Patent GB-734588-A (GB)
Title: Improvements in or relating to secondary radar systems
IPC: H04R1/30
Filed: 1952-05-09
Abstract: 734,588. Radar; pulse signalling. LEVELL, D. A., and HARRIS, K. E. April 27, 1953 [May 9, 1952; May 9, 1952], Nos. 11779/52 and 11780/52. Classes 40 (5) and 40 (7). A secondary radar system comprises an interrogating transmitter generating pulse pairs having a spacing small compared with the recurrence period, the second pulse in each pair being applied to a scanning directional aerial and the first to an aerial giving a field strength in all directions intermediate that of the main and side lobes of the directional aerial, and a responder arranged to be conditioned by reception of a first pulse to give a reply to a second pulse of the correct amplitude and timing. The two pulses may be preceded by a third pulse radiated by the directional aerial and effective to condition responders situated at such a range that the wide-angle intermediate pulse is of insufficient amplitude. In a pulse pair system the first .pulse of each pair may be radiated substantially omnidirectionally and the second pulse generated 10 microseconds later radiated by the scanning directive aerial, the system may be triggered from, and have a combined display with, a primary radar (Fig. 1, not shown). The directive aerial system may comprise a dipole array 20 mounted on the rear face of a metal column 21 which acts as an aperiodic reflector. Energy from the array 20 is reflected by curved reflector 22 to give a directive pattern with one main lobe and a number of small side lobes. Aerial array 24 connected in parallel with a dipole on the rear side of reflector 22 may be used to radiate the conditioning pulses and a substantially omnidirectional pattern may be thereby produced. At the responder the received pulses (of which the second pulses of each pair are larger when the directive aerial at the transmitter is turned towards the responder) are detected, amplified and inverted to give positive-going pulses at V1, and then applied through cathode follower V2 through diode V3A to storage capacitor C1. A diode V3B is connected to fix the minimum level of applied pulse for which V3A will conduct to feed capacitor C1. The charging of C1 causes a voltage rise which is amplified by V4 and applied through anode follower clamp valve V5A to trigger phantastron V6. - V6, when triggered, generates a positive rectangular pulse having a duration a little less than the spacing between the pulses of a pair. The termination of the rectangular pulse is used to initiate the generation of a positive 3 microsecond gating pulse applied to open V7 for its duration. The rectangular pulse is also applied to render V8 non-conducting and prevent the discharge of C1. This discharge -commences just before the second pulse of a pair is due and if the second pulse is received and has an amplitude greater than the potential already on the cathode of V3A then the increase in potential will be passed through V8 which will be now conducting and through the gate circuit to trigger the reply transmitter. Second pulses of insufficient amplitude or incorrect timing relative to a first pulse capturing the responder will not operate the reply transmitter. The responder may be interrogated by another interrogator almost immediately after a reply has been sent to a first. In the arrangement wherein three pulses are used the first and last are transmitted by the directional aerial and, in a typical embodiment, the first may precede the-second by 6 microseconds and the second the third by 10 microseconds. The responder charging circuit is set by the arrival of the first pulse and 3rd (interrogating) pulses of suitable amplitude and timing are passed to trigger the responder. The circuit differs somewhat from that of Fig. 5 and is described in detail with reference to Fig. 6 (not shown).