Space fed phased array antenna with dual phase shifter

Abstract: An array antenna (10) for a radar includes a phase shifter arrangement wherein, in the transmitting mode, ferrite phase shifters (35) are used to collimate and direct a beam of transmitted radio frequency energy originating at a feed horn and, in the receiving mode, amplifiers (45) and diode phase shifters (37) are used to amplify and to focus received radio frequency energy on a feed horn. Each front antenna (31) is coupled by a switch (39), limiter (43), amplifier (45), diode phase shifters (37), and a circulator (41) to the corresponding rear antenna (33), and the rear antenna (33) is coupled through the circulator (41), the ferrite phase shifters (35), and the switch (39) to the front antenna (31).

Description: This invention relates to a radar system having a radio frequency energy source and a radio frequency energy receiver, and a space fed array antenna comprising an array of antenna elements each of which has a first radiating element for receiving radio frequency energy from the source and transmitting radio frequency energy to the receiver, and a second radiating element, the first and second radiating elements of the antenna element being coupled together through controllable phase shifting means, and the phase shifting means of the antenna elements being adapted to shift the phase of radio frequency energy to be transmitted by the second radiating elements by an amount required to collimate and to direct such energy in a desired direction from the array antenna, and to shift the phase of radio frequency energy received by the second radiating elements to enable the first radiating elements to direct the received energy to the receiver. It is known in the art that a space fed phased array antenna may be used to advantage in ground-to-air defense systems, such as the system called "PATRIOT," (a registered trademark of the Government of the United States of America). Thus, in the PATRIOT system, a control radar utilizing a first or main space fed phased array antenna is arranged to illuminate a target (say an attacking aircraft) and to receive echo signals directly reflected from the target and a second antenna is arranged to receive signals retransmitted from a guided missile (referred to hereinafter as the "missile") in flight to intercept the target, such retransmitted signals being analogous to echo signals at the missile. Both the echo signals and the retransmitted signals then are processed to derive guidance command signals that are passed through the second phased array antenna to the missile, ultimately to cause the course of the missile to be adjusted as required to ensure interception of the target. The PATRIOT system, which is described in more detail in "The PATRIOT radar in tactical air defence" by D.R. Carey and W.E. Evans at pages 64 to 70, Proceeding of IEEE EASCON `81, 1981 (reprinted at page 185 to 191 in "Radar Applications" Editor M.I. Skolnik, IEEE Press, 1988) has a radar system of the kind defined hereinbefore at the beginning. Reference should also be made to US-A-3305867 and US-A-3100287. Because of the high levels of radio frequency energy passing through both the first and the second space fed array antennas in the PATRIOT system, controllable ferrite phase shifters are used to determine the phase distribution across the radiating elements making up each one of such antennas. The use of controllable ferrite phase shifters (which are nonreciprocal devices) requires that the control signals for each ferrite phase shifter be changed when the radar is transmitting or receiving radio frequency energy. Further, the noise figure of the radar is degraded by the insertion loss of each one of the ferrite phase shifters. Such loss is particularly important when echo signals are being received. US-A-4791421 describes a transmit-receive module for connection to a radiating element position of a phased-array antenna having a transmit and receive manifold system to connect the modules at all the positions of the array to the common components of a radar system, such as a radar pulse signal generator and a radar echo signal processor. The module includes, in one example, two separate phase shifters located in and respectively dedicated to the transmit and receive paths of the module. The two signal paths are connected to a common radiating element by a single transmit-receive switch. A two-stage power amplifier is used in the transmit path, and a low noise amplifier and a receiver protecting device are used in the receive path. A conducting septum is positioned between the elements of the two paths in order to isolate the paths and to reduce signal leakage between the output of one path and the input of the other. The transmit path phase shifter is a low loss, low duty cycle, high power device with medium resolution, and is exemplified by PIN diodes or an FET operating as a gate controlled resistor. The receive path phase shifter has a higher insertion loss than the transmit path phase shifter, and may be constructed using FETs and have a greater resolution than the transmit path phase shifter. The respective terminals at the input end of the transmit path and the output end of the receive path can be coupled to a common terminal through a circulator or a transmit-receive switch. The receiver protecting device may be a gallium arsenide FET, or back-to-back diodes. In another example, the protecting device is omitted and each phase shifters is replaced by a vector modulator with a control circuit connected to a temperature sensor, for temperature compensation. The use of complex weighting circuits is also proposed to allow active phase shifting. EP-A-0246640 describes a transmit-receive module for connection between a radiating element of a phased array antenna and a system connection. The module is implemented as a printed circuit board unit with coaxial connectors to the radiating element and the system. Separate transmit and receive paths are provided between the coaxial connectors. These paths are coupled to the system connector