In this paper, a miniaturized Ultra High Frequency Radio Frequency Identification (UHF-RFID) tag-based sensor antenna using a magneto- dielectric substrate (MDS) for wireless identification and sensor applications is presented. Two models of RFID tag-based sensors are designed, fabricated and measured. The first model uses two RFID tags; both of the tags are incorporated with two RFID chips. A passive sensor is also integrated in one of the proposed tags to serve as a sensor node, while the other tag is used as a reference node. Based on the difference in the minimum power required to activate the reference and sensor nodes, the sensed data (temperature or humidity) can be determined. The magneto-dielectric substrate layer is placed underneath the patch antenna to reduce the size of the proposed sensor by about 75% compared to a conventional RFID tag-based sensor. The magneto-dielectric layer is thin enough to embed in the planer circuit. To reduce the size of the proposed sensor, a multi-port tag for including the reference and sensor node in one antenna is also presented. The proposed RFID tag-based sensors have several features such as small size, they are completely capable for two objectives at the same time and easy to integrate with a planer circuit.

A circular polarized patch antenna for UHF RFID tag-based sensor applications is presented, with the circular polarization (CP) generated by a new antenna shape, an asymmetric stars shaped slotted microstrip patch antenna (CP-ASSSMP). Four stars etched on the patch allow the antenna’s size to be reduced by close to 20%. The proposed antenna is matched with two RFID chips via inductive-loop matching. The first chip is connected to a resistive sensor and acts as a sensor node, and the second is used as a reference node. The proposed antenna is used for two targets, serving as both reference and sensor simultaneously, thereby eliminating the need for a second antenna. Its reader can read the RFID chips at any orientation of the tag due to the CP. The measured reading range is about 25 m with mismatch polarization. The operating frequency band is 902–929 MHz for the two ports, which is covered by the US RFID band, and the axial-ratio bandwidth is about 7 MHz. In addition, the reader can also detect temperature, based on the minimum difference in the power required by the reference and sensor.