The Cavity Resonator

The cavity resonator is a bandpass filter circuit with a center resonance frequency
related to its physical dimensions. The unloaded Q depends on its physical volume
and at VHF can range between 2000 and 10,000. The loaded Q (that is, the damped
unloaded Q, produced by the effect of the terminating impedances) is usually
designed to be between 500 and 1000.
Asystem using a cavity resonator gives protection to co-sited receivers by reducing
the radiation of wideband noise and spurious products. Cavity resonators may also be
connected together to provide additional isolation when multiple transmitters are
combined to a single antenna. When used in this application, the cavity resonator is
used together with ferrite isolators to ensure unidirectional power flow.