Humidity sensing performance of defected ground structure‐based microwave sensors coated with PMMA, PHEMA, and PVA

This article compares the humidity sensing performance of microwave sensors coated with a poly(methyl methacrylate), poly(2‐hydroxyethyl methacrylate), or poly(vinyl alcohol) (PVA) polymer. A microwave sensor based on a modified interdigital‐capacitor‐shaped defected ground structure (IDCS‐DGS) in a microstrip transmission line with enhanced permittivity sensitivity was designed on an RF‐301 substrate with a thickness of 0.76 mm, which operate at a 1.5 GHz microwave frequency under unloaded conditions. One of the three different polymers was coated onto the IDCS‐DGS of the microwave sensors at a thickness of 0.02 mm. The humidity sensing performance of the microwave sensors coated with each of the three polymers was measured by the shift in the resonant frequency and magnitude level of the transmission coefficient when relative humidity was varied from 40 to 90%, with increments of 10%, at a temperature of 25°C. The experiment results show that the humidity sensing performance of the PVA‐coated microwave sensor is the best among the three polymers.