Patent GB-594765-A (GB)
Title: Improvements in radio distance measuring apparatus
IPC: G01S13/34
Filed: 1944-02-21
Abstract: 594,765. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 21, 1944, No. 3229. Convention date, July 31, 1942. [Class 40 (v)] In a radio distance-measuring apparatus of the frequency modulated type, the rate of change of frequency is periodically varied, and the beat frequency which thus varies periodically for a particularreflecting object, is applied to a sharply tuned circuit which has its output fed to a distance indicating device. A transmitter 10 and dipole 11 radiate waves which are frequency modulated by saw-tooth waves fed to the modulator 19 from a circuit 18, and the signals after reflection are received on dipole 12 and mixed in a heterodyne receiver 13 with the frequency-modulated signal direct from the transmitter, to produce a beat frequency. The circuit 18 is controlled by pulses from a blocking oscillator 21 as described, for example, in Specification 402,629, the frequency being varied cyclically by a variable resistance 24 in parallel with the oscillator grid leak 26, which is rotated by a motor 22. An arm 27 of an indicator 17 is rotated by motor 22 synchronously with the rotation of variable resistance 24, the arm carrying a neon lamp and mask 29 having a slit aperture and being mounted behind a translucent scale 28. As the beat frequency output of receiver 13 is proportional to the distance of the reflecting object and rate of change of the transmitted frequency, the beat frequency produced by each reflected signal will vary as the modulation frequency is changed and the beat frequencies due to reflections from differing distances will assume the same values at different instants during the cycle of variation of the modulation frequency. The beat frequency output from receiver 13 is thus fed through a sharply tuned amplifier 16 to the neon lamp in indicator 28 so that each time the beat frequency assumes a value which enables amplifier 16 to pass signals, the neon lamp lights and produces an indication, the position of which on scale 28 indicates the range of the reflecting object. In a modification, the amplitude of the modulating wave may be varied instead of its frequency. Saw-tooth waves, the frequency of which is set manually, are fed through a motor driven attenuator, Fig. 2 (not shown), to the frequency modulator, the resultant rate of change of frequency of the transmitted wave being proportional to the amplitude of the triangular modulating wave. Both the frequency and amplitude of the modulating wave may also be varied simultaneously. Fig. 2 (not shown) also illustrates a modified indicator in which a movable belt containing an aperture is placed between a scale and tubular neon lamp. In a further modification, Fig. 3 (not shown), a cathode-ray tube indicator is employed. A potential divider, driven in synchronism with the modulation frequency control, generates a time base which is applied to produce a trace on the C.R. tube screen which is intensity modulated by the output from the sharply-tuned beat frequency amplifier to cause an indication at a point on the trace, which is provided with a distance-indicating scale. The change in frequency of reflected waves due to the Doppler effect when relative movement exists between the system and the reflecting body causes indications to be spread slightly along the range scale; and when a triangular wave is employed for frequency modulation, two separate indications may be obtained if the relative speeds are sufficiently high, and may be employed to determine the relative speed approximately. Specification 317,068, [Class 40 (v)], and U.S.A. Specification 2,045,071 also are referred to.