Radio Frequency Beam Forming Network

This invention relates generally to radio frequency antenna beam forming networks and more particularly to active beam forming networks adapted for use in connection with a multibeam antenna system to couple a selected one or ones of the antenna feed elements in the antenna system to receiver/transmitter apparatus thereby to form a desired composite antenna beam.

One application for such active beam forming network is in a satellite communication system. Here a geo-stationary satellite in a synchronous circular equatorial orbit above the earth carries a transponder to enable radio frequency communication between locations on the earth's surface. The transponder carried in the satellite in such application transmits and receives signals using a multibeam antenna system. The multibeam antenna system is coupled to a beam forming network which directs an antenna beam in accordance with command signals transmitted to the satellite from a station on the earth. In the particular satellite communication system herein described, the multibeam antenna system is configured so that each one of a plurality of "spot" beams covers a different, slightly overlapping area on the earth's surface viewed by the satellite. Each one of the "spot" beams is formed from an aperture having, substantially, the gain and bandwidth of the whole antenna. The beam forming network may be used on both transmit and receive to form selected one or ones of the plurality of "spot" beams. The antenna beam could vary anywhere from a single "spot" beam for spot coverage (i.e. as to cover the east coast of the United States for example) to a cluster of "spot" beams capable of covering the entire earth's surface as viewed by the satellite. Further, the beam forming network may be used to couple selected "spot" beams in particular amplitude and phase relationship in order to generate nulls in the antenna beam at angles corresponding to the locations of various jammers which may be present on the earth's surface.

One known beam forming network which is adapted for the application described above includes a metal plate lens and a planar array of n antenna feed elements disposed in the focal plane of the metal plate lens, each one of such antenna feed elements being coupled to a receiver/transmitter through a "tree" network of variable power dividers. The "tree" network has at its apex a single variable power divider having an input coupled to the receiver/transmitter. Each one of a pair of output ports of such power divider is coupled to a corresponding one of two different variable power dividers which, in turn, are coupled to four different power dividers. The "tree" network continues until n/2 variable power dividers are coupled to the n antenna feed elements. The power division ratio of each one of the variable power dividers in the "tree" network is individually set by control signals supplied thereto by the command signals from the station on the earth to synthesize the proper amplitude distribution over the array of antenna feed elements and thereby develop the desired composite antenna beam. Included in the path of each one of the antenna feed elements is a corresponding variable phase shifter which is also set by the command signals to provide a proper phase distribution and thereby develop the desired antenna beam. Preamplifiers are also disposed in the paths of the antenna feed elements to serve as a buffer against insertion losses associated with the beam forming network. While such beam forming network may generate the desired antenna beam, it is noted that failure of a single preamplifier, phase shifter or power divider may result in complete loss of coverage in the associated "spot" beam and, in fact, failure of a single critical variable power divider, specifically that which is connected to the receiver/transmitter at the apex of the "tree" network, results in the complete loss of half of the maximum possible antenna beam coverage. In the satellite application described above replacement of such preamplifier phase shifter or power divider is not practical for obvious reasons and hence the overall performance of the system must be designed to ensure high reliability over many years.

With this background of the invention in mind it is an object of this invention to provide an improved, highly reliable beam forming network.

This and other objects of the invention are attained generally by providing, in a radio frequency communication system, a plurality of antenna feed elements arranged in the focal plane of a radio frequency lens; means, having a plurality of input ports coupled to the plurality of antenna elements for distributing radio frequency energy fed thereto to a plurality of outputs thereof, such energy being distributed to such output ports with a phase distribution in accordance with the fed input port; a power combiner coupled between the output ports and a receiver/transmitter through different paths. Disposed in the different paths are active elements, including variable phase shifters and attenuators, responsive to command signals for providing phase shift and attenuation to radio frequency signals passing through such paths in accordance with such command signal. With such arrangement failure of a single one of such active elements only slightly degrades the performance of the communication system without any complete loss of coverage in any portion of the potential area coverage of such system.