through respective PIN diodes operated as a transmit/receive switch. A circulator couples the other ends of the two paths to the radiating element connector. The transmit path contains a phase adjuster and a high power amplifier. The receive path contains a diode limiter, a low noise amplifier, and a phase adjuster. Each of these phase adjusters is a capacitive open circuit stub transmission line. Phase adjustment is carried out during installation of the modules in the array antenna by increasing or decreasing the length of the stub, or by adding or subtracting capacitive, open circuit stubs in shunt with the through line of each path. According to the present invention, a radar system of the kind defined hereinbefore at the beginning is characterised in that, in each antenna element, (i) the phase shifting means comprises: a ferrite phase shifter for coupling radio frequency energy from the first radiating element to the second radiating element; and a diode phase shifter for coupling radio frequency energy from the second radiating element to the first radiating element; (ii) amplifier means are provided for amplifying radio frequency energy received by the second radiating element to counteract, at least, the insertion loss of the diode phase shifter; and (iii) means are provided for connecting the ferrite phase shifter between the radiating elements when radio frequency energy is to be transmitted by the second radiating element and for isolating the amplifier means and the diode phase shifter from the radio frequency energy coupled to the second radiating element for transmission, and for connecting the amplifier means and the diode phase shifter between the radiating elements and for isolating the ferrite phase shifter from the radio frequency energy received by the second radiating element. A preferred embodiment of the invention has a phase shifter arrangement which is optimized for both the transmitting and the receiving mode of operation; has its insertion loss kept at a minimum; and is adapted to permit performance of the foregoing when signals at widely differing frequencies are received. The noise figure of the radar system is improved by providing amplifiers for received signals before such signals are applied to the diode phase shifters. Brief Description of the Drawings For a more complete understanding of this invention, reference is now made to the following description of the accompanying drawings wherein: FIG. 1 is a sketch illustrating a radar system embodying the present invention in a ground-to-air defense system; and FIG. 2 is a block diagram of an example of a typical one of the phase shifter arrangements used in the radar system in FIG. 1. Description of the Preferred Embodiment Referring now to FIG. 1, it may be seen that a space fed antenna 10 in an embodiment of this invention is actuated to transmit: (a) interrogating pulses of radio frequency energy (referred to hereinafter simply as "interrogating pulses") at a first frequency; and (b) command signals of radio frequency energy (referred to hereinafter simply as "command signals") at a second frequency. The space fed antenna 10 is also actuable to receive: (a) echo signals from an aircraft (hereinafter referred to as "target 12"), the frequency of the echo signals being at the first frequency, shifted by the Doppler effect; and (b) retransmitted signals indicative of the echo signals received by appropriate known equipment (not shown) on a missile 16 in flight to intercept the target 12, the frequency of the carrier of the retransmitted signals here being different from the first frequency or the frequency of echo signals. The space fed antenna 10 here is made up of an array of antenna elements such as the antenna elements 18 illustrated in FIG. 2 and described hereinafter. It will be appreciated that each one of the antenna elements 18 in the array of antenna elements is actuated in the transmitting mode to collimate and direct radio frequency energy from a feed 20, thereby to form a beam (not shown) of radio frequency energy directed toward the target 12. A transmitter/receiver in response to signals from a controller 24, is operative to form pulses of radio frequency energy for transmission and to process received radio frequency energy. The beam is directed toward the missile 16 when command signals are to be transmitted. In the receiving mode the beam is directed toward the target 12 when echo signals are to be received or toward the missile 16 when retransmitted signals are to be received. For a more complete explanation of the principles of operation and structure to scan a beam from a space fed array antenna, reference is made to United States Patent No. 3,305,867, which patent is assigned to the same assignee as the application. Referring now to FIG. 2, details are shown of an exemplary one of the antenna elements 18 (FIG. 1) that is here contemplated to replace each one of the antenna elements in a space fed array antenna such as the antenna shown in United States Patent No. 3,305,867. Thus, in addition to a front antenna 31 and a rear antenna 33, the exemplary one of the antenna elements 18 (FIG. 1) illustrated in FIG. 2 provides different phase shifters in the signal path of radio frequency energy when transmitting or receiving. As indicated, ferrite phase shifters 35 are used in the transmitting mode and diode phase shifters 37 are used in the receiving mode. Switching between the phase shifters is accomplished by a switch 39 and a circulator 41 that are connected as shown to operate as a double-pole, double-throw switch. In the transmitting mode, the ferrite phase shifters 3 are connected between the rear antenna 33 and the front antenna 31; and (b) in the receiving mode, the diode phase shifters 37 (along with a limiter 43 and an amplifier 45) are connected between the front antenna 31 and the rear antenna 33. The actuating signal for the switch 39 is provided (along with control signals for each phase shifting element (not shown) making up the ferrite phase shifters 35 and the diode phase shifters 37) by the controller 24 (FIG. 1). The limiter 43 may be a limiter as shown in European patent application No. 90 303 495.7, publication No. 0 391 635 or any other known type of limiter. Leakage signals passing through the switch 39 during transmission of any pulse of radio frequency energy are thereby prevented from being impressed on the amplifier 45. The amplifier 45, which may be made up of several individual stages, is arranged to provide sufficient gain to received signals (whether echo signals or retransmitted signals) to counteract the insertion loss of the diode phase shifters 37 or any losses suffered by received signals in passing from the front antenna 31 to the first detector (not shown) in the receiver section of the transmitter/receiver 22 (FIG. 1). The pass band of the amplifier 45 is broad enough to cover any difference between the carrier frequencies of the interrogating pulses and retransmitted signals as well as any Doppler shift impressed on any echo signals or retransmitted signals. It will be noted here that the carrier frequencies of the retransmitted signals and command signals need not be, and usually are not, the same as the frequencies of the interrogating pulses or the echo signals. It follows then that the noise figure of a radar with an amplifier such as amplifier 45 is lower than the noise figure of a radar that does not incorporate an amplifier such as the amplifier 45. It will also be noted that the pass band of the ferrite phase shifters 35 need not be as wide as the pass band of the amplifiers 45 is the carrier frequency of the command signals is the same (or nearly the same) as the frequency of the interrogating pulses. It will still further be noted that the diode phase shifters 37 are reciprocal devices, even though the ferrite phase shifters 35 may be nonreciprocal devices, so the same control signals may be applied to both the ferrite phase shifters 35 and the diode phase shifters 37. That is to say, if a single feed (such as feed 20, FIG. 1) is used in both the transmitting mode and the receiving mode, the same control signals would be impressed on the ferrite phase shifters 35 and the diode phase shifters 37. On the other hand, if two (or more) feeds are used: (a) the control signals applied to the ferrite phase shifters 35 would be such as to collimate and direct radio frequency energy from a selected one of the feeds (which, of course, would be connected to the transmitter section of the transmitter/receiver 22 (FIG. 1)); and (b) the control signals applied to the diode phase shifters 37 would be such as to focus received radio frequency energy on the remaining feed, or feeds. It will be noted finally that the switch 39 (FIG. 2) may be replaced with a circulator similar to the circulator 41. Such replacement circulator would, of course, be arranged: (a) to pass radio frequency energy out of the ferrite phase shifters 35 (FIG. 2) to the front antenna 31; and (b) to pass radio frequency energy out of the front antenna 31 to the limiter 43 (FIG. 2). In using the illustrated embodiment, time multiplexing is used to permit a single beam to be scanned from a target to a missile. Alternatively, a multibeam array antenna may be used.

Claims: A radar system having a radio frequency energy source (22) and a radio frequency energy receiver (22), and a space fed array antenna (10) comprising an array of antenna elements (18) each of which has a first radiating element (33) for receiving radio frequency energy from the source (22) and transmitting radio frequency energy to the receiver (22), and a second radiating element (31), the first and second radiating elements (33,31) of the antenna element (18) being coupled together through controllable phase shifting means (35,37), and the phase shifting means (35,37) of the antenna elements (18) being adapted to shift the phase of radio frequency energy to be transmitted by the second radiating elements (31) by an amount required to collimate and to direct such energy in a desired direction from the array antenna (10), and to shift the phase of radio frequency energy received by the second radiating elements (31) to enable the first radiating elements (33) to direct the received energy to the receiver (22), characterised in that, in each antenna element (18), (i) the phase shifting means comprises: a ferrite phase shifter (35) for coupling radio frequency energy from the first radiating element (33) to the second radiating element (31); and a diode phase shifter (37) for coupling radio frequency energy from the second radiating element (31) to the first radiating element (33); (ii) amplifier means (45) are provided for amplifying radio frequency energy received by the second radiating element (31) to counteract, at least, the insertion loss of the diode phase shifter (37); and (iii) means (39,41) are provided for connecting the ferrite phase shifter (35) between the radiating elements (31,33) when radio frequency energy is to be transmitted by the second radiating element (31) and for isolating the amplifier means (45) and the diode phase shifter (37) from the radio frequency energy coupled to the second radiating element (31) for transmission, and for connecting the amplifier means (45) and the diode phase shifter (37) between the radiating elements (31,33) and for isolating the ferrite phase shifter (35) from the radio frequency energy received by the second radiating element (31). A radar system according to claim 1, characterised in that the source (22) and the receiver (22) share a single feed (20) arranged for transmitting and receiving radio frequency energy to and from the first radiating elements (33) of the array antenna (10), and means (24) are provided for applying the same control signals to the ferrite phase shifters (35) and the diode phase shifters (37). A radar system according to claim 1 or 2, characterised in that the ferrite phase shifters (35) are nonreciprocal phase shifters.

Inventors: ROBERGE RAYMOND A, ROBERGE, RAYMOND A.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1990
Main classification: H01Q21
IPC codes: G01S7, H01Q3, F41G7, H01Q21