where λ0 is the wavelength, Pt is the transmit power of
the reader, Gt is the gain of reader antenna, Pth is the
minimum threshold power required by the tag chip to
respond, Gr is the gain of tag antenna, ρ is the polarization
factor, and Γ is the reflection coefficient of tag antenna. In
this case, both the reader antenna and the proposed antenna
are RHCP, ρ = 1 . The calculated reading range of the
proposed tag antenna is about 8.8 m at 915 MHz. As shown
in Fig. 11, to verify that the antenna is indeed circularly
polarized, the reading range has been measured by rotating
the tag along the φ direction. In Fig. 11, the measured
maximum reading range is about 6.5 m.