Cutoff Frequencies of the Dielectrically Loaded Comb Structure as Used in Traveling-Wave Masers

Iii the early development of the traveling-wave maser (TWM), 1 the design procedures used were largely empirical. Short T W M model sections were built, tested and modified in order to meet the desired performance specifications. By this cut-and-try method, a satisfactory design was finally derived which was applied in the construction of full-length TWM's. However, a more satisfying approach is possible if the relevant theoretical aspects regarding the maser active material, the ferrimagnetic isolator and the electromagnetic behavior of the slow-wave structure are known, either rigorously or approximately. Then a T W M can be designed on the basis of analysis before actual construction. Most attractive in the analytical approach is the inherent flexibility and versa* T h i s work WHS s u p p o r t e d in p a r t by the U.S. A r m y Signal Corps under Cont r a c t DA 3G-039-SC-85357. 437 4.38 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , J A N U A R Y 1964 tility. Thus, a large number of design ideas may be explored and a near optimum configuration can be found before any hardware is built. The present paper is a step in the direction of a more analytical approach. Using reasonably accurate approximations to the field pattern at both cutoff frequencies, the equivalent T E M line impedances, the "effective" dielectric constants and, finally, the cutoff frequencies are calculated. This results in a numerical design procedure for the T W M structure. The analysis is made for a comb having fingers of rectangular cross section and for dielectric loading with maser material in the form of one or two rectangular parallelepipeds as shown in Fig.