Multiple scanning beam direct radiating array and method for its use

A phased array antenna system producing multiple beams that can be rapidly and reliably scanned between desired angular beam locations without the need for highly complex hardware. The antenna system includes multiple antenna elements (30) coupled to frequency converters (34) that downconvert received signals to an intermediate frequency. Each frequency converter (34) receives a local oscillator (36) signal that passes through a phase shifting circuit (40). The phase shifting circuits are adjusted only in a calibration mode, to remove any phase errors, but are not used to select beam locations. In a receive mode, the downconverted received signals are input to a matrix network (44), such as a Butler Matrix, which transforms the antenna signals on its input lines (42) to an equivalent set of beam location signals on its outputs (46), of which there is one for each possible angular beam location of the antenna system. A switch network (50) then selects from among this set of beam location signals and associates selected beam location signals with selected beam signals. The switch network (50) has its configuration determined by multiple electronically controllable switches (58), and determines the association of each of multiple communication beams with a selected angular beam location. Thus each communication beam can be conveniently directed or redirected to a desired angular beam location without the need to adjust a large number of phase shifting circuits.

BACKGROUND OF THE INVENTION 
This invention relates generally to phased array antennas and, more 
particularly, to phased array antenna systems that must provide multiple 
beams simultaneously. By adjusting the phase angles of signals received 
from or transmitted to multiple antenna elements in an antenna array, an 
antenna control system effectively steers the antenna beam, whether in a 
receive mode or a transmit mode. In satellite communication systems, it is 
highly desirable to be able to provide phased array antenna systems with 
highly agile beams, which can be scanned both rapidly and accurately 
between beam locations. It is also desirable to provide on-orbit 
re-configurabilty of such an antenna system, to switch rapidly between 
different beam configurations as needed. 
In both commercial and military satellite communication systems, antenna 
arrays must be controlled to produce relatively narrow beams, as small as 
one degree in width. Each narrow beam covers only a relatively small, 
approximately circular area of the earth's surface. Besides being more 
energy efficient, the use of narrow beams permits multiple ground stations 
to use the same radio frequency without conflict. Also modern satellite 
communication systems need the ability to transmit or receive over 
multiple beams simultaneously. As the number of required multiple beams 
increases, so does the complexity of the phased array antenna control 
circuitry. 
In conventional phased array antenna systems, each radiating element in the 
array has to have an independent radio-frequency (RF) phase shifting 
circuit for each independent beam to be produced. In an illustrative 
system to be discussed in more detail below, the array has 547 elements 
and there is a requirement to produce sixteen independent beams. Thus, 
8,752 phase shifting circuits are needed, together with sixteen 547-way RF 
power combiners to produce the sixteen independent beams. Each phase 
shifting circuit has to be connected to an appropriate one of the power 
combiners, creating a maze of crossing lines. Moreover, each of the phase 
shifting circuits requires its own four-bit control line to provide the 
requisite beam steering accuracy. The complexity of implementation 
increases even further as the number of independent beams rises above a 
modest value. 
Accordingly, it will be appreciated that there is a need for a less complex 
technique to provide multiple independent beams from a phased array 
antenna system. The present invention is directed to this end. 
SUMMARY OF THE INVENTION 
The present invention resides in a phased array antenna system in which 
multiple independent beams are conveniently directed or redirected to 
desired angular beam locations. Briefly, and in general terms, the phased 
array antenna system of the invention comprises a first plurality of 
antenna elements operable at radio frequencies (RF) in a receive mode or a 
transmit mode; an equal plurality of frequency converters coupled to the 
antenna elements to effect a frequency conversion of received RF signals 
to an intermediate frequency; a local oscillator providing a local 
oscillator frequency signal to the frequency converters; an equal 
plurality of phase shifting circuits, connected between the local 
oscillator and each of the frequency converters, to permit phase 
adjustment of the local oscillator frequency signal provided to each of 
the frequency converters; a matrix network having a first plurality of 
input ports equal in number to the number of antenna elements, and a 
second plurality of output ports equal in number to a desired number of 
possible angular beam locations, wherein the matrix network effects a 
transformation from a set of antenna element signals to a set of beam 
location signals; and a switch network having a second plurality of input 
ports coupled to respective output ports of the matrix network, and a 
third plurality of output ports equal in number to a selected number of 
beams used as separate communication channels. The switch network selects 
a beam location from the second plurality of beam locations, and couples 
signals from the selected beam location to a selected beam output port; 
and each beam can be quickly assigned to any one or more angular beam 
locations. 
