Design and characterization of a tunable open TE₀₁₁ resonator for Q-band pulse EPR experiments

Electron Paramagnetic Resonance (EPR) is an important technique for the investigation of the structure and function of metalloproteins and enzymes. The variety of questions in this line of research requires versatile instrumentation. In this work, we explored the utility of the open resonator concept for a general-use highly tunable TE₀₁₁ resonator design at Q-band frequencies (≈ 34 GHz). Using proof-of-concept calculations, we establish a viable range of critical parameters compatible with the desired instrument specifications. We then present the resonator design, targeting ease of execution and handling. Experimental characterization of the built resonator shows high tunability. Specifically, we show that the resonator can be critically coupled and overcoupled with a three-fold change in the bandwidth using a matching short. We also show that the resonator can be incorporated with frequency tuning by means of movable axial plungers, allowing it to work with a wide range of samples using relatively narrow-bandwidth microwave instrumentation. Furthermore, because of its high tunability, the resonator is very tolerant of manufacturing imperfections, which makes it affordable and easy to execute with minimal tooling. We also discuss the long-term use of the resonator in our research, highlighting its versatility.