A microwave microstrip ring resonator as a moisture sensor for biomaterials: application to wheat grains

A miniaturized, non-destructive sensor employing a microwave microstrip ring resonator (MRR, f(r) = 10.2 GHz, mean diameter 3.69 mm) was developed for estimating the moisture content of a single wheat (Triticum aestivum L) grain. A single wheat grain with a known amount of moisture was placed on the MRR at two different orientations (0 degrees and 90 degrees) with respect to the feedline. The resonance frequency (f(r)), bandwidth (B) and quality factor (Q(r)) of the MRR were calibrated against the moisture content. The measurements were made with a scalar network analyser. The sensor was studied in the normal useful moisture range of 11-32% (on a wet-weight basis), the actual moisture values being obtained by an oven-drying method. The 0 degrees orientation was more sensitive to moisture than was the 90 degrees orientation. The total changes in f(r) for a 21% change in moisture content for 0 degrees and 90 degrees orientations were 235 and 150 MHz, respectively. The errors in moisture estimation with f(r) for 0 degrees and 90 degrees were +/-2.06% and +/-2.09%, respectively. The corresponding values with B and Q(r) for 90 degrees orientation were +/-2.14% and +/-2.19%, respectively. The proposed sensor is more sensitive than a reported waveguide resonator and is easy to operate, for the microstrip offers an open structure, thereby facilitating easy loading and unloading of the samples.