High-Sensitive Microwave Humidity Sensor Using Polyvinyl Alcohol/Carboxymethyl Cellulose (PVA/CMC) Composites

This study investigates the humidity sensing characteristics of microwave sensors coated with polyvinyl alcohol/carboxymethyl cellulose (PVA/CMC) composites with different weight percentages. The microwave sensor has a band-stop filter characteristic and consists of a microstrip transmission line with an interdigital capacitor-defected ground structure (IDC-DGS). To evaluate performance, PVA/CMC composites were prepared in 100/0 (pure PVA), 90/10, 80/20, 60/40, and 0/100 (pure CMC) weight percentages. The humidity sensing capability of the IDC-DGS-based microwave sensors coated with the PVA/CMC composites with different weight percentages was compared by measuring the variations in the resonant frequency and magnitude level of the transmission coefficient. The relative humidity (RH) was changed from 40% to 90% with increments of 10% at a temperature around 25 °C. The experimental results demonstrate that the humidity sensing capability of the microwave sensor in terms of the variations in the resonant frequency and magnitude level of the transmission coefficient increased as the weight percentage of CMC content increased. Pure CMC shows enhanced humidity sensing performance compared to gelatin and PVA in terms of the percent relative frequency shift and effective relative permittivity.