Note: Descriptions are shown in the official language in which they were submitted.
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MULTI BEAM ANTENNA WITH REDUCED SIDE LOBES
Technical Field
The present invention relates to a multi beam
antenna with reduced side lobes and, more particularly, to a
multi beam earth-station antenna comprising multiple feeds
and a network arrangement disposed at the outputs of the
feeds for coupling to the output path of each feed of
interest a signal from each other feed aiming a beam in the
same direction as a side lobe which is above a predetermined
side lobe level at the feed of interest, thereby
substantially reducing the effects of interfering signals
from other beams at the feed of interest.
Background of the Invention
In a circularly symmetric antenna for use at, for
example, an earth-station, one of the chief causes of
enlarged side lobes is beam blockage. Blockage, in turn, is
caused by obstacles located in front of the main reflector.
Typical obstacles are feeds and struts in a front-fed
design, or a sub reflector and struts in a Cassegrainian
design.
In multi beam antennas, a single feed is normally
used for each beam Feeds in, for example, an earth-
station antenna are located such that the beam from each
separate feed is aimed at, for example, a separate
satellite, which is closely spaced on the geosynchronous
arc with other satellites associated with beams from other
feeds. The main signal passing through each feed is
intended for transmission to, or reception from, only one
satellite. However, side lobes of a given beam generally
point towards adjacent satellites Consequently, a small
part of each main signal is inadvertently radiated toward
the wrong satellites. Conversely part of the signal
radiated by each satellite, which is intended for just one
associated feed of an earth-sta~ion antenna, is
inadvertently received by adjacent feeds. Such inadvertent
transmission and reception is defined as interference.
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Various techniques have been devised to reduce
side lobes. For example, to provide separation for o'er-
lapping area coverage and spot beams in the far field of
an antenna, the transmitting antenna includes networks
which couple a portion of the area coverage signal into
each of the spot beam feeds to provide cancellation in a
selected area in the far field of the antenna. In this
regard see, for example, US. patent 4,145,658 issued to
A. Acampora et at on March 20, 1979.
Another technique for reducing side lobes in a
single beam antenna is to provide, for the principal feed
of interest, means in the plane of the side lobes of the
principal feed to suppress such side lobes only. Such
means can comprise two auxiliary feeds and associated
networks, in the plane of the side lobes to be suppressed,
for launching separate beams of the signal being launched
by the principal feed whereby the central ray of each beam
impinges the same point on the main reflector as the
central ray of the principal feels beam as shown in US.
patent 4,364,052 issued to E. A. Ohm on December 14,
1982. Such means can also comprise suppression means
disposed adjacent and on symmetrically opposite sides of
the main antenna in the plane of the side lobes to be sup-
pressed at approximately the width of the aperture of the
main reflector. The suppression means can alternatively
comprise separate auxiliary antennas ox subsections of the
main reflector as disclosed in US. patent 4,376,940 issued
to H. Madame on March 15, 1983.
Still another technique is the use of adaptive
interference suppression arrangements as disclosed, for
example, in US. patent 4,320,535 issued to D. M. Brady et
at on March 167 1982. There a main antenna picks up the
desired and interfering signal while a small auxiliary
antenna essentia71y picks up only the interfering signal.
A feedback arrangement adaptively adjusts the phase and
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amplitude of the interfering signal at the output of the
auxiliary antenna to cancel the interfering signal in the
overall signal received by the main antenna.
The problem remaining in the prior art is to
provide a ~ultibeam antenna capable of receiving beams
from a plurality of remote transmitters at separate feeds
with reduced interference at each feed using simple and
cost effective means for new antennas or for retrofitting
existing antennas.
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The foregoing problem in the prior art has been
solved in accordance with the present invention which
relates to a multi beam antenna with reduced side lobes
in directions served by each of the feeds and, more
particularly, to a multi beam earth-station antenna
comprising multiple feeds and a network arrangement
disposed at the outputs of the feeds fur coupling to the
output path of each feed of interest a signal from each
other feed that aims a beam in the same direction as a
side lobe which is above a predetermined side lobe level
at the feed of interest, thereby substantially reducing
effects of interfering signals from nearby satellites at
the feed of interest. The network arrangement functions
to couple a weighted signal, which is 180 degrees out of
phase from the output of each other feed, to the output
of the feed of interest when each side lobe of the feed of
interest exceeds a predetermined side lobe level in the
direction served by one of the other feeds.
