Design of drop-in microstrip circulator

The operation of a ferrite drop-in circulator is theoretically predicted and experimentally confirmed. The cavity model is applied both to the ferrite disc and the dielectric region directly below the ferrite disc. The standing-wave condition required by the dielectric cavity plus the two ferrite circulation conditions determine uniquely three design parameters for the circulator: the magnitude biasing field, the radius of the ferrite disc, and the dielectric constant of the dielectric sleeve surrounding the ferrite disc. In addition, the characteristic impedance of the microstrip feeder lines was purposely designed for 50 /spl Omega/ in order to eliminate the need for impedance transformers. We found that for a thin dielectric substrate with dielectric constant smaller than that of the ferrite, the circulation frequency can be slightly above FMR resonance. Three circulators were fabricated using design parameters calculated by our theory. The measurements are in reasonable agreement with predicted performance of the device.<>