  A low-cost chipless RFID sensor tag to determine the dielectric properties of
materials is presented. The sensor is based on a depolarizing dipole resonator
loaded with printed capacitors whose capacitance depends on the permittivity of
the material located on contact with the tag. The permittivity is obtained from
the shift of the resonance frequency. The resonator is tested both individually
as well as placed on a periodic arrangement so as to form a frequency selective
structure (FSS) to increase the cross- polar radar cross section, thus
facilitating the tag detection. The proposed structure and measurement
procedure is particularly suitable for the characterization of civil materials
as a nondestructive testing (NDT).
