Electrically Small Circularly Polarized Antenna Based on Capacitively Loaded Loop

Abstract

This letter presents a novel design of an electrically small, circularly polarized (CP), capacitively loaded loop (CLL)-based antenna for UHF radio frequency identification (RFID) applications. A simplified CLL working as a magnetic dipole is proposed. It is analyzed by the equivalent circuit and field distribution. Owing to the strong <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -resonant character of the CLL structure, an electrically small size of 0.137 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.137 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.046 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ka</i> = 0.67) is easily achieved. Two sets of CLL structures with different loop dimensions are orthogonally arranged, with the crossed slot and 45°-rotated coupling feedlines. As a result, the dual-mode operation is realized and the RHCP wave is radiated. A 3.71% measured −10 dB impedance bandwidth (899–933 MHz) is achieved, which is well poised for the 902–928 MHz UHF reader applications. The measured peak gain and radiation efficiency are 4.83 dBic and −1.26 dB (74.8%), respectively. The proposed electrically small CLL-based CP antenna exhibits excellent overall performance, including easy fabrication, good bandwidth, high gain and efficiency and wide beamwidth. Therefore, it is suitable for RFID reader applications and other Internet of Things (IoT) systems.