Patent GB-842720-A (GB)
Title: Radio-frequency electrical power measuring apparatus
Inventor: WOODS DONALD
IPC: G01R21/00
Filed: 1956-09-18
Abstract: 842,720. Power measurements. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Sept. 17, 1957 [Sept. 18 1956], No. 28457/56. Class 37. Radio-frequency power measuring apparatus includes a coaxial line terminated by a resistor 19 and having a portion 14 of different characteristic conductance from the remainder, the ends of the portion being connected respectively to rectifiers 10, 10 of a voltage doubler circuit, and the arrangement being such that the input impedance of the apparatus is constant and purely resistive throughout the frequency range of the apparatus. The radio-frequency input is fed to the main inner coaxial conductor, Fig. 3, comprising an input socket 12, a short undercut portion 14, a plug and socket 15, and a conical portion 18 connected to the coaxial outer conductor by terminating resistor 19 ; the conical portion and resistor being removable. Silicon crystal rectifiers 10, 10 are connected by leads 11, 11 to respective ends of the portion 14, the output from each crystal being fed to output sockets 24. The basic circuit diagram of this part of the apparatus is shown in Fig. 5. The characteristic conductance Gt of the conical portion is chosen so that Gt = G d  + Go, where G d  is the sum of the mean crystal conductances, and Go is the characteristic conductances, and Go is the characteristic conductance at the input socket. The required characteristic impedance Zu of the short portion is stated to be given by Zu = 1/Gtd [N + (1 + N<SP>2</SP>)<SP>¢</SP>] where Gtd = Gt + Gd/2 N = VC<SP>1</SP>d/21GTD V = 2À997 x 10<SP>10</SP> cm. sec<SP>-1</SP> C<SP>1</SP>d = The sum of the crystal capacitances together with the capacitance change in the co-axial line produced by the undercut short portion. For such a characteristic impedance of the portion 14, the resulting input impedance of the apparatus is substantially independent of frequency. The output from sockets 24 is fed to an indicator unit, Fig. 8, comprising ganged resistors V  1 , V 2 , and pre-set resistors F, W, T ; a calibrating switch shorts resistors F, W, V 2 , and in this position V  1  and T are adjusted to provide : (a) a current through the micro ammeter which is a linear function of the radio-frequency power to be measured ; (b) full scale deflection of the micro-ammeter for a given standard value of power, 0À6325 watts in 50 ohms. Several pairs of values of V  1  and T are obtained corresponding to various temperatures of the crystals 10, 10 as measured by a thermometer mounted in the crystal housing, Fig. 4 (not shown). Two constants i, S are derived graphically from the variation of V  1  with T ; and these constants determine the appropriate settings for F, T, W so that, with the calibration switch at 'use', the conditions (a) and (b) above hold at any temperature of the crystals, the appropriate temperature being selected by suitably setting V  1  and V 2 , a temperature calibrated scale being provided for this purpose. Condition (a) is only approximately realised by the above method. For improved accuracy it is desirable that the microammeter indication should be slightly non-linear with respect to the controlling current, figures giving the desired variation are provided. Specification 670,339 is referred to.