A low cost flexible passive UHF RFID tag for sensing moisture based on antenna polarization

Cost optimization and performance are the continuous improvement areas in any technology. In this paper a flexible dipole antenna on a passive UHF RFID tag is designed on an inexpensive paper substrate which can sense the moisture based on the polarization of the antenna. An inexpensive paper substrate and copper (Cu) layer are used for the flexibility and the optimization of manufacturing cost. The dipole antenna is matched with the complex conjugate of the power harvesting circuit of the Higgs2 Integrated Circuit (13.88 -j143.6 at the frequency of 915 MHz). The key characteristic of this design is the sensitivity of the antenna polarization on the passive RFID tag to the moisture content in the paper substrate. In simulations, the antenna is circularly polarized when the substrate is dry (relative permittivity, ϵ_r= 2.38) and the antenna is linearly polarized when the substrate is wet (relative permittivity, ϵ _r= 35.35). The read ranges of the fabricated dipole antenna (i.e., prototype RFID moisture sensor) are measured and the desired performance is achieved with overall tag dimensions of 0.144λ_o × 0.133λ_o where λ_o is the free space wavelength at 915 MHz.