Telecommunications antenna intended to cover a large...

Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite

Reexamination Certificate

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C342S373000

Reexamination Certificate

active

06650281

ABSTRACT:

The invention relates to a telecommunications antenna which is installed on a geosynchronous satellite and is intended to relay communications over an extensive territory.
BACKGROUND OF THE INVENTION
A geosynchronous satellite which carries a send antenna and a receive antenna, each of which has a reflector associated with a multiplicity of radiating elements or sources, is used to provide communications over an extensive territory, for example a territory the size of North America. In order to be able to re-use communications resources, in particular frequency sub-bands, the territory to be covered is divided into areas and the resources are assigned to the various areas so that when one area is assigned one resource adjacent areas are assigned different resources.
Each area has a diameter of the order of several hundred kilometers, for example, and its extent is such that, to provide a high gain and sufficiently homogeneous radiation from the antenna in the area, it must be covered by a plurality of radiating elements.
FIG. 1
shows a territory
10
covered by an antenna installed on board a geosynchronous satellite and n areas
12
1
,
12
2
, . . . ,
12
n
. This example uses four frequency sub-bands f
1
, f
2
, f
3
, f
4
.
The area
12
i
is divided into several sub-areas
14
1
,
14
2
, etc. Each sub-area corresponds to one radiating element of the antenna.
FIG. 1
shows that some radiating elements, for example the radiating element
14
3
at the center of the area
12
i
, correspond to only one frequency sub-band f
4
, while others, like the radiating elements at the periphery of the area
12
i
, are associated with a plurality of sub-bands, i.e. the sub-bands which are assigned to the adjacent areas.
FIG. 2
shows a prior art receive antenna for a telecommunications system of the above kind.
The antenna includes a reflector
20
and a plurality of radiating elements
22
1
, . . . ,
22
N
close to the focal plane of the reflector. The signal received by each radiating element, for example the element
22
N
, is passed first through a filter
24
N
intended in particular to eliminate the (high-power) send frequency, and then through a low-noise amplifier
26
N
. The signal at the output of the low-noise amplifier
26
N
is split into several parts by a splitter
30
N
, possibly with coefficients that can differ from one part to another; the object of this splitting is to enable a radiating element to contribute to the formation of more than one beam. Thus an output
32
1
of the splitter
30
N
is assigned to an area
34
p
and another output
32
i
of the splitter
30
N
is assigned to another area
34
Q
.
The splitters
301
, . . . ,
30
N and the adders
36
P
, . . . ,
36
q
intended to define the areas are part of a device
40
referred to as a beam or pencil beam-forming network.
The beam-forming network
40
shown in
FIG. 2
includes a combination of a phase-shifter
42
and an attenuator
44
for each output of each splitter
30
i
. The phase-shifters
42
and the attenuators
44
modify the radiation diagram, either to correct it if the satellite has suffered an unwanted displacement or to modify the distribution of the terrestrial areas.
Also, each low-noise amplifier
26
N
is associated with another low-noise amplifier
26

N
which is identical to it and which is substituted for the amplifier
26
N
should it fail. To this end, two switches
46
N
and
48
N
are provided to enable such substitution. It is therefore necessary to provide telemetry means (not shown) for detecting the failure and telecontrol means (also not shown) to effect the substitution.
An antenna system of the type shown in
FIG. 2
includes a large number of low-noise amplifiers, phase-shifters and attenuators. A large number of components is a problem on a satellite because of their mass. Also, a large number of phase-shifters
42
and attenuators
44
causes reliability problems.
OBJECTS AND SUMMARY OF THE INVENTION
The invention significantly reduces the number of low-noise amplifiers, phase-shifters and attenuators.
To this end, a receive antenna according to the invention includes:
at least one first Butler matrix, each input of which receives the signal from a radiating element and each output of which is associated with a low-noise amplifier in series with a phase-shifter and preferably with an attenuator,
a second Butler matrix which is the inverse of the first Butler matrix and has a number of inputs equal to the number of outputs of the first Butler matrix and a number of outputs equal to the number of the inputs of the first Butler matrix, the outputs of the second Butler matrix being combined to form the area beams, and
control means for controlling the phase-shifters and, where applicable, the attenuators, to correct or modify the beams.
In a Butler matrix, which is made up of 3 dB couplers, the signal at each output is a combination of the signals at all the inputs, but the signals from the various inputs have a particular phase, different from one input to another, so that the input signals can be integrally reconstituted, after passing through the inverse Butler matrix, followed by amplification and phase-shifting, and where applicable attenuation.
The number of outputs of the first Butler matrix is preferable equal to the number of inputs. In this case, the number of low-noise amplifiers is equal to the number of radiating elements, whereas in the prior art, as shown in
FIG. 2
, the number of low-noise amplifiers is twice the number of radiating elements. Furthermore, the number of phase-shifters is also equal to the number of radiating elements, whereas in the prior art the number of phase-shifters and attenuators is significantly greater, because the output signal of a radiating element is split and the phase-shifting and the attenuation
42
,
44
are applied to each channel of the beam-forming network.
Controlling the phase-shifters in series with the low-noise amplifiers to correct or modify the beams is particularly simple in a receive antenna according to the invention.
Because Butler matrices are used, if a low-noise amplifier fails the signal is reduced uniformly at all the outputs.
To reduce the effect of an amplifier failure on the output signals, in one embodiment the low-noise amplifier which is associated with each output of the first Butler matrix includes a plurality (for example a pair) of amplifiers in parallel, for example interconnected by couplers. In this case, the degradation due to failure of only one of the two amplifiers of a pair is half or less than that if a single amplifier were associated with each output.
It can be shown that the degradation is equal to −0.56 dB if 8
th
order Butler matrices are used with a pair of amplifiers in parallel associated with each output. The degradation is −0.28 dB with 16
th
order Butler matrices and with a pair of amplifiers associated with each output of the first Butler matrix.
One embodiment uses a plurality of associated two-dimensional matrices, for example matrices in different planes, so that each signal received by a radiating element is distributed over n×n low-noise amplifiers, n being the order of each two-dimensional matrix. In one example n=8 and in this case each signal received by a radiating element is distributed over 64 low-noise amplifiers. In this example, if only one amplifier is associated with each output, failure of one amplifier leads to a loss of only −0.14 dB.
The invention equally applies to a send antenna with a similar structure. In this case, the inputs of the first Butler matrix receive signals to be sent and the outputs of the second Butler matrix are connected to the radiating elements. Power amplifiers are provided for send antennas instead of low-noise amplifiers, of course.
In one embodiment that applies to sending and receiving, one of the Butler matrices and the beam-forming network constitute a single device.
It is already known in the art to use a structure with two Butler matrices for send antennas in order to distribute the

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