Electrically Small, Low-Q, Wide Beam-Width, Circularly Polarized, Hybrid Magnetic Dipole Antenna for RFID Application

A novel electrically small, wide beamwidth circularly polarized (CP) antenna using two orthogonal smaller hybrid magnetic dipoles (HMDs) for radio frequency identification (RFID) reader application is presented, which achieves a quality factor (
$Q$
-factor) approaching the
Chu-limit
. First, a HMD is implemented by the combination of two single-slit magnetic dipoles (S-MDs) and one dual-slit magnetic dipole (D-MD), which is carefully analyzed by equivalent circuit models and field distribution. Then, by implementing two orthogonal HMDs with slightly size difference and a 45° angle tilted coupling feeding line, two orthogonal magnetic components with equal magnitude and 90° phase difference are obtained, which lead to a CP radiation. Finally, an electrically small CP magnetic dipole antenna with a size of
$0.12\,\,\lambda _{0} \times 0.12\,\,\lambda _{0} \times 0.04\,\,\lambda _{0} (ka< 0.55)$
(
$\lambda _{0}$
is the free-space wavelength at its center frequency) is fabricated and measured. The measurement shows that a −10 dB impedance bandwidth of 3.17% (cover 902–928 MHz UHF band well), a half power beamwidth (HPBW) of 104°, and a peak radiation gain of 4.57 dBic are achieved. Generally, it features a small size, wide beamwidth, simple design, and good radiation performance, exhibiting good application potential usage in internet of things (IoTs) systems.