GB640000A - Improvements in and relating to pulsed radar systems - Google Patents

Abstract

640,000. Radiolocation; pulse circuits; radio signaling. MINISTER OF SUPPLY. Feb. 20, 1947, No. 5023. [Classes 40 (v), 40 (vi) and 40 (vii)] In a radar system of the interrogator-transponder type the interrogator transmits pairs of pulses, separated by a predetermined time interval, respectively on each of two radiating elements having divergent directions of maximum radiation, and the transponder is adapted to transmit a characteristic signal in response to the reception of a pair of pulses of substantially equal amplitudes and separated by said predetermined time interval. In the system described the interrogator is situated adjacent to and used in conjunction with a ground base primary radar system for location of aircraft, and transponders are provided on specified aircraft for identification purposes. A suitable interrogator (Fig. 1, not shown) consists of a transmitter controlled by the primary radar system and feeding pairs of equal amplitude pulses, separated by a fixed time interval, to an electronic switch whose operation is synchronized with the pulses by means of signals sent over a separate lead. The arrangement is such that pulses of each pair are radiated respectively from two dipoles disposed symmetrically about the axis of a common reflector. The pulses are thus radiated alternately on two beams whose axes lie at an angle to each other and the angle within which pulses of substantially equal amplitudes can be received is much narrower than the width of a single beam. Fig. 4 shows the preferred form of transponder in which the aerial 15 is omni-directional in azimuth and feeds signals, directed towards it from the interrogator, to receiver 17 (arranged to provide A.G.C. but to amplify the two pulses of a pair equally). The output of receiver 17 is fed to the grid of valve 18 and via a delay circuit 19 to the grid of valve 20, the valves 18 and 20 being so arranged that the difference between their anode voltages is proportional to the difference between their control grid voltages. Thus should the pulses of a pair be unequal in amplitude or have a time interval different from the delay time of circuit 19 a difference anode voltage occurs and is rectified by circuit 22 to give a negative bias on the control grid of coincidence valve 24. Valve 24 is arranged to pass anode current only when its first grid is at earth potential and the second and third grids are fed simultaneously with positive pulses from amplifier 27. Thus the anode circuit of valve 24 generates a signal only on reception at aerial 15 of two pulses of equal amplitudes and spaced by the predetermined time interval, and when this occurs the transmitter 25 is triggered to send out a characteristic reply signal from aerial 15.