Study of wood samples positioning on two microwave planar coupled ring resonators for water content measurements

Microwave resonance-based techniques have become increasingly popular for the non-destructive testing of wood materials, as they enable the measurement of moisture content without causing damage. This paper presents a preliminary study focusing on the most suitable way of placing wood samples on two microwave planar coupled ring resonators, specifically the capacitive and inductive configurations, for water content measurements. The study aims to assess the impact of sample positioning on measurement accuracy, considering two different positions corresponding to the coupling points: between the ring gap and the feed line and the inner gap. Results indicate that, for capacitive coupling (c.c.), placing the sample in proximity of the inner gap is more suitable for detecting variations in permittivity, as evidenced by the regression analysis with a R2 value exceeding 0.98 for the two peaks. Instead, for the inductive coupling (i.c.) configuration, both positions present a good response. The regression analysis reveals a R2 value greater than 0.98 for the two peaks in proximity of the outer gap, and a R2 value of 0.9 for the inner one.