Portable Microwave Sensor Based on Frequency-Selective Surface for Grain Moisture Content Monitoring

A novel portable, lightweight, and compact microwave sensor for monitoring the grain moisture content (MC) is proposed in this letter. The proposed sensor is a frequency-selective surface (FSS) antenna consisting of 21 square-shaped complementary split-ring resonators (SCSRRs). The proposed sensor was printed directly on a lightweight and flexible container made of PF-4 with the size of 60 mm x 28 mm x 50 mm. Each SCSRR consists of two cocentered square-shaped rings with 0.3-mm spacing. The length, width, and thickness of the conductive section are 52 mm, 20 mm, and 0.017 mm, respectively. The operating frequency band was chosen as 1-6 GHz due to the insensitivity of the water's dielectric constant to the temperature variation. Monitoring variations in S-parameters, notably the resonance frequency, indicate the MC variations. The proposed sensor detects a 160-MHz shift in the resonance frequency for barley MC ranging from 10 to 25%. The high sensitivity, compactness, flexibility, low weight, portability, small cross-interference sensitivity, simplicity of operation, environment-friendly fabrication, and low cost of the proposed sensor make it attractive for monitoring grain MC.