A multi-port amplifier system for multi-beam communication antennas in which redundancy is achieved without using RF switches. The multi-port amplifiers are divided into sets of stages arranged in tree fashion and the amplifier stages can be turned off without degrading port-to-port isolation. In one embodiment there are thirty-two input ports 1 through 32 and thirty-two corresponding output ports 1 through 32 wherein input port 1 is functionally connected to output port 1 and so on. An input signal to each of the input ports 1 through 32 is connected to a separate one of a first group of sixteen, four stage multi-port hybrid circuits. The outputs of the first group of multi-port hybrid circuits are connected through solid-state power amplifiers to a second group of sixteen, four stage multi-port hybrid circuits which are also labeled according to the input port numbers to which they are functionally connected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to multi-beam communication systems using a 
plurality of electromagnetic wave beams, and more particularly, to a 
communication transmitter including multi-port redundant amplifier 
configurations for amplifying a plurality of signals for transmission by 
multi-beam antennas. 
2. Background Art 
Modern communication satellite systems employ multi-beam antenna 
technology. One common method as shown in FIG. 1 for implementing a 
multi-beam antenna for a satellite 100 is to provide multiple primary 
input terminal feeds 1--1 through 1-N that illuminate a common reflector 9 
so that each feed forms a beam spatially distinct from that of every other 
feed. The other feeds are arranged so that the other spatially distinct 
beams 6-1 through 6-N will cover the desired area 5-1 through 5-N on the 
Earth's surface. Once the antenna design is determined the next step is to 
design the transmitter or power amplifier suite to drive it. 
The usual configuration for driving multiple feeds is to dedicate a power 
amplifier such as amplifiers 2-1 through 2-N respectively connected to 
input feeds 1--1 through 1-N and having output terminals 3-1 through 3-N 
connected to antenna 4. While this straight forward approach works it has 
drawbacks. In FIG. 1, the transmission output at each output terminal is 
limited by the output P of each of the amplifiers 2-1 to 2-N. That is to 
say, the path between each input terminal and the corresponding output 
terminal is constructed completely independently of the other paths. On 
account of this, for instance, even if the amplifiers 2-1 has a margin in 
its power thereto from the input terminal 1--1 and amplified thereby for 
output to the output terminal 3-1, the margin cannot be used for signals 
which are provided to the other output terminals. 
In multi-beam satellite communication, the number of carriers which each 
beam transmits varies with the amount of communication traffic in each of 
the service areas 5-1 through 5-N, therefore, it is necessary that each of 
the amplifiers 2.sub.1, 2.sub.2 . . . for the beams have a power 
amplification capacity large enough to sufficiently amplify the input 
signal when it is assigned the largest number of carriers. To meet this 
requirement, an expensive amplifier of a large power capacity must be 
prepared for each beam, but when the number of carriers assigned to the 
input signal is small, such capacity of the amplifier is not effectively 
utilized. Furthermore, even in the case where the amplifier has a margin 
in its output but another amplifier wants more output, the surplus power 
cannot be assigned to the beam of insufficient power. Since space borne 
equipment must be redundant, usually two amplifiers are provided for each 
feed, one active, one standby redundant. This essentially doubles the cost 
of the amplifiers. Also, each amplifier is totally independent, providing 
no means of power sharing or combining. The advantage of this approach is 
that since the power amplifiers are totally independent, they do not have 
to be matched for gain, phase or power output, nor must they track each 
other over time or temperature. This is most advantageous if the output 
stage consists of a single device. 
To maximize reliability, it is desirable to implement microwave power 
amplifiers with solid state devices. Solid state device characteristics 
generally require that multiple devices be used in the output stage, and 
sometimes in the driver stage of a solid state microwave power amplifier. 
The common method for implementing this approach is to use a hybrid tree 
to split the input into multiple paths to feed multiple solid state 
amplifiers and use a reciprocal hybrid tree to combine the outputs of the 
multiple stages into a single output. A typical example of this type of 
amplifier, referred to as a combined amplifier, is shown in FIG. 2. The 
implementation shown in FIG. 2 combines the capability of eight amplifier 
stages. Elements 110 are amplifier modules and elements 112 are ninety 
degree hybrids. One port of each of the hybrids 112 is terminated by a 
load resistor. If the amplifiers are well matched for gain and phase, and 
the hybrids are properly designed and implemented, then if Po is the power 
output capability of a single stage, the combined amplifier has a power 
output capability of eight Po less some small combining loss. 
