where Pt is the transmitted power f

where Pt is the transmitted power from RFID reader, Gr/t is the gain from the reader and the tag, Pth is the threshold power and τ is the power transfer coefficient. From Fig. 19 it can be seen that the maximum tag reading range for the tag attached on metal object is slightly higher than the tag in free air. With the threshold power of -15 dBm and the effective isotropic radiated power (EIRP) of the reader is 4W, the maximum reading range obtained by the proposed tag is 2.9 m in free air and 3.1 m on metal object.