........................................................................................ 84
4.5.4 Bow-Tie Antenna with a Series Tuning Inductor ................................................ 85
4.6 Conclusions.......................................................................................................................... 91
Acknowledgment ......................................................................................................................... 91
References..................................................................................................................................... 91
4.1 Introduction
Antennas used in the HF region operate at 13.56 MHz whose frequency has an electromagnetic
wavelength of around 22 m giving a near-?¬?eld far-?¬?eld boundary of around 3.5 m.
Thus, given reading distance requirements of <3 m and using the regulated radiation
power at the HF ISM band, reader antennas are almost always near-?¬?eld creation structures
that aim to create large energy density ?¬?elds with the minimum amount of radiation.
However, at UHF frequencies the scenario is different. At UHF frequencies, the near-?¬?eld
far-?¬?eld boundary is around 50 mm. Thus the region of operation in the UHF spectrum
65
is almost always in the far ?¬?eld, and therefore reader antenna designs are far-?¬?eld creation
structures that aim to operate at the highest possible ef?¬?ciency.
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