Humidity and Temperature Dual Flexible Microwave Sensor

A dual flexible microwave sensor is presented for humidity and temperature sensing. The sensor consists of two microstrip interdigitated resonators printed on a flexible Kapton substrate. One resonator is coated with polyethyleneimine silane 1:0.5 and the other is left uncoated. The coated resonator at 3.26 GHz showed a good sensitivity of 0.45 MHz/%RH and -0.01 dB/%RH in a range of 35 - 85 % RH, with low hysteresis (<5%), and -0.192 MHz/degrees C and 0.01 dB/degrees C in a range of 30 - 45 degrees C. On the other hand, the uncoated resonator showed a linear response and high hysteresis (> 30%) with a low sensitivity of -0.045 MHz/%RH and similar to 0 dB/%RH under humidity. It showed a temperature sensitivity of 0.07 MHz/degrees C and 0.01 dB/degrees C. RH sensitivity does not vary with temperature for both the coated and uncoated resonators. The uncoated resonator showed a distinct RH-independent temperature sensitivity in magnitude which makes it easier to decorrelate variations of both parameters.