This paper presents a small wideband circularly polarized patch antenna printed on the low-cost FR-4 material for radio-frequency-identification smart bookshelves in libraries. The antenna is composed of four top-loaded patches sequentially rotated with a phase difference of 90° and double shorted to the ground. It operates at a center frequency of 0.915 GHz. The impedance bandwidth (SWR < 2) is 26.8%, the axial-ratio (AR) bandwidth (measured AR < 3 dB) is 11.6%, and the 3-dB beamwidth is 115°. The antenna gain is about 4 dBi, and the overall dimension is reduced by ~60% in length, in comparison with that of the conventional sequential patch antenna.
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Lau et al. (2009) studied this question.
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