For a more complete understanding of the invention reference is now made to the following description and to the drawings in which

FIG. 1 is a general sketch of a satellite communication system according to this invention;

FIG. 2 is a block diagram of transponder equipment carried on the satellite shown in FIG. 1;

FIG. 3 is a block diagram of an alternate embodiment of the transponder equipment according to the invention;

FIG. 4 is a simplified block diagram of a controller means used in the transponder equipment shown in FIG. 2; and

FIG. 5 shows an exemplary one of the preamplifier sections included in the transponder equipment.

Referring now to FIG. 1, a satellite spacecraft 10 is shown in synchyronous, geo-stationary, circular equatorial orbit above the earth (not numbered). The satellite spacecraft 10 carries radio frequency communication transponder equipment 12, the details of which will be described in connection with FIG. 2. Suffice it to say here that the radio frequency communication transponder equipment 12 is adapted to enable radio frequency communication between desired locations on the earth as viewed by the satellite. The radio frequency communication transponder equipment 12 includes a multibeam antenna system, to be described, for synthesizing, in response to command signals from an earth station, a desired antenna beam which may vary from "spot" coverage so as to cover, for example, the east coast of the United States, to multiple "spot" coverage so as to cover, for example, the west coast of the United States and also eastern Europe, to complete coverage of the surface of the earth as viewed by the satellite spacecraft 10. As indicated here in FIG. 1, four "spot" beams, 11a-11d, are shown to cover continental United States.

Referring now to FIG. 2, the radio frequency communication transponder equipment 12 is shown to include the multibeam antenna system 13 and coupled thereto a receiver/transmitter 14. Such receiver/transmitter 14 may be any wide band radio frequency receiver/transmitter such as for example similar to that described in U.S. Pat. No. 3,710,255, "Satellite Communication System", Francis A. Gicca, issued Jan. 9, 1973 and assigned to the same assignee as the present invention. The multibeam antenna system 13 includes a radio frequency lens, here a metal plate lens 20, a plurality of antenna feed elements 22.sub.1 -22.sub.n, here radio frequency horns, suitably mounted by any convenient means (not shown) in the focal plane of the metal plate lens 20, a radio frequency energy distributing means 24, here a Butler matrix, having n inputs 26.sub.1 -26.sub.n (each one thereof coupled to a corresponding one of the horns 22.sub.1 -22.sub.n) and n outputs 28.sub.1 -28.sub.n. Also included is an n:1 power combiner 30, of any conventional design, here having n input ports 32.sub.1 -32.sub.n coupled to corresponding ones of the output ports 28.sub.1 -28.sub.n through different paths. Disposed in corresponding ones of such different paths are variable phase shifters 36.sub.1 -36.sub.n, variable attenuators 38.sub.1 -38.sub.n and preamplifier sections 37.sub.1 -37.sub.n (to be described in detail in connection with FIG. 5) as shown. The power combiner 30 has a single output port 40 coupled to a receiver/transmitter 14 through a conventional transmit/receive switch (not shown). Also, a controller means 44 (the details of which will be described) is included, such controller means 44 being adapted to supply control signals to the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n (and preamplifiers 37.sub.1 -37.sub.n) in response to radio frequency command signals transmitted from the station on the earth and received by the satellite spacecraft 10 via radio frequency antenna 45.