More specifically, the matrix network is implemented in the form of a 
Butler Matrix, a Blass Matrix Network, or Rotman Lens Network. The switch 
network includes a second plurality of splitters, a third plurality of 
switches for each of the splitters, and a third plurality of combiners. 
The splitters are equal in number to the number of input ports in the 
switch network, each having a single input port connected to an output 
port the matrix network and a third plurality of output ports, equal in 
number to the number beams. Each of the switches is connected to a 
separate output port of a splitter. The combiners are also equal in number 
to the number of beams. Each combiner has a single output port that is an 
output port of the switch network, and has a second plurality of input 
ports, equal in number to the number of input ports to the switch matrix. 
Therefore, each input port of the switch matrix is connectable to any of 
the output ports of the switch matrix, through one of the splitters, one 
of the switches and one of the combiners. The switches are operable to 
associate any selected beam with any selected beam location. 
The antenna system is also operable in a transmit mode in which the switch 
network functions to associate selected beam signals to selected beam 
location signals; the matrix network functions to transform a plurality of 
beam location signals to antenna array signals; and each frequency 
converter performs an upconversion from an intermediate frequency to a 
radio frequency. 
In method terms, the invention, comprises the steps of receiving 
radio-frequency (RF) signals through a first plurality of antenna elements 
in an array; downconverting the received signals to an intermediate 
frequency in an equal plurality of frequency converters, wherein the 
downconverting step includes generating a local oscillator signal, 
splitting the local oscillator signal into a first plurality of local 
oscillator signals for connection to the frequency converters, and 
adjusting the phase of the local oscillator signals applied to the 
frequency converters to compensate for any phase errors; outputting from 
the frequency converters a first plurality of downconverted received 
signals; transforming the first plurality of downconverted signals to a 
second plurality of signals, corresponding in number to a selected number 
of angular beam locations to which the phased array antenna is capable of 
being pointed; and selecting from the second plurality of signals a set of 
beam signals, of which there is one for each of a desired plurality of 
communication channels. The selecting step provides for rapid and reliable 
switching of beams to different angular beam locations. 
More specifically, the selecting step includes splitting each of the second 
plurality of signals into a third plurality of signals; connecting the 
third plurality of signals from each splitting step to input ports of a 
third plurality of signal combiners, through a third plurality of 
controllable switches; controlling the switches to select which of the 
second plurality of signals, corresponding to different angular beam 
locations, are connected to the signal combiners. The selected signals are 
then output as beam signals from the signal combiners. 
There are various possibilities for associating beam signals with beam 
locations. One possibility is that the controlling step selects a single 
angular beam location signal to assign to each beam signal. Alternatively, 
the controlling step selects multiple angular beam location signals to 
assign to each of some of the beam signals. Or the controlling step 
selects a single angular beam location signal to assign to multiple beam 
signals. 
Other aspects and advantages of the invention will become apparent from the 
following more detailed description, taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings by way of illustration, the present invention 
pertains to phased array antenna systems for producing multiple 
independent beams simultaneously. In satellite communication system, it is 
often a requirement for antennas to be able to handle multiple beams 
directed toward different ground stations or communication terminals. As 
shown in FIG.1, coverage of the earth's surface as viewed from a 
geosynchronous orbit can be achieved with a total of 313 beam locations 
using a one-degree beam diameter. The angular diameter of the earth as 
viewed from geosynchronous orbit is approximately 18E. The large circle in 
FIG. 1 represents the earth and each of the small circles represents a 
beam location with a one-degree diameter. When the 313 beam locations 
shown are arranged in a hexagon pattern with eleven beam locations along 
each side, the pattern approximately overlaps the earth's disk in the 
field of view. 
