Instrumentation for remote control of the unloaded quality factor,Q0, of a loop‐gap resonator is described. The value ofQ0can be lowered by weak inductive coupling of the resonator to a loop of wire that contains a carbon resistor. Replacement of the resistor by a PIN diode permits remote control. Theoretical analysis and experiments are carried out in a context of electron paramagnetic resonance (EPR) spectroscopy atXband. Equivalent circuits are solved numerically and predicted performance confirmed experimentally. Two applications are demonstrated: (a) superfine control of critical coupling of the incoming transmission line to the resonator in excess of −80 dB, and (b) dynamicQspoiling for reduction of the dead time after a microwave pulse. Possible degradation of EPR system performance by shot noise from the PIN diode is considered. Scant literature suggests that it is very low, and noise from this source could not be detected experimentally. ©1995 American Institute of Physics.