A more general and flexible solution is to use a "Hybrid Transponder" or 
"Multi-port Amplifier" which is analyzed in the publication entitled "An 
Adaptive Multiple Beam System Concept" by Shunichiro Egami and Makoto 
Kawai, IEEE Journal on Selected Areas in Communications Vol SAC-5, No. 4, 
May 1987. 
The multi-port amplifier uses multiple amplifier modules as does the 
combined amplifier discussed previously. In the multi-port amplifier, the 
input and output hybrid trees are replaced by suitable multi-port hybrids. 
An embodiment of an eight port multi-port amplifier is shown in FIG. 3 and 
includes amplifier modules 114 and multi-port ninety degree hybrids 116. 
It can be shown that there is a one to one correlation between each input 
and its corresponding output for the multi-port amplifier. Specifically, 
if the amplifier stages in the multi-port amplifier are matched for gain 
and phase, exciting one input port of a multi-port amplifier will produce 
an output at only one unique corresponding output port, and insignificant 
output signal levels at the other output ports. Therefore, if different 
multiple beam antenna feeds are attached to different output ports, the 
beam can be switched simply by exciting the proper input port. This allows 
a single amplifier to be used for all feeds, providing maximum flexibility 
in the allocation of power output to each beam. Isolation between beams 
depends upon the gain and phase matching of the amplifier stages. 
Increasing the number of stages increases the output signal isolation in 
the presence of amplifier stage mismatch. Therefore, it is desirable to 
use many more ports than the required number of beams to gain tolerance 
for amplifier stage mismatch. 
Given the desirability of the multi-port amplifier, the implementation of 
redundancy becomes an important consideration. Given the large number of 
amplifier modules it does not appear efficient to simply switch in a new 
multi-port amplifier if a single module fails. This not only requires a 
large number of redundancy switches, but also fails to take advantage of 
the flexibility and graceful degradation characteristics of the Multi-Port 
Amplifier. 
FIG. 4 shows still another implementation of a multi-port amplifier 
circuit. In FIG. 4, T switches are used to switch in a standby redundant 
amplifier stage for either of two adjacent amplifier stages. This 
implementation requires four extra stages and twenty-four "T" switches. It 
is also complicated by the fact that all path lengths must not change when 
switching between an active and redundant stage. This solution has both 
high weight and high recurring touch labor costs and is undesirable. 
Background references that illustrate the state of the art include the 
following. 
U.S. Pat. No. 5,055,798 issued Oct. 8, 1991 to Heinzelmann entitled "Hybrid 
Matrix Amplifier Systems And Methods For Making Thermally-Balanced Hybrid 
Matrix Amplifier Systems" discloses a hybrid matrix amplifier system that 
yields balanced thermal loads with minimal input signal constraints and 
includes an input multi-port hybrid coupler system having outputs joined 
to a plurality of amplifiers, and an output multi-port hybrid coupler 
system joined to outputs of the amplifiers. The input multi-port hybrid 
coupler system, and the output multi-port hybrid coupler system, each 
include n stages where n is an integer equal to or greater than 1, with 
each stage including 2.sup.n-1 couplers. These coupler systems may also 
include strategically-placed phase shifters. Such systems also include N 
amplifiers where N is equal to 2.sup.n, with N inputs from the outputs of 
the input multi-port hybrid coupler system, and with the outputs of he 
amplifiers connected as inputs to the output multi-port hybrid coupler 
system. This configuration permits a division of the N amplifiers into two 
groups that each dissipates substantially the same quantity of heat 
energy. 
U.S. Pat. No. 5,033,108 issued Jul. 16, 1991 to Lockwood entitled "Signal 
Repeater Using Shared Amplification With Selectable Input/Output 
Connections" discloses a signal repeater assembly for a spacecraft that 
includes a channel selectively network (CSN) having a plurality of first 
input ports connected by selectively operable filter circuits to a 
plurality of first output ports to enable a signal at any selected first 
input port to be passed to any selected first output port. The CSN is 
connected to a shared power amplification module (SPAM) having a plurality 
of second input ports for receiving signals from the first output ports, a 
plurality of second output ports and a network of balanced amplifier 
devices operable to couple the second input ports to the second output 
ports so that any amplification of a signal received at any one of the 
second input ports is shared by all the amplifier devices to allow the 
total available amplification power to be used by any one or a combination 
of second output ports. 