Before describing the operation of the radio frequency communication transponder system 12, an initial programming procedure will be described, such procedure being performed prior to the launching of the satellite spacecraft 10 in order to program the controller means 44 and thereby enable such controller means 44 to provide, in response to the transmitted command signals, proper control signals for the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n thereby to generate a commanded, desired, antenna beam. In such programming procedure a radio frequency signal is fed into a first one of the n antenna feed elements 22.sub.1 -22.sub.n. Such radio frequency signal is divided into n signals by the energy distributing means 24, such divided signals appearing at output ports 28.sub.1 -28.sub.n. The signals at such output ports 28.sub.1 -28.sub.n will have a substantially linear phase relationship from output port to output port. The slope of the linear phase relationship is related to the particular one of the antenna feed elements 22.sub.1 -22.sub.n which is excited. Therefore, as succeeding ones of the antenna feed elements 22.sub.1 -22.sub.n are so energized the slopes of the linear phase relationships will have correspondingly different slopes. Now, with any particular one of the antenna feed elements 22.sub.1 -22.sub.n excited, the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n are controlled so that maximum power is transferred from the excited one of the antenna feed elements 22.sub.1 -22.sub.n to the receiver/transmitter 14. The Table below is thereby generated to provide an indication of the relationship between the excited one of the antenna feed elements 22.sub.1 -22.sub.n and an associated control word which, when such associated control word is supplied to the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n, enable maximum power to be transferred from the excited one of the antenna feed elements 22.sub.1 -22.sub.n to the receiver/transmitter 14. The control words are then stored in a memory 50 (FIG. 4) included in the controller means 44. A selected one or ones of control words are read from such memory in response to the command signals in a manner to be described. It follows then that, in operation, when the controller means 44 supplies signals to the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n, which signals represent one of the stored control words, a "spot" antenna beam associated with such control word is formed by the multibeam antenna system 13. For example, on transmission (realizing that principles of reciprocity apply during reception) radar frequency energy transmitted by receiver/transmitter 14 will pass through power combiner 30 to the ports 28.sub.1 -28.sub.n of the energy distributing means 24, the gain and phase of such signals being adjusted by the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n in accordance with the control word supplied thereto by the controller means 44. The signals applied to the energy distributing means 24 are then directed to the one of the antenna feed elements 22.sub.1 -22.sub.n associated with such supplied control word. The energy radiates from such selected one of the antenna feed elements 22.sub.1 -22.sub.n and is focused by the metal plate lens 20 in the proper direction to provide a "spot" beam at the location on the earth's surface selected by the command signal.

It is noted that the control word is presented above in polar notation, the magnitude portion, R.sub.a,b representing the setting of the variable attenuator and the angle portion .theta..sub.a,b representing the variable phase shifter setting. The control words described above are stored as digital words in the conventional memory 50 included in controller means 44 (FIG. 4). One portion (one set of bits) of each one of the stored digital words (i.e. the phase portion ".theta.") is for use in developing control signals for the variable phase shifters 36.sub.1 -36.sub.n and the other portion (i.e. the magnitude portion "R") of each one of such stored digital words is for use in developing signals for the variable attenuators 38.sub.1 -38.sub.n. The locations of the stored digital words correspond to the antenna feed elements 22.sub.1 -22.sub.n. Hence, for example, location 1 of the memory stores the digital word equivalent of the control words associated with antenna feed element 22.sub.1, as shown in the Table.

It follows then that any one of the n beams may be individually generated during either transmit or receive by controller means 44 supplying, in response to the command signal from the location on the earth, the control words associated with such one of the beams to the variable phase shifters 36.sub.1 -36.sub.n and variable attenuators 38.sub.1 -38.sub.n.

For a composite beam, that is where two or more of the antenna feed elements 22.sub.1 -22.sub.n are to be excited during, say transmit, (realizing that principles of reciprocity apply when considering reception of receiver signals) the control words associated with each one of the beams are vectorially added in the controller means 44 by any conventional microprocessor or computer 52 (FIG. 4) in the following manner: For example, if it is desired to form an antenna beam made up of "spot" beams a, b and c, a radio frequency signal representing this command signal is transmitted from a station on the earth to the satellite spacecraft 10. Such signal is fed to receiver 51 (FIG. 4) via antenna 45. The received signal is detected, decoded and converted into a digital word by conventional processing equipment 47 and then passed to computer 52, all in a conventional manner. The digital word causes the computer 52 to vectorially add the control words associated with each one of the "spot" beams a, b and c. The control signals supplied by the controller means 44 to the variable attenuators 38.sub.1 -38.sub.n and variable phase shifters 36.sub.1 -36.sub.n are calculated by the computer 52 as follows:

The magnitude portions (i.e. R.sub.a-c,1 . . . R.sub.a-c,n) are fed to the variable attenuators 38.sub.1 -38.sub.n and the phase portions (i.e. .theta..sub.a-c,1 . . . .theta..sub.a-c,n) are fed to the variable phase shifters 36.sub.1 -36.sub.n.

