Patent GB-581980-A (GB)
Title: Improvements in or relating to searchlight control installations
IPC: G01S13/42
Filed: 1942-12-11
Abstract: 581,980. Radiolocation. COCKCROFT, J. D., CHICK, D. R., EASTWOOD, W. S., and OXFORD, A. J. H. Dec. 11, 1942, No. 17680. [Class 40 (v)] A searchlight control installation for facilitating the rapid laying of a searchlight on a mobile target comprises a transmitting aerial 2 radiating a pulsed exploring beam in a direction parallel with the axis of the projector 9, a receiver having a directional aerial system 5, 6, 7, 8 mounted symmetrically about the projector axis, and means for feeding the echo signals reflected from the target to cathode-ray tube indicating means 35, 36 which indicate the alignment of the searchlight in relation to the target. The direction of maximum sensitivity of the receiving aerial system is continuously varied by a phasing switch 18 so that it rotates over the surface of a cone coaxial with the projector axis, the cathode-ray tubes 35, 36 being used to compare, and indicate equality of, the amplitudes of the two echo signals received from opposite cardinal directions. The transmitting aerial is supplied from the pulsed oscillator 1. The dipoles 10 of the receiving aerials 5, 6, 7, 8 are connected by bifurcated matching transformers 13 and feeders 14, 15, 16, 17 of equal electrical length to points on the periphery of a phasing ring 18 coupled by a rotating arm 19 to a common input lead 20. The phasing ring continuously varies the comparative feeder lengths and thereby the directivity of the aerial system. The received signals are fed from lead 20 through amplifier 27, mixer 29, I.F. amplifier 30, rectifier 31, to video-amplifier 32, part of the output being fed to the Y-plates of a " signal-selecting "cathoderay tube 33, the X-plates of which are supplied from a time-base generator 34 controlled by the pulsed-oscillator 1. The tube 33 indicates the range of the target, but its main purpose is to enable the operator to select one of several possible traces, corresponding to different possible targets, so as to ensure that only the echo-signal corresponding to the selected target is passed to the tubes 35, 36 indicating bearing and elevation respectively. To this end, part of the output of time-base generator 34 is fed to a signal-selecting circuit 37 comprising a valve whose cathode positive bias is adjustable manually by a potentiometer, to determine the instant at which the valve conducts and produces pulses of short duration, the timing of these pulses relative to the pulses from transmitter 1 being determined by the setting of the potentiometer. A portion of the output of circuit 37 is applied (by a circuit not shown in Fig. 1) to the grid of the signal selector tube 33, and the operator adjusts the potentiometer until the particular echo trace which he wishes to select appears brighter than the remainder of the trace. The output of circuit 37 is also fed to the outer grid of a separator valve 38, the inner grid of which receives echo signals from the amplifier 32. Valve 38 is cut off except when its outer grid receives a pulse from 37, and during such periods the amplitude of the signal on its anode will be determined by the inner grid potential. Consequently, signals corresponding to the selected target only are passed by valve 38 and led through rectifier 39, amplifier 40 and distributer 43 (synchronous with phasing switch 18) to the deflecting plates of the tubes 35, 36. Diametrically opposite contacts of the distributer 43 are connected as shown to the deflecting plates, the arrangement being such that one tube responds to the echo signals from aerials 5, 6, say, and the other to signals from 7, 8. The effect of such signals on the cathode beam is prolonged by the capacity of a smoothing circuit, so that if the signals received from aerials 5 and 6 are equal, the spot will be in the centre of the screen on tube 36, and similarly with aerials 7, 8 and tube 35. The searchlight is adjusted to keep both spots in the centre of the respective screens. Aerial construction. The transmitting aerial comprises a folded dipole 2, Fig. 2, in line with four shorter director dipoles 3 backed by a wire mesh reflector 4. Each unit of the receiving aerial comprises a dipole 10, Fig. 3, in line with four shorter director dipoles 11 and backed by a wire mesh reflector 12. Transmitter oscillator. This comprises a pair of triodes 50, 51, Fig. 6, in push-pull acting as a self-squegging oscillator with tuned anode and cathode circuits. The frequency is determined by the setting of the bridge 53 on the Lecher 54, and the feed-back by condensers 55. The grid circuit includes a condenser 56, inductance 57 and resistance 58 connected to H.T.+. Output is taken by a tuned coupling Lecher 59 to the aerial feeder 60. Superhet receiver. The received carrier is mixed with the third harmonic of the frequency generated by the local oscillator. Automatic volume control, Fig. 8. A peak rectifier 96 develops a gain control voltage across a load resistance arranged in series with a variable resistance 98 for manual control. The control voltage is passed to a lowimpedance valve 99 by which it is led back to the first three stages of the I.F. amplifier. Video-frequency amplifier. The valve 92, Fig. 8, of this amplifier is de-generatively backcoupled as regards direct current by a resistance 93 which in combination with a cathode load 94 cuts down the D.C. output and keeps the mean grid bias independent of the mean signal input. Features of the system described form the subject-matter of Specifications 581,996, 581,998 and 581,999.