Design of RF sensor for simultaneous detection of complex permeability and permittivity of unknown sample

In this paper, a novel microwave planar resonant sensor is designed and developed for simultaneous detection of permittivity and permeability of an unknown sample using a nondestructive technique. It takes advantage of two-pole filter topology where the interdigitated capacitor (IDC) and spiral inductor are used for placement of a sample with significant relative permittivity and permeability values. The developed sensor model has the potential for differentiating permittivity and permeability based on the odd mode and even mode resonant frequencies. It operates in the ISM (industrial, scientific and medical) frequency band of 2.2–2.8 GHz. The sensor is designed using the full wave electromagnetic solver, HFSS 13.0, and an empirical model is developed for the accurate calculation of complex permittivity and permeability of an unknown sample in terms of shifts in the resonant frequencies and transmission coefficients (S₂₁) under loaded condition. The designed resonant sensor of size 44×24 mm² is fabricated on a 1.6 mm FR4 substrate and tested, and corresponding numerical model is experimentally verified for various samples (e.g., magnetite, soft cobalt steel (SAE 1018), ferrite core, rubber, plastic and wood). Experimentally, it is found that complex permeability and permittivity measurement is possible with an average error of 2%.