A direct-reading waveguide standing-wave detector for use at low power levels

Abstract

A method is described of directly measuring voltage standing-wave ratios (v.s.w.r.) at a fixed input power and frequency by means of a directional coupler, in which the errors introduced by the coupler are compensated. The directional coupler is used to measure the power reflected by the load, which is a function of its v.s.w.r. The main purpose of this development has been to afford a means of directly measuring the v.s.w.r. of low-level detector crystals. In this application it is necessary to have an input power level of not more than about 5?W, below which the v.s.w.r. is substantially constant, and it is required to measure v.s.w.r.'s of the order of 0.80 to an accuracy of ±0.01. Unless the coupler has infinite directivity and one arm is perfectly matched, an error is introduced into the measurement which depends on the phase of the voltage wave reflected by the load. Since it is difficult to obtain a tightly-coupled directional coupler suitable for use at low power levels (of the order of 5?W) in which the error due to imperfect directivity is negligible, the treatment adopted is to cancel out the voltage waves which produce the error. An instrument developed on these principles is used to measure the v.s.w.r. of crystal rectifiers at 3.20 cm over a range of power levels from 5 to 650 ?W. The accuracy is better than ±0.01 from 0.90 to zero v.s.w.r.