Patent GB-544452-A (GB)
Title: Improvements in distance determination by wave reflection
IPC: G01S13/36
Filed: 1940-10-25
Abstract: 544,452. Radio echo apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Oct. 25, 1940, No. 15714. Convention date, Oct. 25, 1939. [Class 37] An arrangement for determining the distance of an object by means of waves reflected therefrom includes a transmitter for sending a wave towards the object and a receiver for picking up the reflected wave, the transmitted wave being periodically varied in 'frequency, and the frequency modulated reflected wave being converted into an amplitude modulated wave, which is detected and passed to an indicator to indicate the required distance. As shown in Fig. 1 a thermionic oscillator 10 radiates energy to the object 0 by means of an antenna 1. This energy is reflected by the object O, picked up on an antenna 2, and amplified by an automatic volume controlled amplifier 4 which feeds differential detectors 18, 19 through a push-pull transformer 5. Energy from the oscillator is also fed to the common leg of the differential detectors 18, 19 through a variable phase-shifter 3 and a transformer 6. The oscillator 10 is frequency modulated by a re-actance valve 17 which is modulated by a voltage source 11 and a transformer T1. A condenser 15 and a resistor 14 form a phase shifter for the feed-back voltage of the reactance valve. A resistor 12 furnishes a D.C. path from grid to earth and permits application of the modulating voltages to the control grid of the re-actance valve. A radio frequency choke 16 is used to apply plate voltage to the reactance valve and the oscillator. The detected frequency modulation is passed through an amplifier 20 and applied to a voltmeter 9, which may be calibrated in terms of distance. Milliammeters 7, 8 indicate balance of the detector currents and are adjusted for balance of the phase adjuster 3. In a modification, Fig. 2, the received energy is amplified by an amplifier 42 which is automatic volume controlled by rectified energy from a resistor 45 in the circuit of a diode 44. The plate current meter 46 indicates the strength of the reflected wave and therefore indicates the reflection coefficient of the object O for known distances, enabling the nature of the terrain to be deduced. The detected output is passed through a rectifier 43 and controls the gain of an amplifier 40, which applies the modulating voltages to the frequency modulating re-actance valve. An ammeter 41 in the plate circuit indicates the amplitude of the frequency modulation detected by the detectors 18, 19. The meter may alternatively be placed in the screen or cathode lead. The dial of the phase shifter 3 may be calibrated to give the correction to be applied to the distance reading at each setting, or a potentiometer may be ganged to the phase adjuster to insert a compensating voltage in the circuit of the distance meter. The antennae 1, 2 may be replaced by a sound projector and receiver. The frequency modulated oscillator may be replaced by a phase modulated source of oscillations, the wave generated by which is made the same as that generated by the frequency modulated system by applying frequency correction circuits to the modulating voltages. Manual variation of frequency may be applied instead of employing an alternating wave. In this case the reactance valve 17 is omitted and the frequency of the oscillator 10 is varied by varying its tuning. The change in the detector currents as indicated by meters 7, 8 for a given frequency variation is a measure of the distance.