Highly sensitive differential moisture sensor based on a novel multi-mode resonator

Abstract

This paper presents a highly sensitive differential microwave sensor for moisture content (MC) measurement, demonstrated using whole-wheat flour as the sample under test. The proposed design incorporates a pair of novel symmetrical multi-mode resonators placed on either side of a 50 Ω microstrip transmission line. Fabricated on a low-cost FR-4 substrate, the sensor offers an optimal balance between high sensitivity and economic feasibility. Differential sensing is realized by employing one resonator as a reference (έr = 1) and the other as the sensing element exposed to the sample. The sensor is designed to generate four transmission zeros (TZs) within the frequency range of 0.1–4 GHz, with individual sensitivity of each TZ analyzed and compared. The fourth mode demonstrates a peak sensitivity of 168.8 MHz/έr. The sensor’s operational mechanism is examined through odd- and even-mode analysis, validated via full-wave electromagnetic simulations using CST Studio Suite. The fabricated prototype has compact physical dimensions of 8.4 × 4 × 0.16 cm3. Experimental verification was performed using both a vector network analyzer and a custom low-cost readout circuit, with results showing excellent agreement. The proposed sensor’s compact size, low cost, and high-performance position it as a strong candidate for precision MC monitoring in food industry applications.