In accordance with an aspect of the invention
there is provided a multi beam antenna feed arrangement
comprising: a plurality of N feeds capable of receiving
signals from a plurally of N remote, spaced-apart,
transmitters and transmitting the received signals along N
separate associated paths characterized in that the feed
I
arrangement further comprises: network means comprising a
plurality of at least N cross-coupling paths Norway Lo is an
even integer, or N-l cross-coupling paths where is an
odd integer, each cross-coupling path including means for
providing a predetermined phase and amplitude adjusted
version of a signal propagating in an output path of an
associated first one of the plurality of N feeds to an
output path of an associated second one of the plurality
of N feeds where a side lobe associated with the second one
of the feeds exceeds a predetermined side lobe level in a
beam direction associated with the first one of the reeds
for substantially reducing interference from the signal
destined for the associated first one of the feeds which
is also received by the associated second one ox the feeds
via the side lobe associated with the second one of the
feeds.
It is an aspect of the present invention to
provide a multi beam antenna comprising an interference
reducing network at the output of the Leeds of the
multi beam antenna. The network functions to couple to a
feed of interest, a weighted signal 180 degrees out of
phase from the output line of each other feed, where each
side lobe of the feed of interest is above a predetermined
side lobe level in the direction served by one of the other
feeds. In general, in an N-beam antenna, maximum of N-l
cross-coupling paths are needed to couple to each feed of
interest signals from feeds that aim teams in directions in
which swaddles ox a feed of interest exceed predetermined
levels. However, in a particular embodiment, for an Beam
antenna, where N is an even integer then a total of only
cross coupling paths are needed, while where N is an odd
integer then a total of only N-l cross-coupling paths are
needed.
It is a further aspect of the present invention
to provide an antenna feed arrangement or use with either
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front-fed or subreflector-included antennas wherein the
antenna employing the feed arrangement can be used to
receive signals from closely spaced satellites in
geosynchronous orbit with reduced interference at earn
feed associated with a separate one of the satellites.
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.
Brief Description of the Drawings
Referring now to the drawings, in which like
numerals represent like parts in the several views:
It. 1 is an illustration of an exemplary
display of principal lobes and certain side lobes of beams
associated with an exemplary four-beam antenna where feeds
?3~j
are not located on the foresight axis and feeds aim beams
at separate satellites
FIG. 2 is a block diagram of an exemplary four-
beam antenna feed arrangement with a side lobe reducing
network in accordance with one aspect of the present
invention which couples to a feed output of interest a
; separate weighted and phase shifted signal from each of tic
other feed outputs; and
FIG. 3 is a block diagram of an exemplary four-
beam antenna feed arrangement including a side lobe reducing network in accordance with the present invention which
cross-couples to each feed of interest, a weighted and
phase shifted signal from only one adjacent feed where each
feed of interest has a side lobe above a predetermined level
in the direction served by the adjacent feed.
Detailed Description
The present invention is described hereinafter
with regard to an exemplary four-beam antenna feed
arrangement to cause reduced side lobe interference at each
of the four feeds. It is to be understood that such
arrangement is described herein for purposes of exposition
only and not for purposes of limitation and that the
multlbeam feed arrangement can comprise any arrangement of
two or more feeds and still fall within the spirit and
scope of the present invention. The present multi beam
antenna feed arrangement is designed for use with either
front-fed or subreflector-included antennas. In such
antennas, each feed is positioned on the focal surface of
the antenna to receive signals either from a separate one
of a plurality of satellites positioned near each other on
the Geosynchronous Equatorial Arc GUY) or from a separate
one of a plurality of terrestrial transmitters within the
field of view of the antenna.
As was described herein before, in multi beam
antennas, interfering signals are characterized by low
antenna gain. However, each signal is also available with
high antenna gain at the principal feed of each beam. or
example ! in FIG. 1, an interfering signal from satellite 12
can enter feed 1 via the third side lobe 10 of the beam
associated with feed 1. Simultaneously, a relatively large
signal from satellite 12 is available at feed 2. In a
similar manner an interfering signal from satellites 13 and
14 can also enter feed 1, but because tune side lobe Lyle of
the beam associated with feed 1 is very low at feeds 3 and
4, these interfering signals will be very low compared to
the interfering signal from satellite 12. A similar
explanation can be provided for the interfering signals at
feeds 2-4 from all satellites 11-14 except the one which is
the principal source of signals for each feed of interest.
