Cutoff frequencies of the dielectrically loaded comb structure as used in traveling-wave masers

The subject of traveling-wave maser design is reviewed and a first step towards an analytical design procedure is presented. A method is derived for calculating the upper and lower cutoff frequencies of a comb-type slow-wave structure of simple geometry. It is based on the electromagnetic field pattern and the equivalent impedances which are calculated for these frequencies, both for the dielectrically loaded and the empty comb structure. The design procedure resulting from these calculations permits the prediction of a dielectric loading geometry that shifts the upper and lower cutoff frequency of the empty comb to new, lower values which can be arbitrarily specified within certain limitations. Frequencies calculated by this procedure are compared with the results of measurements, and it is found that cutoff frequencies can be predicted to better than 10 per cent.