The development ofin vivoelectron paramagnetic resonance spectrometers has generated a need for a better method of inductive coupling between a microwave resonator and a transmission line. We describe here an approach, based on introducing an additional impedance transformer between a coupling loop and the transmission line. This additional transformer decreases (by a factor of 20) the resonance frequency shift that occurs during adjustment of the coupling. The calculated optimal length is slightly greater than a quarter of the wavelength of the electromagnetic waves. Experimental measurements of the return loss for the matching circuit has shown the validity of the calculations and the usefulness of the proposed theoretical model. ©1997 American Institute of Physics.