In accord with the present invention, the above-
described interference at each feed can be substantially
reduced by the arrangements shown in Figs 2 and 3. In
FIG. 2, feeds 1-4 are connected at their outputs, via
separate wave guide means 6-9, respectively, to respective
optional low-noise amplifiers 25-28 which, in turn, are
connected to the respective receivers associated with feeds
1-4. disposed between feeds 1-4 and optional amplifiers
25-28 is a network arrangement according to the present
invention for substantially reducing interfering signals
from other than the principal satellite 11, 12, 13 or 14 of
each respective feed 1, I 3 or 4 of interest via an
associated side lobe exceeding a predetermined level in each
direction served by another feed.
In Figs 2 and 3, it is desired, inter alias to
receive in the output path of feed 1 only the signal from
satellite 11. As described hereinabove, interference
signals are also picked up by feed 1 from transmissions
from satellites 12-14 via sîdelobes of the beam associated
with feed 1, as shown in FIG. 1. The arrangement of FIG.
2 is designed to substantially reduce at each feed J
interference from all satellites which are not the primary
satellite of interest for each feed. However, only the
network means for substantially reducing side lobe
interference at feed 1 is shown for simplicity. It is to
f43~
be understood that similar additional network means are
needed for each of feeds 2-4 to substantially reduce
interference at these feeds. In FIG. 2, interference in
the output of feed 1 from satellite 12 is substantially
reduced by means of a passive cross-coupling path
comprising an amplitude and phase adjusting means 16, a
directional coupler 17 disposed in wave guide means 7
associated with feed 2, and a directional coupler 18
disposed in wave guide means 6 associated with feed 1.
Directional couplers 17 and 18 are disposed to transmit a
portion of the signal propagating in wave guide means 7
received by feed 2 through the amplitude and phase
adjusting means 16 and into wave guide means 6 associated
with feed 1. For purposes of illustration, the amplitude
and phase adjusting sections of means 16 have been labeled
Aye ~21 to indicate amplitude adjustment (A) and
phase adjustment (~) from the output of feed 2 to the
output of feed 1.
Three essential steps are: (a) part of the
signal originating at satellite 12 and received with high
earth-station antenna gain at feed 2 is coupled out of
wave guide means 7 of feed 2 via directional coupler 17 (b)
amplitude and phase of the coupled signal are adjusted by,
for example, attenuator Aye and phase shifter ~21
in means 16, and (c) the adjusted signal is coupled into
wave guide means 6 of feed 1 via coupler 18. Reduction of
interference of the signal from satellite 12 in the
wave guide means 6 associated with feed 1 is achieved by
initially adjusting the amplitude to be somewhat less than,
and the phase to be 180 degrees out of phase with the
interfering signal from satellite 12 arriving at feed 1 via
the pertinent side lobe of the beam associated with feed 1,
which in the exemplary illustration of FIX 1 is the third
side lobe 10. Similarly. an unlink signal propagating in
wave guide means 6 associated with feed 1 can be reduced in
the direction of satellite I However if unlink and
down link frequencies are substantially different, a second
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Aye ~21 network that is in parallel with the
first network, and which has different Aye ~21
settings, is needed to accommodate the somewhat different
side lobe of an unlink beam.
Extension of this technique allows other
interfering signals due to side lobes of the beam associated
with feed 1 pointing toward satellites 13 and 14 to also be
reduced by the addition of a directional coupler 20 in
wave guide means 8 associated with feed 3 which is connected
to an amplitude AYE) and phase adjusting (~31)
means 21 r and a directional coupler 23 disposed in
wave guide means 9 associated with feed 4 which is connected
to an amplitude (Aye) and phase adjusting (~41)
means 24. The outputs from amplitude and phase adjusting
means 21 and 24 are introduced into wave guide means 6,
associated with feed 1, by either directional coupler 18 or
by separate directional couplers (not shown) disposed in
wave guide means 6 associated with feed 1. Again side lobes
associated with the beam of feed 1 pointing towards
satellites 13 and 14 can be reduced by proper adjustment of
the amplitude and phase of a cross-coupled signal coupled
out of means 20 and 23, respectively, and introduced into
wavegulde means 6. Such adjustments may require a minor
readjustment of the amplitude and phase in means 16 to
maintain a given level of reduced interference from
satellite 12.
The network portion shown in FIG. 2 reduces
interfering signals only received in the output of feed 1.
A similar network is needed to reduce interference in each
of the other outputs of feeds 2-4 caused by side lobes from
each of the associated beams Networks such as the one
shown in FIG. 2 can be implemented in different ways. Fur
example, the wave guide means 6 9 from feeds 1-4,
respectively, and the associated directional couplers, can
be short in length and made of low-loss wave guide. This
minimizes the noise temperature of each receiver. However,
a somewhat larger loss can be tolerated in pathways that
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cross-couple energy between wave guide mean 6-9.
