Resonant inductive coupling of RF EPR resonators in the presence of electrically conducting samples

A new method of matching for radio frequency EPR resonators is proposed. It uses resonant inductive coupling and is particularly suitable for studies with lossy conducting samples. The equivalent circuit of the resonant inductive coupling method is presented and a theoretical analysis of the equations that produce the critical coupling coefficient is discussed. A prototype resonant inductive coupling suitable for loop gap resonators operating at about 200 MHz has been developed and tested. Experimental results obtained with saline solutions show the advantages of resonant inductive coupling in terms of the coupling coefficient, RF magnetic field amplitude and field homogeneity. This matching method has been especially designed for CW EPR applications, in which the adoption of critical coupling is important for optimization of sensitivity. Moreover, resonant inductive coupling should also produce benefits for pulsed EPR studies.