An instrument for accurate determination of relative surface losses in the millimetre band

The instrument consists of one reference cavity and a second test cavity fed from the same source. The rectified outputs from these are combined in a difference amplifier so that the whole arrangement behaves like a bridge whose output can be balanced out for any particular setting. The cavities are operated in the H01 modes and are identical except that one end-wall of the test cavity is removable and forms the sample under test. The measurement is carried out by feeding the bridge from a swept-frequency source and displaying the output on an oscilloscope. When the cavities are balanced at the mid-resonant frequency the response curve of the bridge becomes a measure of the difference in the Q-factors of the two cavities, and, in this way, small changes in the state of the sample can be easily detected. The use of the instrument is illustrated by various experimental results obtained at a frequency around 35 Gc/s.