Consequently, adjustable attenuators (A), adjustable phase
shifters (~) and connecting lines between the associated
directional couplers and amplitude and phase shifting means
can be coaxial, thereby resulting in a network which is
more compact ! and easier to assemble, than a network
composed entirely of wave guide parts.
In FIG. 2, cross-coupled energy can be
increased, to allow for further reduction of interference,
by increasing the coupling of each directional coupler.
However, such increase of coupling can degrade receiver
noise temperatures significantly. This problem can be
avoided by inserting an identical low-noise amplifier 25-28
at the output of each feed as shown in the arrangement of
FIG. 3 rather than as shown in the arrangement of FIG. 2.
Modest losses in subsequent feed lines Jo and from the
transmitter and receiver can then be tolerated. Although
the herein before description has been directed to radio
frequency (RF) interference reduction, it is to be
understood that similar results can be achieved at
intermediate frequencies IF) if etch low-noise amplifier
25-28 in the arrangement of FIG. 2 is disposed directly at
the output of each feed 1-4 as shown in FIX. 3, rather than
after the network arrangement as shown in FIG. 2, and the
amplifiers are equipped with an RF-to-IF converter. It is
to be understood that with such RF-to-IF conversion, the
cross-coupling network portions should also be designed to
operate at IF.
Side lobes shown in FIG. 1 assume that beam
blockage is negligible. Actually, blockage can be
significant resulting in an additional component of the
antenna pattern known as a "blockage pattern". In
comparison to each main Lowe each blockage pattern (not
shown) is (a) very wide, by low in E-field amplitude, I
opposite in E-field sign, and Ed) centered about each main
lobe. ~ur~hermore, even-numbered side lobes of each
beam have the same R-field sign as the main lobe, odd-
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numbered side lobes have the opposite Elude sign Adding
such E-fields algebraically odd-numbered nearside lobes
are larger and even-numbered side lobes smaller than the
ones shown in FIG. 1. Adjacent side lobes that are near
each main lobe are opposite in sign because they are larger
in amplitude than the blockage antenna pattern.
Conversely, adjacent side lobes that are far from each main
lobe (not shown) have the same sign because they are
smaller than the blockage pattern.
In FIG. 2, coupling of the beam associated with
feed 2 into the beam associated with feed 1 via directional
couplers 17 and 18 and amplitude and phase adjusting means
16 is also constant in sign. If the phase, ~21~ of
the signal coupled out of wave guide means 7 is adjusted
such that the main lobe of the beam associated with feed 2
is opposite in sign to the third side lobe 10 of the beam
associated with feed 1, then adjustment of the amplitude
Aye in means 16 is the equivalent to subtracting a
variably attenuated portion of the pattern of the beam
associated with feed 2 prom that of the beam associated
with feed 1.
Simultaneously, sides of the main lobe of the
beam associated with feed 2 add to the patterns of the
second and fourth side lobes of the beam of feed 1 because
both of these side lobes have the opposite sign. Effects on
other side lobes of the beam associated with feed 1 are
negligible because the gain of the beam associated with
feed in those side lobe directions is essentially zero.
Consequently, even when nearside lobes are enlarged by
aberrations and beam blockage, the side lobe showing the
greatest interference, e.g., the third side lobe 10 in
FIG. 1, can be reduced at least I dub.
For largest cancellation bandwidth, the principal
and cross-coupling path lengths associated with each
satellite are chosen to be substantially equal.
Moreover, if satellite 12 in FIG. 1 is located
between near side lobes, then Aye and ~21 can be
I
adjusted to achieve a deep null. The null, however, moves
toward the main lobe as frequency is increased This, in
turn, increases interference from satellite 12, and
illustrates that interference cancellation is bandwidth
limited even when principal and cross-coupling pain lengths
are substantially equal. Nonetheless, worst-case
interference at the edges of, for example, the 3.7-4.2 Go
band, can still be reduced about 5 dub it respect to
adjacent nearside lobe peaks. Additionally, suppose that
feed apertures are small, as needed to accommodate closely
spaced beams. Then the null moves more slowly as a
function of frequency. Consequently, worst-case
interference can probably be reduced another 5 do, thereby
giving a total reduction of 10 dub.