The 313 beam locations shown in FIG.1 represent the possible angular 
locations of multiple beams generated at a phased array antenna on a 
communication satellite in geosynchronous earth orbit. FIG. 2 shows a 
phased array antenna system of the prior art, for generating up to sixteen 
independent beams directed to angular beam locations selected from the 
ones shown in FIG. 1. 
The phased array antenna system of FIG. 2 has 547 radiating antenna 
elements, indicated by reference numeral 10. For simplicity, only the 
first two and the last elements are shown. In this description, it is 
assumed that the antenna system is operating in a receive mode. Each 
antenna element 10 is coupled through an amplifier 12 to a 16-way splitter 
14, which provides sixteen parallel connections to the antenna element. 
Each of the sixteen lines from the 16-way splitter 14 is coupled to a 
phase shifting circuit 16. Therefore, there are sixteen phase shifting 
circuits for each antenna element 10, or a total of 8,752 phase shifting 
circuits 16. 
Finally, the phased array antenna system includes sixteen 547-way RF power 
combiners 20, only the first and last of which are shown. The first power 
combiner 20, shown in the lower position in the drawing, receives as 
inputs the RF signals from each of the phase shifting circuits 16 that are 
in the first position as shown in the figure. This set of 547 phase 
shifting circuits is controlled by appropriate control signals to the 
separate phase shifters, to produce a beam designated "beam 1." Similarly, 
each other set of 547 phase shifters is connected to its own power 
combiner 20 to produce an independent beam, of which there are sixteen in 
all in this illustration. 
There are a number of significant problems associated with the conventional 
phased array antenna system of FIG. 2, one of which is its complexity. A 
large number of phase shifting circuits 16 must be accurately adjusted and 
connected to appropriate RF power combiners 20. Wiring to control the 
phase shifters 16 and the interconnecting wiring to the power combiners 
both present significant challenges because the inter-element spacing of 
the antenna elements 10 is fixed and is relatively small. A second major 
concern with the conventional system is its potential slowness to switch 
or reconfigure beams to different angular locations. In the system of FIG. 
2, beam scanning or switching is achieved by changing the settings of the 
phase shifting circuits 16. Inevitably, there is a delay or "settling 
time" involved when the settings of a group of 547 phase shifting circuits 
16 are changed to move a beam to a new location. A related difficulty is 
that RF phase shifting circuits are notoriously susceptible to 
inaccuracies attributable to various causes, such as manufacturing 
tolerances or changes in temperature. 
In accordance with the present invention, the foregoing difficulties are 
completely avoided. Specifically, only one phase shifting circuit is 
required for each antenna element, for purposes of calibration only, and 
scanning or switching beam locations is accomplished practically 
instantaneously by switches instead of phase shifting circuits. 
As shown in FIG. 3, the phased array antenna system of the present 
invention also has 547 antenna elements 30, but it will be understood that 
the invention is not limited to the numerical values used in this 
illustrative embodiment. Coupled to each antenna element 30 is a low-noise 
amplifier (LNA) 32 and a downconverter 34, which shifts the frequency of 
received radio-frequency (RF) signals, at 44 gigahertz (GHz), for example, 
to an intermediate frequency (IF). Associated with the downconverters 34 
is a local oscillator 36, which supplies a local oscillator (LO) signal to 
a power divider 38 that splits the LO signal into 547 paths, one for each 
of the downconverters 34. Each of the 547 LO signals passes through a 
separate phase-shifting circuit 40. Adjustment of the phase of the LO 
signal also serves to adjust the phase of the intermediate frequency (IF) 
signal output from the downconverter 34 on line 42. These phase 
adjustments are performed only during a calibration procedure to ensure 
phase tracking along all signal paths, and not for beam steering as in the 
conventional system of FIG. 2. This approach greatly reduces demand on the 
antenna control system. Also, because the phase shifting circuits 40 
operate at the LO frequency, which is lower than the radio frequency, they 
are less sensitive to manufacturing tolerances and changes in operating 
temperature. Moreover, packaging is greatly simplified because the LNA 32 
and downconverter 34 adjacent to each antenna element 30 occupies much 
less space than the sixteen phase shifters required in the conventional 
system of FIG. 2. 