U.S. Pat. No. 4,868,520 issued Sep. 19, 1989 to Terakawa et al. entitled 
"High-Frequency Synthesizing Apparatus" discloses a high-frequency power 
synthesizing apparatus having a plurality of distributors arranged in a 
plurality of stages, a plurality of amplifiers which are supplied with 
outputs from a final stage of said power distributors and which amplify 
these outputs to a predetermined level, and a power synthesizing unit 
including initial-stage power synthesizers supplied with outputs from said 
amplifiers and a final-stage synthesizer which outputs power to an 
external load. The final-stage power distributors and the initial stage 
synthesizers are constituted by 0.sup.0 -hybrid modules. The output levels 
of the amplifiers can be displayed on an external display. 
U.S. Pat. No. 4,831,619 issued May 16, 1989 to Rosen entitled "Satellite 
Communications System Having Multiple Downlink Beams Powered By Pooled 
Transmitters" describes a satellite communications system that employs 
separate subsystems for providing broadcast and point-to-point two-way 
communications using the same assigned frequency band. The broadcast and 
point-to-point subsystems employ an integrated satellite antenna system 
which uses a common reflector. The point-to-point subsystem achieves 
increased communication capacity through the reuse of the assigned 
frequency band over multiple, contiguous zones covering the area of the 
earth to be serviced. Small aperture terminals in the zones are serviced 
by a plurality of high gain downlink fan beams steered in the east-west 
direction by frequency address. A special beam-forming network provides in 
conjunction with an array antenna the multiple zone frequency address 
function. The satellite employs a filter interconnection matrix for 
connecting earth terminals in different zones in a manner which permits 
multiple reuse of the entire band of assigned frequencies. A single pool 
of solid state transmitters allows rain disadvantaged users to be assigned 
higher than normal power at minimum cost. The intermodulation products of 
the transmitters are geographically dispersed. 
U.S. Pat. No. 4,825,172 issued Apr. 25, 1989 to Thompson entitled "Equal 
Power Amplifier System For Active Phase Array Antenna And Method Of 
Arranging Same" discloses an equal power amplifier system for amplifying a 
plurality of signals for transmission by an active phase array antenna, 
and a compact plural level beam-forming network for forming a plurality of 
such signals, for example on excitation patterns for frequency scanned 
virtual beams, are disclosed. The amplifier system uses at most only a few 
sizes of power amplifiers to amplify efficiently numerous signals having 
significantly different amplitudes. This is accomplished by distributing 
the task of amplifying signal pairs composed of one large amplitude signal 
and one small amplitude signal to equally sized power amplifiers. The 
first hybrid coupler divides the two signals for input into the two power 
amplifiers. The second hybrid coupler receives the intermediate amplified 
signals from the power amplifiers and through constructive and destructive 
interference produces amplified output signals corresponding to the input 
signals. 
U.S. Pat. No. 4,780,685 issued Oct. 25, 1988 to Ferguson entitled 
"Composite Power Amplifier With Redundancy" describes a power combiner 
that includes an input power splitter driving a plurality of amplifiers. 
The amplifier outputs have Z.sub.0 output impedance and are coupled by 
transmission lines of Z.sub.0 characteristic impedance to the junction 
point of a loss less power combiner. Short-circuiting switches disconnect 
failed amplifiers from the combiner. 
U.S. Pat. No. 4,644,301 issued Feb. 17, 1987 to Hecht entitled "Redundancy 
System and Switching Network" discloses a redundancy switching system 
which provides four reserve devices is disclosed which is suitable for use 
with from eight to twelve channels. The switches in the input switching 
network of this redundancy system are connected in first and second 
sections. Each section includes a ring of switches and two switches 
connected as appendages to the ring. Only as many switches are needed to 
control the input connection to the redundant devices as there are 
redundant devices. A similar number of switches are needed to control the 
output connection of the active devices. 
