by Daniel M. Dobkin, Steven M. Weigand and Nathan Iye
Conventional radiatively coupled, ultra-high frequency radio-frequency identification (UHF RFID) tags have certain limitations: they are physically large and do not operate well in or close to aqueous fluids. Recently, the use of tags configured to use inductive instead of, or as a supplement to, radiative coupling has been advanced as an alternative application model for UHF RFID. Inductively coupled tags require reader antennas that provide significant magnetic fields.
The mutual inductance between two loop antennas is strongly influenced by the antenna diameter. A small loop produces a strong magnetic field along its axis, but the field falls rapidly when the sensing coil is moved more than approximately one diameter above the larger loop. A large loop produces a more modest peak field, but the extent of the field is larger, scaling with its diameter. Therefore, there is an optimal size for best read range for any given peak loop current and coupling requirement.1 Two obstacles are encountered when one attempts to design simple loop antennas for UHF (approximately 900 MHz) near-field readers. Antennas of intermediate diameter (between 2 and 5 cm) are awkward to match to a 50 Ω input, due both to low radiation resistance and high input impedance: loop antennas have a parallel resonance in this size range. As the antenna diameter approaches the series resonant size (approximately λ/π) the current distribution shifts to produce a change in sign of the current flow, and the antennas produce relatively little magnetic field on axis.2,3
In order to address the problems cited above, a segmented loop antenna has been constructed. Each segment is composed of a metal line forming a polygonal face and a series capacitor to the next segment.
Each segment forms a resonant line, avoiding the accumulation of inductive reactance that otherwise impedes matching.
Because at resonance the voltage across the segment is real, phase does not accumulate around the loop, and the current around the loop remains approximately in phase and of the same sign, thus producing a substantial magnetic field along the axis.
Example antennas are shown in Figures 1 and 2.
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