Patent GB-587390-A (GB)
Title: Improvements in and relating to automatic radio range finders and altimeters
IPC: G01S13/14
Filed: 1940-07-04
Abstract: 587,390. Radiolocation ; electric dischargeapparatus. FORMAN, J., and PYE, Ltd. July 4, 1940, No. 11218. [Classes 39 (i) and 40 (v)] In range-finder or altimeter a wave energy signal is transmitted and received after reflection from the object or ground of which the distance is to be determined, the received wave operating a triggering device to produce a pulse of constant magnitude. The transmission of the signal initiates the building up of a gradually increasing voltage, the pulse of constant amplitude being combined with the gradually increasing voltage to give in the resultant voltage an indication of the distance of the object. A pulse generator 1, Fig. 1, modulates or keys a short-wave radio transmitter 2 and simultaneously synchronizes a saw-tooth voltage generator 3. Waves transmitted from a directional antenna 4 are received, after reflection, by a dipole 15 and fed to a receiver 14, which operates a trigger circuit comprising a magnetron 5 arranged in the magnetic field of permanent or electromagnets 7. A dropping resistor 11 is connected to the anode and a load resistor 20 to the cathode. Anode potential is derived from a potentiometer 12 connected across the H.T. supply 13. The received signal impulse is applied through a condenser 9 across the resistor 11 and momentarily increases the anode potential so that the magnetron is triggered and passes a saturation current. This current causes a potential difference across the resistor 20 in the form of a pulse which is applied to the grid of a valve 6. Owing to the voltage drop in the resistors 11, 20, the anode voltage drops to a point below cut-off so that the magnetron ceases to pass current, also a condenser 10 in parallel with the magnetron. discharges and then recharges slowly through the resistor 11 so as to maintain the magnetron below cut-off until a short time before the next pulse arises, when the condenser is fully charged. The valve 6 is biassed to cut-off by a resistor 18 and is periodically taken to below cut-off by the saw-tooth voltage applied across the bias resistor 18 by a condenser 19. The effect of a pulse C, Fig. 2, at the end of the range, together with the sawtooth voltage S and the constant bias, is' to bring the valve just up to the cut-off point, and so no current flows in the load circuit 16 and meter 22. A pulse B at an intermediate point brings the valve to a point above cutoff where current flows and is shown on the meter 22. A pulse A received almost at the beginning of the saw-tooth cycle causes the greatest current in the meter 22. Alternatively, the valve 6 may be biassed to below cut-off and the saw-tooth voltage applied in a positive direction so that the closest pulse gives a minimum reading and the farthest pulse a maximum. For changing the range, the frequency of the pulse generator 1 is made variable in steps, each corresponding to a separate scale on the meter 22. Simultaneously, the time constant of the trigger circuit is also changed to make at suitable 'for the range utilised. An alternative form of trigger circuit is shown in Fig. 3 in which a cathode-ray tube contains an electron gun 31, a pair of deflecting plates 34 and a target 35. 'The applied pulse from the receiver is fed to the plates 34 from the input 43 across a resistor 42 and deflects the beam on to the target 35 so that, after a certain minimum amplitude, the current flowing between the cathode and target is constant for any deflection. This current causes a voltage drop across a resistor 39 which is applied to the grid of the measuring valve through a condenser 37 and also to the plates 34 through a condenser 36, which charges up. In a period depending on the time constant of the condenser 36 and the resistor 42, the condenser discharges slowly and after an interval allows the beam to return to normal. During this lag any echo signals only deflect the beam more and as the current through the resistor 39 remains constant, no pulse is transmitted to the grid of the measuring valve.