U.S. Pat. No. 4,618,831 issued Oct. 21, 1986 to Egami it el. entitled 
"Power Amplifying Apparatus" relates to a system wherein a first 
multi-port hybrid coupler is formed by hybrid couplers alone, which are 
divided into n stages, each including 2.sup.n-1 hybrid couplers. A signal 
input to any one of N (=2.sup.n) input terminals of the first stage is 
distributed equally to N output terminals of the nth stage. The N output 
terminals of the nth stage of the first multi-port hybrid coupler are 
connected to N amplifiers at the input side thereof, respectively, and the 
N amplifiers are connected at the output side to a second multi-port 
hybrid coupler. The second multi-port hybrid coupler is identical in 
construction with the first multi-port hybrid coupler, but its input 
terminals and output terminals are reverse from those of the latter and 
the input terminals of the second multi-port hybrid coupler are connected 
to the N amplifiers in an order reverse from the order of arrangement of 
the output terminals of the first multi-port hybrid coupler. 
U.S. Pat. No. 4,477,781 issued Oct. 16, 1984 to Reuss, Jr. entitled 
"Combined Microwave Parallel Amplifier-RF Attenuator/Modulator" discloses 
a parallel channel microwave amplifier comprising a plurality of 
amplification channels interconnecting a power-dividing matrix and a 
power-combining matrix. Each amplification channel includes a phase 
shifter, an attenuator and a power amplifier connected in series. The 
device functions as a combination of amplifier and an r.f. 
attenuator/modulator. 
U.S. Pat. No. 4,198,611 issued Apr. 15, 1980 to Eng entitled "Redundancy 
System With Eight Devices For Five Channels" describes a redundancy system 
for use in a communications satellite. Eight traveling wave tube 
amplifiers are provided for five communications channels in a manner such 
that full service is maintained even though any three of the amplifiers 
have failed. A ten-switch network connects the five channels to the inputs 
of the eight TWT amplifiers, and a mirror-image ten-switch network 
connects the outputs of the eight TWT amplifiers to the five channels. 
U.S. Pat. No. 4,010,426 issued Mar. 1, 1977 to Rambo entitled "Re-Power 
Amplifier Parallel Redundant System" teaches an RF power amplifier 
parallel redundant circuit that employs three amplifiers. Under normal 
operation when all three amplifiers are functioning, the output from a 
comparator holds a O.degree., 60.degree. two-bit phase shifter in the 
60.degree. position. When anyone of the amplifiers or any pair of 
amplifiers fail, the comparator output causes the two-bit phase shifter to 
switch to the 0.degree. position. The system holds the power output 
constant at one-fourth the value of each individual amplifier for single 
or double failures. Only one simple switching position operation is 
required. 
U.S. Pat. No. 3,928,806 issued Dec. 23, 1975 to Canter et al. entitled 
"Power Dividing And Combining Techniques For Microwave Amplifiers" 
discloses a novel microwave power divider that is a combination of an N 
port junction circulator and a novel isolator-mismatch device for each 
circulator port other than one input port and divides microwave signal 
power incident to the input port into any desired ratio among the output 
ports and isolates all the ports from microwave power reflected back 
toward the divider after being propagated through the divider and thus 
prevents interaction among line devices, e.g., amplifier units, that are 
fed by the output ports. Also a novel microwave power combiner is 
disclosed that is a combination of an N-port junction circulator and a 
novel isolator-reflector device for each port other than one output port 
to combine at the one output port identical microwave signals or microwave 
signals that differ in frequency of phase incident to the 
isolator-reflector devices. The microwave power combiner, like the micro 
wave power divider, provides isolation among the signal transmission 
lines. Also, a novel solid-state microwave power amplifier is disclosed 
that includes the novel power divider, a solid-state power amplifier unit 
for each power fraction from the outputs of the divider and the novel 
power combiner for the amplified outputs of the power divider. 
SUMMARY OF THE INVENTION 
The present invention provides a multi-port amplifier system for multi-beam 
communication antennas in which redundancy is achieved without using RF 
switches. The multi-port amplifiers are divided into sets of stages 
arranged in tree fashion. The amplifier stages can be turned off without 
degrading port-to-port isolation.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention provides a multi-port amplifier system wherein it is 
possible to trade the high weight and high recurring touch labor cost of 
prior art systems as described and shown in FIGS. 2, 3 and 4 for a 
redundancy scheme that uses somewhat higher power. The exact loss of 
efficiency and loss of isolation versus the reliability of the multi-port 
amplifier is complex so that the exact configuration choices must be made 
in the context of an actual application. For purposes of explanation of 
the present invention, it is assumed that there are four separate antenna 
beams. To help insure good isolation and graceful degradation, it is 
assumed that the basic multi-port amplifier has thirty-two amplifier 
device stages. An embodiment of a thirty-two stage multi-port amplifier 
with thirty-two ports according to the present invention is shown in FIGS. 
