Microwave Probe Based on an Inverted Microstrip Line for Broadband Electron Paramagnetic Resonance Spectroscopy

This work presents a broadband inverted microstrip line (IMSL) as a microwave probe for electron paramagnetic resonance (EPR) spectroscopy, a technique used to detect unpaired electron spins in biological and chemical samples. The IMSL geometry enables sample placement in the most homogeneous region of the microwave magnetic field, overcoming limitations of standard coplanar waveguide (CPW) and microstrip line (MSL) probes. Simulated and measured S-parameters confirm broadband operation up to 67 GHz. Improved field homogeneity relative to MSLs is demonstrated by a reduced coefficient of variation of the simulated magnetic field and by EPR measurements showing signal amplitudes independent of sample orientation. The IMSL geometry offers a compact microwave probe with straightforward fabrication for broadband EPR sensing in biological and healthcare applications.