GB645818A - Improvements in and relating to detection and measurement devices for high frequencyelectromagnetic waves - Google Patents

Abstract

645,818. Wave meters; power meters. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 21, 1948, No. 27391. Convention date, Oct. 24, 1947. [Class 37] A system for detecting or measuring highfrequency electromagnetic waves comprises a bolometer element, a wave-reflecting member arranged near it at a distance which is substantially an integral number of quarterwavelengths of the waves which are to be detected or measured, and a circuit means for measuring the change in impedance of the bolometer. The bolometer is adapted to provide an impedance per square, i.e. an ohmic impedance or resistance between opposite edges of any square-shaped section, substantially equal to the wave impedance in the medium, i.e. air, in which the waves are propagated. The bolometer 1, Fig. 2, is supported by conductors 7, 8 within an hermetically-sealed metal casing 4. Projecting from the casing 4 is a tube 12 within which slides a rod 13 carrying a silver reflector tip 15 which has a flat reflecting surface 2. The rod 13 is connected to the casing 4 by means of a bellows 16 so that the casing may be evacuated to minimize heat loss by convection. The outer end of the rod 13 is finely threaded to engage a knob 14, which also engages the inner surface of an enlarged part of the tube 12 by means of coarse threads. The outer surface of the enlarged part of the tube 12 is calibrated so that the distance through which the reflecting surface 2 has been moved by turning - the knob 14 may be measured. The front wall of the casing 4 has an opening in which a collimating lens or window 17 is hermetically sealed. The bolometer 1 comprises a thin mica or glass support 19, Fig. 3, coated with bolometer material 19a, such as tin, nickel, cobalt, or uranium oxide, or tin chloride, so that its resistance between opposite edges is equal to the wave impedance of air or vacuum. Materials with positive temperature coefficients of resistance, such as cobalt, iron and nickel, may alternately be used. Wire members 18a, 18b may be cemented to the support 19 or they may take the form of painted strips of silver, platinum, or gold. The wire members 18a, 18b are supported by wire members 20a, 20b which are attached to the conductors 7, 8. The wire members 18a, 18b are fastened to the opposite edges of the mica support 19 and are connected to a resistance measuring circuit. The mica is then coated with tin by evaporation or sputtering and the tin is oxidized in a furnace until the resistance reading is equal to the wave impedance of free space or air (377 ohms). To measure the wave-length of ultra-short waves entering the window 17 the leads 7, 8 are connected to a Wheatstone bridge so that the resistance of the bolometer 1 can be measured: The reflector surface 2 is adjusted until a minimum resistance is obtained, indicating that the reflector is an odd number of wavelengths behind the bolometer 1. By measurement of the distance between successive positions giving minimum resistance the wave-length may be determined. The temperature change of the bolometer 1 may alternatively be measured by incorporating a thermocouple in the bolometer material 19a. The thermocouple may then be used to measure the power absorbed by the bolometer. The sensitivity may be increased by using a concentrating means such as a lens or parabolic mirror. For measuring large amounts of power a bolometer element may be used having a negative coefficient of resistance with an impedance greater than the wave impedance of air or vacuum. By passing a steady D.C. through the bolometer it can be heated until its wave impedance is equal to that of air or vacuum. As ultra-high-frequency power is absorbed by the bolometer the D.C. is decreased to keep the bolometer element matched to the air or vacuum. The decrease in the D.C. power supplied is equal to the power absorbed by the bolometor from the high-frequency source. As shown in Fig. 6, a D.C. source 23 is arranged in series with a variable resistance 24 to control the power fed to the bolometer 1 to vary its impedance. The bolometer forms the fourth arm of a bridge of which the other three arms are formed by resistances 25, 26, 27. A voltmeter 29 measures the change in power absorption of the bolometer 1 from the D.C. source 23 with changes in the resistance 24. The resistance 25 is made equal to the wave impedance of the medium in which the measurement is being conducted. When the bridge is balanced the impedance of the bolometer 1 is equal to that of the resistance 25. The difference of the squares of the voltages measured by the voltmeter 29 when no electromagnetic energy is being absorbed and when it is being absorbed, divided by the resistance 25 gives the electromagnetic wave energy absorbed by the bolometer.