GB644290A - Improvements in and relating to microwave power measuring instruments - Google Patents

Abstract

644,290. Power measurements. BRITISH THOMSON-HOUSTON CO., Ltd. June 17, 1946. No. 18140. Convention date, June 19, 1944. [Class 37] [Also in Group XL (b)] In a microwave-power-meter wherein the H.F. concentric transmission line for the electromagnetic wave to be measured is series connected with a bolometer resistance across which a voltage is developed proportional to the wave power, a stub-transmission line (or lines) connected across the main line effects reactance compensation at the mean operative frequency so that the impedance presented to the line by the bolometer and matching section is purely resistive, and a further tunable stubtransmission line connected across the main line between the stub-line and the bolometer resistance provides reactance compensation at other frequencies on the band of operation. As shown, the main transmission line 1 comprises concentric conductors 4, 5 and the bolometer 2 is mounted in a cartridge 7 resiliently held between fingers 6 terminating the conductor 4 and fingers 8 of a contact block 9 which is supported by insulation 11 within a conducting ring 10 secured to the end of conductor 5 to form a concentric capacitor short-circuiting the main transmission line for H.F. currents. Two identical stub-lines 12, 13 are connected across the main line 1. Each comprises an outer tubular conductor 17 seated in a block 18 supported by conductor 5, and an inner hollow conductor 14 screwed at 15 to the inner conductor 4. Tuning plugs threaded at 22 into the conductors 14 are adjusted to cancel out the input reactance of the transmission line 1 at the point of connection of lines 12, 13 at the mid-frequency of the operating band. Between the bolometer 2 and the stub-lines 12, 13 a tunable concentric transmission line 24 is connected across the main line 1. Tuning is effected by a capacitortype plunger 29 of length=“γ at the midband frequency, slidable along the inner conductor 25 in spaced relation with the outer conductor 27. The plunger 27 is carried by a piston 30 supported by an anchor block 32 and is adjusted through a rack 34 and pinion 35 by manually rotating a shaft 36. The length of line 24 is adjusted to a quarter wavelength of the operating frequency which may be indicated on a graduated disc 39 by a pointer 38 on the shaft 36. The length of the main line between the stub-lines 12, 13 and the line 24 is chosen so as to transform the conductance of bolometer 2 to the characteristic conductance of the line 1 at the point of connection of lines 12, 13 thereto, so assuring that the wave power is all absorbed in the bolometer 2. The voltage developed in the bolometer 2 and hence the power of the wave, is measured by a bridge 3 the meter 46 of which may be used to check the voltage of the battery 43 supplying the bridge by operating a push-button 49.