DESIGN OF DROP-IN MICROSTRIP CIRCULATOR

The operation of a ferrite drop-in circulator is theoretically predict ed and experimentally confirmed. The cavity model is applied both to t he ferrite disc and the dielectric region directly below the ferrite d isc. The standing-wave condition required by the dielectric cavity plu s the two ferrite circulation conditions determine uniquely three desi gn parameters for the circulator: the magnitude biasing field, the rad ius of the ferrite disc, and the dielectric constant of the dielectric sleeve surrounding the ferrite disc. In addition, the characteritic i mpedance of the microstrip feeder lines was purposely designed for 50 OMEGA in order to eliminate the need for impedance transformers. We fo und that for a thin dielectric substrate with dielectric constant smal ler than that of the ferrite, the circulation frequency can be slightl y above FMR resonance. Three circulators were fabricated using design parameters calculated by our theory. The measurements are in reasonabl e agreement with predicted performance of the device.