A Novel Sensor-Integrated Aperture Coupled Microwave Patch Resonator for Humidity Detection

Wireless sensor elements integrated with miniaturized antennas are useful in various applications such as wearable chemical and environmental sensors and Internet-of-Things (IoT) sensor nodes. A major problem in antenna operation is detuning of the antenna bandwidth due to loading by the sensor element. In this article, we report on the integration of an interdigitated capacitor (IDC), acting as a sensor, into an aperture coupled patch (ACP) antenna, such that weak coupling is established between the IDC and the rectangular patch resonator. Because of low mutual coupling, during the sensing process the antenna is not detuned out of its operational bandwidth and its performance is not compromised by the presence of the sensor and vice versa. A sensing material (barium titanate film) is deposited on the IDC located at the edge of the microstrip line used to slot-feed the ACP antenna. A change in the material permittivity is transduced into a variation of resonant frequency of the antenna. We describe the design and fabrication of the IDC sensor-integrated ACP antenna and demonstrate the measured sensing performance at different temperatures and relative humidity concentrations.