A Miniaturized Wireless Passive Frequency Selective Surface Sensor for High-Temperature Applications

A passive and wireless sensor based on frequency selective surface (FSS) is presented for high-temperature applications. The sensor consists of a bent square loop array deposited in an alumina dielectric substrate. By inserting four symmetrical slots in the square loop structure, the dimension of the FSS unit cell is as compact as
$0.13 \lambda _{{0}}$
. The results show that the resonant frequency of the sensor decreases from 13.96 to 13.172 GHz with an average sensitivity of 0.81 MHz/°C when the temperature rises from 25 to 1000 °C. It was found that the resonant frequency shift is mainly attributed to the variations in permittivity of the substrate material. The sensor has a stable resonant frequency for both TE and TM polarizations under incident angles with a range of 0°–30°. The proposed sensor, which has the advantages of miniaturization, low profile, high sensitivity, and excellent angular stability, can be potentially applied in various harsh environments.