Impact of the Characteristic Impedance of Coaxial Lines on the Sensitivity of a 750-MHz Electronically Tunable EPR Resonator

A 750-MHz electronically tunable resonator was investigated in terms of the sensitivity of electron paramagnetic resonance (EPR) signal detection. The conversion efficiency of the radio-frequency magnetic field was calculated for resonators with 50- and 100-Ω coaxial coupling lines using three-dimensional (3D) microwave field and microwave circuit simulators. Based on the simulation results, two tunable resonators were physically constructed and compared in terms of EPR signal sensitivity using a nitroxyl radical solution. While the resonator with 100-Ω coaxial lines provided 14% greater signal intensity, its signal-to-noise ratio was lower than that of the resonator with 50-Ω lines. To demonstrate the capability of the constructed tunable resonator for EPR imaging experiments, a solution of nitroxyl radical and the leg of a tumor-bearing mouse were visualized.