A wideband semi re-entrant microstrip coupler

The semi re-entrant microstrip (SREM) section is a useful structure for the realization of high-levels of coupling between two microstrip lines. A full-wave spectral domain method is used to calculate the dispersion characteristics and the frequency-dependent line impedances of an SREM section. Results obtained in this way are interpreted in a novel way by relating them to the parameters of a simple equivalent directional coupler. This facilitates the design of a stepped asymmetric coupler through application of well-known TEM transmission line coupler design principles. This paper describes the design of such a coupler. The theoretical and measured performance of a prototype is presented.