High-Resolution Dielectric Constant Measurement Using a Sensor Antenna With an Allocated Link for Data Transmission

A dual-band low-profile patch antenna, which operates based on aperture-coupling mechanism, is exploited to measure the permittivity of a sample under test (SUT) in the microwave frequencies and send the measured data simultaneously. The upper resonant frequency, which is considerably affected by changing the permittivity of the SUT, can be utilized for sensing purpose. In contrast, the lower resonant frequency is almost independent of the samples hence can be used as the communication link. The samples are placed on top of the feeding side of the structure. The variation in the sample's permittivity leads to changes in the input impedance of the sensor antenna. While this change results in having different feedline-dependent resonant frequencies, the cavity-dependent resonant frequency is almost preserved intact for different samples. The sensor antenna is designed using electromagnetic solver and an equivalent circuit is proposed to model its behavior. The effect of varying parameters in the equivalent model is investigated. Moreover, by fabricating the sensor, the measurement results show that by changing the permittivity of samples from 1 to 12.85, the upper band resonant frequency (feedline-dependent) shifts from 12.30 to 11.37 GHz (78 MHz per unit dielectric constant). Nevertheless, the small variation in resonant frequency and radiation characteristics of the antenna in the lower band (cavity-dependent) can provide a stable communication channel with respect to the SUT. This link with a reasonable 2.8% (9.54-9.81 GHz) 10-dB impedance bandwidth can be reserved for sending the measured and pilot data in a wireless sensor system.