GB1316562A - Secondary radar systems - Google Patents

Abstract

1316562 Pulse radar; aerials A C COSSOR Ltd 21 Feb 1972 [1 March 1971] 5697/71 Headings H4A and H4D The interrogating station of a secondary radar system has a directional aerial with two halves, which are coupled to respective ports of a sum and differencing circuit. The circuit also has a sum port and a difference port, and these are coupled to respective inputs of a differential amplifier. As described, an aerial 20 comprises two halves 21, 22 each formed from a linear array of radiators and having respective ports L, R, which are coupled to two ports of a hybrid ring 23. The sum port of said hybrid ring is coupled through a T/R switch 31 (also coupled to a transmitter 30), an R.F. amplifier 24, a mixer 25, and an I.F. amplifier 26 to one input of a differential amplifier 27. The difference port of the hybrid ring 23 is coupled through an R.F. amplifier 24, a mixer 25, and an I.F. amplifier 26 to the other input of the differential amplifier 27. Output from the amplifier 27 is fed to a detector 28, to give a video signal for application to decoding circuits 29. In operation, a coded train of pulses from the transmitter 30 is fed via the T/R switch 31 and the sum port of the hybrid ring 23 to the aerial 20, the two halves of which radiate equal powers in phase to produce maximum field strength in the boresight direction. Coded replies from a responder along the boresight direction give rise to equal signals in phase in the aerial halves 21, 22 and, in consequence, to an output from the sum port of the hybrid ring 23 but not from the difference port thereof. However, a reply from an offboresight direction results in an output from said difference port, and this can be arranged to predominate over the output from the sum port at, for example, angular differences of just above 1 degree (Fig. 1, not shown). Said predominance causes a change of polarity of output from the detector 28, against which the decoding circuits 29 can be set to discriminate, thereby effecting suppression of side lobe responses (Figs. 3(a), 3(b), not shown). The circuit enables degarbling to be effectively performed, Fig. 4 (not shown). In another embodiment separate detectors are provided in the sum and difference channels, and the differential amplifier receives the resulting video signals as its inputs.