The Measurement of Power at Centimetric and Decimetric Wavelengths

In order to assess the performance of transmitters both for radar and other applications it is necessary to be able to measure the power given out and the present paper deals with the methods adopted to measure mean powers of the order of 1¿100 W at wavelengths ranging from 3 to 50 cm. The power may be in the form of a continuous wave or may consist of short pulses of high peak-power. Methods of measurement may be divided into two distinct classes, one where all the power is absorbed in the measuring Jevice, and the other where the measuring arrangement forms part of the transmission system between the transmitter and the load and a known percentage of the power is abstracted and measured. The first method has the greater accuracy and has therefore been used as a standard measurement and for calibrating instruments depending on the second method. Fundamentally, the procedure adopted for the first method has been to absorb all the power in a water load and to measure the amount of heat produced. The paper also describes auxiliary gear for the provision of a constant-head water supply. Systems of the second class depend on the abstraction of a known percentage of the power and this has been measured by means of a bolometer, the whole of the apparatus being calibrated against an instrument utilizing the first method. Apparatus has been designed for operation in bands in the region of 50, 10 and 3 cm wavelength and whilst no attempt has been made to give universal coverage in between these bands it is obvious that a compromise between the varying methods adopted would give satisfactory results. In fact it is shown that at 10 cm the methods used at 50 cm and 3 cm tend to overlap and either may be used depending on whether wave-guide or cable transmission is used.