GB493708A - Improvements in and relating to high frequency electrical apparatus for use in determining the relative velocity of moving bodies - Google Patents

Abstract

493,708. Speed indicators ; directive wireless signalling; thermionic valves. BRITISH THOMSON-HOUSTON CO., Ltd. April 9, 1937, No. 10206. Convention date, April 9, 1936. [Class 37] [Also in Groups XIX and XL] In a method of determining the relative velocity of two bodies by using the Doppler effect, high frequency electric waves are radiated from one body towards the other and are received on the first body after reflection from the second, the relative velocity being determined by measuring the beat frequency produced by reacting the reflected waves with the radiated waves. Waves not exceeding a few centimetres in length are used, in order that a large percentage of the radiation may be scattered at the second body, part of it returning along the line of sight. Apparatus for applying the method to the determination of the velocity of an aircraft relatively to the ground comprises a high frequency radio transmitter 1 of the kind described in Specification 491,444, [Group XL], and a high frequency receiving apparatus 2, of the kind described in Specification 491,445, [Group XL], mounted on opposite sides of a block 11 rigidly secured to a vertical shaft 12. The radiating member 3 of the transmitter is placed at the focus of a parabolic metal reflector 4 which has a diameter many times the wave length of the radiation, thus producing a highly directional beam. A small disc reflector 5 is placed in front of the member 3. The antenna 7 of the receiver is similarly placed at the focus of a parabolic reflector 8. In order to obtain a heat frequency in the receiver, two small metal reflectors 13, 14 are secured to an adjustably mounted supporting rod 15, and are arranged at angles of 45‹ to the line of propagation of the transmitter, so that a small portion of the wave is directly reflected from the transmitter to the receiver The intensity of this portion may be adjusted by mounting an iris metal diaphragm 17 in its path or, alternatively, a polarized filter may be used. A frequency responsive device 18, having a scale calibrated in miles per hour, is included in the output circuit of the receiver 2, and a telephone 19 permits the operator to listen to the beat note of the receiver. By rotating the transmitter and receiver about the axis 12 by means of cables 22, 23 connected to a control rod 21 until a note of maximum pitch is heard in the telephone, it is possible to determine the true direction of motion of the aircraft with respect to the ground, since maximum pitch occurs when the transmitter faces the direction of motion. A pointer 24 on the control rod 21, co-operating with a scale 25, gives the angle between the true direction of motion and the fore-and-aft line of the craft. Both transmitter and receiver are shielded by a metal casing 30 having polarized filters 28, 32 placed over the open end ; these consist of spaced parallel conductors which may be electrically connected together at one end and insulated from each other at the opposite end, or alternately connected together at opposite ends to form a zigzag conducting path. In a modified shielding arrangement, Fig. 5, the discharge device 35 with antenna member 36, which is placed at the focus of a parabolic reflector 37, is mounted in a cylindrical metal shield can 40. The polarized filter 41, comprising a plurality of conductors 43 arranged perpendicular to the plane of polarization of the high frequency wave, is mounted on a sleeve 42 which can be moved in and out of the cam 40. A window 44 made from a material such as mica transparent to short radio waves is placed across the outer end of the sleeve. The temperature within the shield can is controlled by an electric heater 49 under the control of a thermostat 49<1>. An adjustable plane reflector 45 is provided inside the shield can. When the spacing between the filter and the discharge device is small, two or more filters may be employed.