A Dual-Band Rotational Feed Circularly Polarized RFID Reader Antenna Array With Stable Gain

In this letter, a dual-band rotational feed circularly polarized (CP) antenna array is proposed. The dual-band antenna element consists of two pairs of symmetric dipoles. The low-frequency CP radiation is achieved by the longer dipole pair, and the high-frequency radiation is mainly superimposed by the shorter dipole pair of the fundamental mode and the longer dipole pair of the higher-order mode. A compact and low-profile antenna array with a 1-to-4 rotational feeding network is introduced. The measured |<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$S_{11}$</tex-math></inline-formula>| < −10.0 dB of the array is in the frequency range of 0.8 GHz to 1.26 GHz and 2.15 GHz to 2.82 GHz. The measured axial-ratio (AR) bandwidth is approximately 36.3% (3 dB) in the low-frequency band and 24.5% (1.5 dB) in the high-frequency band. The realized gain reaches approximately 7.7 dBic and 12.3 dBic with symmetric radiation patterns.