Design of planar transmission line microwave probes for broadband EPR spectroscopy

In broadband transmission-based electron paramagnetic resonance (EPR) spectrometers, non-resonant planar microwave probes play a key role, but very few systematic explorations of their design space exist. We develop design guidelines for two common types of transmission lines, microstrip and coplanar waveguide, to achieve a high effective microwave magnetic field strength, which ultimately leads to a more sensitive EPR measurement set-up. We compare the optimized transmission line structures and show that the coplanar waveguide achieves a higher simulated effective magnetic field strength. The simulation results are confirmed by EPR measurements up to a microwave frequency of 45 GHz.