The 547 outputs on lines 42 from the downconverters 34 are input to an IF 
matrix network 44, which may be a Butler Matrix, a Blass Matrix Network or 
a Rotman Lens Network. The matrix network 44 functions to convert, in the 
receive mode, the set of 547 "feed" signals to an equivalent set of 313 
"beam" signals, one for each possible angular beam location. In a transmit 
mode, the matrix network 44 performs the opposite conversion function. The 
matrix network 44 is best disclosed in U.S. Pat. No. 5,734,345 issued to 
Chen et al., assigned to the same assignee as the present application and 
having the title, "Antenna System for Controlling and Redirecting 
Communications Beams," and in U.S. Pat. No. 5,760,741 issued to Huynh et 
al., assigned to the same assignee as the present application and having 
the title, "Beam Forming Network for Multiple-Beam-Feed Sharing Antenna 
System." Both of these patents are hereby incorporated by reference into 
this specification. The beam forming network (14 in FIG. 7 of U.S. Pat. 
No. 5,734,345) performs the same function as the matrix network 44 of the 
present invention. 
The outputs of the matrix network 44 operating in a receive mode, on lines 
46, correspond to the 313 possible angular beam locations of the antenna 
array. The other principal component of the invention is an intermediate 
frequency (IF) switch network 50, which associates selected output lines 
46 with beams #1 through #16, as indicated by lines 52. The switch network 
50 includes a plurality of 1:16 splitters 54, one for each of the lines 46 
from the matrix network 44. Each splitter 54 has one input and sixteen 
outputs, indicated by lines 56, most of which have been omitted for 
clarity. Each of the lines 56 passes through a separate electronically 
controllable switch 58. Finally, the IF switch network 50 includes sixteen 
313H1 combiners 60, each having 313 inputs, on lines 56, and a single 
output, on one of the lines 52. The connecting lines 56 between the 
splitters 54 and the combiners 60 are routed such that each combiner 
receives a potential signal contribution from every one of the splitters 
54. For example, the first combiner 60 is connected to the first output 
position of each of the splitters 54; the second combiner is connected to 
the second output position of each of the splitters, and so forth. 
In operation in a receive mode in which all sixteen beams are enabled, each 
combiner 60 will have only one of its associated input switches 58 closed. 
In other words, each combiner 60 is associated with one particular beam 
location. Typically, the sixteen combiners 60 will be associated with 
sixteen different beam locations selected from the 313 possible locations, 
but other associations of the beams and beam locations are also possible. 
A single beam, which constitutes an independent communication channel, may 
be associated with multiple beam locations at the same time, or multiple 
beams may be associated with a single beam location. Switching a beam from 
one angular location to another is accomplished by control of the switches 
58. No readjustment of phase delays of the antenna elements is needed. 
Once the switches 58 have settled in their new positions, the antenna 
beams immediately assume their new configuration. 
It will be well understood by those familiar with the antenna art that 
phased array antennas may be operated in either a transmit mode or a 
receive mode. For convenience, the invention and the prior art have been 
described primarily as operating in the receive mode, but could have been 
described as operating in the transmit mode. For example, in the transmit 
mode the combiners 60 would function as splitters, and the splitters 54 
would function as combiners. The matrix network 44 would, as mentioned 
above, operate in the transmit mode to perform a transformation from 313 
beam location inputs to 547 antenna element outputs. Also the 
downconverters 34 would function as upconverters, and the low-noise 
amplifiers 32 would be replaced by solid-state power amplifiers in the 
transmit mode. 
It will be appreciated from the foregoing that the present invention 
represents a significant advance in the field of phased array antennas for 
satellite communication systems. In particular, the invention provides a 
less complex technique for switching multiple communication beams from one 
angular beam location to another, without the need for thousands of RF 
phase shifting circuits and associated interconnected control wiring. The 
solution provided by the present invention allows more rapid and reliable 
switching between beam locations, with substantially less hardware 
complexity. It will also be appreciated that, although a specific 
embodiment of the invention has been described in detail by way of 
illustration, various modifications may be made without departing from the 
spirit and scope of the invention. Accordingly, the invention should not 
be limited except as by the appended claims.