In a similar manner, further-out side lobes of the
beam associated with feed 1. which point in the same
directions as beams associated with feeds 3 and 4, can be
reduced by proper adjustment of amplitude and phase
adjusting means 21 and 24, respectively, of FIG. 2. If
further-out side lobes have the same sign, such adjustments
can reduce the amplitudes of adjacent sid~lobes
simultaneously. This is equivalent to substantial
cancellation of the beam-blockage pattern in directions of
beams associated with feeds 3 and 4. resulting side lobes
in such directions are approximately equal to those of an
antenna without blockage, and are small enough to be
neglected.
Worst interference is due to the higher
side lobes, which are located near the main lobe on the
foresight axis side of each off-boresight beam. These
side lobes are higher, relative to those of a foresight
beam due to the aberration called coma, and increase in
amplitude as a function of off-boresigh~ direction of a
beam. Thus, the side lobes of the beam associated with feed
1 in FIG. 1 pointing in the same general direction US the
beam associated with feed 2 are coma lobes, and these
- 0 side lobes Are larger than similar coma lobes snot shown) of
beam 2 associated with feed 2 which point in the same
general direction as the beam associated with feed 3
because the beam associated with feed 2 is closer to the
foresight axis of the antenna In contrast, coma lobes of
beam 1 associated with feed 1 which point in the directions
of beams 3 and 4 associated with Leeds 3 and 4,
respectively, are relatively small, and in most
applications are probably small enough to be neglected.
Similarly, coma lobes ox beam 2 associated with feed 2
pointing in the direction of the beam associated with feed
4 r and side lobes of beam 2 pointing in the direction of the
beam associated with feed 1, are probably small enough to
be neglected.
Assuming that such side lobes are small enough to
be neglected, the entire cross-coupling network can be made
relatively simple as, for example, shown in the arrangement
of FIG. 3. In such simpler arrangement according to the
present invention, only four cross-coupling paths are
needed and additionally, nearly all parts can be coaxial,
cross-coupling adjustments can be made outside the antenna
aperture, noise temperature is not degraded, and additional
amplifiers are not needed since amplifiers 25-28 shown are
required anyhow for normal signal reception. As shown in
FIG. 3, for an exemplary four-beam feed arrangement,
directional couplers 17 and 18 and amplitude and phase
adjusting means 15 (Aye, ~21j disposed between
wave guide means 7 and 6 used in the arrangement of FIG. 2
will still be required in the arrangement of FIG 3.
To reduce side lobe interference at feed 2 from
satellite 13 associated with feed 3, a cross-coupling path
including directional couplers 30 and 31, disposed in
wave guide means 8 and 7, respectively, and an amplitude and
phase adjusting means 32 (Aye ~32)
introduce an amplitude and phase adjusted version of the
signal propagating in wave guide means 8 into wave guide
means 7. Similarly, to reduce side lobe interference at
feed 3 from satellite 12 associated with feed I, a cross
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coupling path including directional couplers 34 and 35,
disposed in wave guide means 7 and I respectively, and an
amplitude and phase adjusting means 36 (Aye ~23)
are used to introduce an amplitude and phase adjusted
version of the signal propagating in wave guide means 7 into
wave guide means 8. Finally, to -reduce side lobe
interference at feed 4 from satellite 13, a cross-coupling
path including directional couplers 38 and 39, disposed in
wave guide means 8 and 9, respectively, and an amplitude and
phase adjusting means 40 (Aye, ~34)
introduce an amplitude and phase adjusted version of the
signal propagating in wave guide means 8 into wave guide
means 9.
It is to be understood that the above-described
embodiments are simply illustrative of the principles of
the invention. Various other modifications and changes
may be made by those skilled in the art which will embody
the principles of the invention and fall within the spirit
and scope thereof. For example, as was stated
herein before, bandwidth is limited by fre~uency-sensitive
changes on direction of the side lobes which point toward
the largest sources of interference. Nonetheless,
bandwidth is sufficient for, for example, 4 GHz receive-
only applications provided that the principal and cross-
coupling path lengths associated with each satellite readjusted to be substantially equal, e.g., as indicated in
FIG. 3 by the use of optional path length 41, or optional
path length 42, each associated with satellite 11.
Similarly, unlink interference suffered by adjacent
satellites can be also reduced across the 6 GHz band by
applying a second cross-coupling network to the same earth-
station antenna. In the arrangement of JIG, 3, with N
feeds, where N it an even number only N cross-coupling
paths are required. In similar arrangements where the
number of feeds is an odd number, only N-1 cross-
coupling paths may be needed.
