Sensor Antenna Transmitter System for Material Detection in Wireless-Sensor-Node Applications

This paper presents a fully combined sensor-antenna transmitter system for sensing relative permittivity of material at microwave frequencies. The transmit (Tx) antenna senses the relative permittivity of specimen and loads the radio frequency (RF) oscillator which translates the relative permittivity to the operational frequency of the carrier signal. At the same time, independent data, which can be the Radio Frequency Identification (RFID) information, are ON-OFF keying modulated over the carrier. At the receiver, a demodulator system extracts the data stream as well as recovers the carrier frequency using a zero-crossing detector. Therefore, the relative permittivity of the sample can be obtained from the extracted frequency shift as a digital number. As all the sensing elements in the proposed structure are passive, the sensor does not increase the power consumption of the system. The sensor-transceiver capability at low powers makes it ideal for sensor nodes in the Internet of Things applications. Since the sensing and demodulation are implemented at the same time, the complexity for detection reduces significantly compared with traditional RF/microwave sensing techniques which need complicated frequency spectrum monitoring equipment. The output of the system is a digital number correlated to the dielectric constant of the specimen and the independent data stream for communication. The proposed sensor node is fabricated at the 2.45-GHz ISM band as an evaluation and the measurement results with some known samples are presented.