Radio frequency reactance properties of the Bruker ENDOR cavity

The practice of electron‐nuclear double resonance (ENDOR) requires that simultaneous microwave and radio frequency irradiation of the sample be performed. A common arrangement for achieving this requirement is to place a nonresonant coil (rf structure) into a cylindrical TM110 resonant microwave cavity. We examined the vector impedance properties of the radio frequency circuit used in these experiments for the commercially available TM110 ENDOR cavity available from Bruker. Our findings indicate that there is a parasitic capacitance between the cavity wall and the coil, and this capacitance undergoes resonance at frequencies that coincide with distortions in the spectral baseline. We are able to eliminate this resonance condition and the associated baseline aberrations by using a broadband impedance match technique that incorporates a series capacitance and an inductive shunt into the rf circuit.