High‐Frequency Transmission Lines – Waveguides

Summary

High‐frequency transmission lines belong among the basic elements of microwave circuits. In lower microwave frequency bands, coaxial lines are used with transverse dimensions significantly smaller than the transmitted wavelengths. Consequently, in the transverse plane of such lines, the electromagnetic field is stationary and transverse electromagnetic waves (TEM) propagate along the line. On the contrary, in higher microwave frequency bands, smaller wavelengths become comparable to the transverse dimensions of the transmission lines, and the field distribution has a wave nature not only in the longitudinal, but also transverse direction. In waveguides, different transverse electric (TE) and magnetic (TM) modes can be excited. Successful usage of waveguides is based on a good knowledge and deep understanding of their characteristic properties, including critical (cutoff) frequency, behavior of individual modes in different waveguides, their dominant modes, and single‐mode bandwidths. Attention is paid also to microwave lines realized on dielectric substrates (microstrip lines, coplanar waveguides) and to nonreciprocal microwave structures (isolators, circulators).