Patent GB-679692-A (GB)
Title: Improvements in or relating to radio direction finding equipment
Inventor: STRONG CHARLES ERIC
IPC: G01S3/54
Filed: 1950-06-21
Abstract: 679,692. Radio direction-finding. STANDARD TELEPHONES & CABLES, Ltd. June 21, 1950, No. 15486/50. Class 40 (vii). Relates to means for automatically blanking out the directional indication provided by a radio direction-finder when the indication would be erroneous due to the simultaneous reception of waves of varying relative phase which have travelled by different paths, e.g. direct and sky waves. Under such conditions the relative amplitudes of the resultant signals at adjacent points varies, i.e. fading is experienced, and according to the present invention a plurality of similar spaced aerials is provided, the indication being blanked out whenever the signal from any of the aerials drops below a predetermined level. The invention is described as applied to a direction-finder of the type disclosed in Specification 594,530, in which the signals received by a plurality of non-directional aerials equally spaced around the circumference of a circle are applied sequentially to a receiver by an electronic switch so that the receiver output is phase-modulated at the switching frequency, the phase modulation being converted to amplitude modulation by a discriminator and the amplitude modulation being compared with the switching voltage to give a bearing indication. As shown, the circumferentially-spaced aerials 1-9 are rendered operative sequentially by time-displaced gating pulses from sources 11-19 and the signals from these aerials of radio frequency f1 are applied through R.F. amplifier 21 and frequency-changer 26 to I.F. amplifier 33. The signal received by another non-directional aerial 10 is applied through similar stages 24, 28, 35 and heterodyned at 38 with the 20 kc/s. output from oscillator 39, the resultant difference signal being heterodyned at 44 with the output from 33 to give a 20 kc/s. signal whose amplitude varies as the outputs from aerials 1-9 and this signal is applied through amplifier 46 to circuit 49 where four D.C. voltages are produced having amplitudes proportional to Œ sin # and Œ cos #, where # is the direction of the incoming signal. The D.C. voltages are applied to charge condensers coupled to the deflector plates of a cathode-ray oscilloscope 58 and a radial trace of angular position # is produced by intermittently shorting the four plates together at a frequency of 50 c/s. as described in Specification 590,260, [Group XXXVI]. In accordance with the present invention the output from amplifier 46 is detected at 59 and applied in parallel to the suppressor grids of nine pentode gate circuits 71-79, gating pulses from the sources 11-19 being applied to the control grids of corresponding gate valves via equal time-delay circuits (not shown) so that the anode output from each gate valve, e.g. 79, comprises a negative D.C. pulse of amplitude proportional to the amplitude of the signal received by the corresponding aerial. A corresponding capacitor 103-111 is connected in parallel across the anode load of each gate valve the time constant being such that the anode voltage is substantially constant and inversely proportional to the amplitude of the corresponding aerial signal and the anodes of the gate valves are connected through respective diodes 112-120 to the grid of a cathode follower 123 so that its grid voltage and hence its cathode output is determined by the weakest signal received by the spaced aerials. The cathode follower is coupled to the grid of a triode 127 and a 50 c/s. voltage synchronized with the oscilloscope " shorting " frequency is also applied to the grid, the cathode bias being adjusted by potentiometer 128 so that the valve is cut off except when the positive D.C. voltage output from the cathode follower exceeds a predetermined level and when this occurs a 50 c/s. output is produced in the anode circuit which is D.C. restored by diode 135 and applied between the grid and cathode of the oscilloscope to blank out the trace during its radial excursions. If the fading conditions are intermittent a continuous display may be obtained by providing the oscilloscope with a long persistent screen; if required, the screen may be masked out except for an annular ring. In a modification, Fig. 2 (not shown), the oscilloscope is normally blanked out and the output from V127 brightens the trace when the output from V123 is less than a predetermined level, i.e. when the weakest aerial signal is greater than a predetermined level.