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The circuit properties of a resonant cavity are effectively described by its loaded Q, unloaded Q, and shunt resistance. One method of Q measurement depends on an accurate knowledge of the variation of VSWR as a function of frequency in a transmission line terminated by the resonant cavity. A method of measuring shunt resistance along any path in the cavity entails accurate observation of the resonant-frequency shifts caused by an obstacle placed at points along the path. Therefore, both measurements require an AFC system with good stability and high resolution. The parameters of a re-entrant cavity with apertures are considered and the associated experimental setup is described.