Circularly Polarized Stacked Annular-Ring Microstrip Antenna With Integrated Feeding Network for UHF RFID Readers

A novel circularly polarized (CP) annular-ring microstrip antenna (ARMSA) is proposed for American ultrahigh frequency (UHF) radio frequency identification (RFID) readers. The annular ring is fed by an equal-split power divider with 90$^{\circ}$ phase difference. For the compactness of the entire antenna, the divider with the shape of Archimedes gradual-change lines is designed at the inside of the annular ring. A circular parasitic patch is suspended above the ring to improve the impedance matching and bandwidth. It also enhances the performances of axial ratio (AR). The impedance bandwidth for ${\rm S} _{11} < -10~$dB and 3-dB AR bandwidth are 870967 MHz (10.6% at 915 MHz) and 893948 MHz (6%), respectively. The CP gain is 8.9 dBi at 915 MHz with the front-to-back ratio (F/B) of 16.5 dB. The proposed antenna is low profile as the total height is $0.043 \lambda _{0}$ ($\lambda_{0}$ is the free-space wavelength ).