Measurement of TE01-mode attenuation in short lengths of circular waveguide

A description is given of equipment for the determination of the attenuation coefficient of the TE01 mode in circular waveguide at 35Gc/s. The Q-factor of a resonator formed by a sample length of the waveguide is measured, and the attenuation coefficient deduced therefrom. Q-factors of the order of 106 are measured by comparing the resonance curve of the waveguide sample with that of a reference cavity of known and much lower Q-factor. The method consists in scanning a klystron oscillator over a narrow band with a gated sawtooth signal, so that the ratio of alternate scan rates can be accurately adjusted with a precision voltage divider. The resonance curves of the waveguide and reference cavities are displayed as alternate traces on a cathode-ray tube having a screen of long persistence. By adjusting the two scan rates and the signal amplitudes, the resonance curves can be matched, and the precision voltage divider then indicates the ratio of unknown to known Q-factor. The accuracy in matching the curves is improved by differentiating the resonance responses and adjusting the scan-rate ratio for equality of amplitudes, i.e. for identical slopes at the points of inflection.