5A and 5B. 
In FIGS. 5A and 5B, there are thirty-two input ports labeled 1 through 32 
and thirty-two corresponding output ports similarly labeled 1 through 32 
indicating that input port 1 is functionally connected to output port 1 
and so on. An input signal to each of the input ports 1 through 32 is 
connected to a separate one of a first group of sixteen, four stage 
multi-port hybrid circuits. The connective paths are designated by the 
input port numbers shown on each of the hybrid circuit in FIGS. 5A and 5B. 
The outputs of the first group of multi-port hybrid circuits are connected 
through solid-state power amplifiers to a second group of sixteen, four 
stage multi-port hybrid circuits which are also labeled according to the 
input port numbers to which they are functionally connected. 
FIGS. 6A and 6B is an illustration of the same embodiment of FIGS. 5A and 
5B with the same components but with the signal paths from input ports 
numbers 1 and 2 to output ports numbers 1 and 2 shown in bold lines. 
Topologically this signal path follows one of the thirty-two stage hybrid 
tree paths, and in fact, each of the other signal paths of the thirty-two 
paths follows its corresponding hybrid tree path embedded in the 
multi-port amplifier. Therefore, if only four input/output pairs are 
required it should be possible to achieve this topology with the 
equivalent of four embedded hybrid trees. Any hybrids that are not part of 
the four embedded hybrid trees will carry no signal and can be replaced 
with load resistor terminations as shown in FIGS. 7A and 7B. FIGS. 7A and 
7B show the thirty-two path multi-port amplifier embodiment of FIG. 6 with 
the excess hybrids removed. The multi-port amplifier of FIGS. 7A and 7B 
contains four, eight stage hybrid trees as shown in the dotted boxes 118, 
120, 122 and 124. Note that all the hybrids that are part of these eight 
stage hybrid trees all have one port terminated by a load resistor. This 
means that any imbalance within an eight stage hybrid tree can not reduce 
the isolation of the output ports per se since all products of the 
imbalance within that tree are dissipated by the terminating resistors 
connected to the hybrids. Therefore, any stage within the eight stage 
hybrid tree multi-port amplifier can be turned off without affecting 
output port isolation if a stage (any one stage) in each of the other 
three, eight stage hybrid tree amplifiers are turned off. 
Redundancy can be achieved therefore by turning off one amplifier device 
within each eight stage hybrid tree multi-port amplifier as a stand-by 
redundant replacement. The cost of this method of redundancy is increased 
power consumption. If one of each eight stages is turned off, the output 
of the remaining seven stages must be increased by a factor 1.306 which 
leads to a total power increase of 14.3%. When the first failure occurs 
the standby amplifier within the group of eight is turned on with no 
adverse effects on the multi-port amplifier's performance. Successive 
failures will also have no impact upon the multi-port amplifier's 
performance as long as the failure occurs within a group of eight stages 
that has a stand by redundant stage. Assuming there are multiple (two) 
failures in a group of eight devices it is possible to minimize the effect 
of the failure. Assuming that the multi-port amplifier is designed to 
deliver thirty-two watts with one device shut off in each group of eight, 
assume two stages fail in group one with the other groups intact. With the 
second failure the output of group one is reduced from eight watts by a 
factor of 0.735 to 5.88 watts. To compensate for this loss, the standby 
redundant stage in group two can be powered on raising the output of group 
two by a factor of 1.31 from eight watts to 10.45 watts. Since the power 
going into hybrid H is unbalanced, there will be power appearing in both 
ports A and B. The output at port A will be 0.327 watts. The power at port 
B is 16 watts which is unchanged from its initial value. The power output 
at port one remains at 32 watts while the leakage signal at A is divided 
equally between Ports 3 and 4. This still provides 22.9 db isolation 
between the beams at port 1 and ports 3 and 4, with the isolation at port 
2 unchanged. The strategy of turning off strategic amplifier stages allows 
redundancy for multiple failures without resorting to RF switches.