It is here noted that, at the expense of some degradation in the synthesized beam, it is possible to perform such synthesis solely with phase control thereby eliminating the bank of variable attenuators 38.sub.1 -38.sub.n. The principle degradation will appear as increased sidelobe levels in the synthesized beam. The programming procedure is equivalent to that described above except that only the phase portion of the control word is retained while the magnitude portion of such word is set equal to unity.

Referring now to FIG. 3, an alternate embodiment of the energy distributing means 24 is shown. Here energy distributing means 24' is particularly useful where the number of antenna feed elements 22.sub.1 -22.sub.n is so large that a single Butler matrix is impractical because of excessive size and complexity. Here the plurality of antenna feed elements 22.sub.1 -22.sub.n is divided into sets, the antenna feed elements 22.sub.1 -22.sub.n in each one of such sets being coupled to a different one of the plurality of Butler matrices. For example, with sixty-four antenna feed elements 22.sub.1 -22.sub.64 divided into eight sets, eight Butler matrices 24.sub.1 -24.sub.8 are used as shown in FIG. 3. The eight antenna feed elements in each set are coupled to one of the eight matrices. Each set is coupled to a corresponding one of the matrices. Each one of the Butler matrices 24.sub.1 -24.sub.8 is fed to a corresponding one of eight power combiners 30.sub.1 -30.sub.8. Variable phase shifters 36.sub.1 '-36.sub.8 ' , attenuators 38.sub.1 '-38.sub.8 ' and preamplifier sections 37.sub.1 '-37.sub.8 ' are disposed in the lines between each one of the Butler matrices 24.sub.1 -24.sub.8 and a corresponding one of the power combiners 30.sub.1 -30.sub.8 as shown. That is, here eight sets of eight preamplifiers, eight variable phase shifters and eight variable attenuators are used. The output of the power combiners 30.sub.1 -30.sub.8 is fed to an 8:1 power combiner 30, the output of which is fed to the receiver/transmitter 14 as described in FIG. 2. An equivalent programming procedure to that described in connection with FIG. 2 is used to develop the control words which are stored in a memory (not shown) included in the controller means 44'. Composite beams are formed by vectorially adding the control words associated with the individual "spot" beams making up the composite beams as described above in connection with FIG. 2. It is here noted that the magnitude portion of each control word is sent to 64 variable attenuators and the phase portion of such control word is sent to 64 variable phase shifters. Further, the 64 attenuators and 64 phase shifters are independently controlled in order to produce the desired antenna beam.

Referring now to FIG. 5, an exemplary one of the preamplifier sections, here preamplifier section 37, is shown to include a "receive" preamplifier 60, a "transmit" preamplifier 62 and a pair of switches 54, 56 arranged and shown, and responsive to transmit/receive control signals supplied by controller 44 to, on transmit, couple transmit preamplifier 62 between port 28.sub.1 and variable phase shifter 36.sub.1 and, on receive, couple receive preamplifier 60 between port 28.sub.1 and phase shifter 36.sub.1.

Having described preferred embodiments of the invention it will now be immediately evident to a person of skill in the art that the concept of this invention may be implemented in different ways than those illustrated and explained hereinbefore. For example, the phase shift and attenuator command signals may be modified in order to provide a desired amplitude and phase distribution to the antenna feed elements to further shape the antenna beam. Such modification in the amplitude and phase distribution may be used, for example, if it is desired to "null out" a particular region on the earth, as if a jamming source is located in such region. In any event, during "transmit" or "receive" radio frequency energy passes between the antenna feed elements and the receiver/transmitter through all of the active elements (i.e. preamplifier sections, phase shifters and attenuators) and therefore failure of one of such elements will not result in a complete loss of any one of the "spot" beams. It is felt, therefore, that this invention should not be restricted to the disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims.