Unloaded Q of Single Crystal Yttrium-Iron-Garnet Resonator as a Function of Frequency (Correspondence)

The practical feasibility of constructing magnetically tunable broad-tuning range microwave filters using single crystal yttriumiron-garnet resonators was demonstrated in a recent paper. Experimental results were presented on one- and two- resonator filters which can be tuned by varying a dc magnetic field bias over a full waveguide bandwidth and greater, at the same time maintaining an insertion loss performance which is comparable to mechanically-tuned cavity filters. The crucial parameter of the resonant elements in a band-pass filter is the unloaded Q, Q_u. With a spherical single crystal of yttrium-iron-garnet the Q_u decreases with frequency below X-band frequencies reaching very low values at frequencies around 2000 Mc.