing and width as the
conductors in the reflectors 13 and 15. Again, grid 23a
i5 adjacent each horn in array 23. See Figure 3b. This
grid, again, .is preferably a sheet of dielectric such as
mylar with the shallow depth conductors printed or
otherwise formed thereon. The sheet may be mounted to a
frame 23b which is fixed to the horn support arm 30 via
flange 33.
The cross-polariæation rejection has been
experim~ntally shown to improve when using grids at the
aperture of the horn feed arrays. Figure 4 illustrates
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this cross-polarization effect without the grids at the
horns for a western-half United States heam and Figure 5
illustrates the reduction in cross-polarizakion effects
with the grids at the horns. What is plotted in each of
these figures is the copolarization minus the
crosspolarization -- i.e., the isolation contours. The
operating frequency is 11.76 GHz. The 44db, 40db, 36db
and 32 db isolation contours are plotted in both figures.
To illustrate the improvement by mounting the grids in
front of the horns the followiny observations are made:
1). In Figure 4 for the case where there is no
grid in front of either horn, the isolation is worse than
32 db for most of the western United States, as
represented by the area outside all of the dashed
lS contrours;
2). In Figure 5 where grids are used in front
of the horns, the isolation is better than 32db over the
western part of the United States and is 36db over a major
portion of the coverage area.
Further, Figures 6 and 7 show the co-polarized
measured patterns for the two beams with and without the
srids in front of the horns. These indicate that the
grids in front of the horns do not substantially affect
the co~polarized signal.
