Optically Transparent 24 GHz Analog Front-End Based on Meshed Microstrip Lines for the Integration in a Self-Sufficient RFID Sensor Tag

This paper presents a 24 GHz Radio Frequency IDentification (RFID) sensor tag including an optically transparent patch antenna and an RF circuit. These components are placed on a single efficient solar cell and the transparency is achieved by realizing metal patches and lines as grid structure. Since the analog front-end is implemented in the microstrip line technology, the impact of the grid structure on the characteristic impedance and the propagation constant of microstrip transmission lines is studied in detail. The knowledge derived are taken into account in the design process of the meshed patch antenna as well as the modulator and demodulator circuit. Furthermore, the influence of the solar cell on the characteristics of the analog front-end is investigated. The overall dimensions of the sensor tag are $13\times 13\times 4$ mm3 including the aforementioned components, a microcontroller, an acceleration sensor and an energy storage. The perforated analog front-end fabricated on quartz glass has an overall transparency of 75 % and an antenna efficiency of 49 %. The achieved sensitivity value of the developed RFID sensor tag is −36 dBm resulting in a maximum communication range of approximately 1.3 m.