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 LC -resonant character of the CLL structure, an electrically small size of 0.137 λ ₀ × 0.137 λ ₀ × 0.046 λ ₀ ( ka = 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.

中文翻译：

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基于容性负载环路的电小圆极化天线

这封信介绍了一种用于 UHF 射频识别 (RFID) 应用的基于电小圆极化 (CP)、基于电容负载环路 (CLL) 的天线的新颖设计。提出了一种用作磁偶极子的简化 CLL。通过等效电路和场分布对其进行分析。由于强CLL结构的LC-谐振特性，电小尺寸为0.137 λ ₀ × 0.137 λ ₀ × 0.046 λ ₀ ( ka = 0.67) 很容易实现。两组不同环路尺寸的CLL结构正交排列，具有交叉槽和45°旋转耦合馈线。结果，实现了双模操作并辐射了RHCP波。实现了 3.71% 的实测 -10 dB 阻抗带宽（899–933 MHz），这对于 902–928 MHz UHF 阅读器应用来说是很好的准备。测得的峰值增益和辐射效率分别为 4.83 dBic 和 -1.26 dB (74.8%)。所提出的基于电小型 CLL 的 CP 天线表现出优异的整体性能，包括易于制造、良好的带宽、高增益和效率以及宽波束宽度。因此，它适用于 RFID 阅读器应用和其他物联网 (IoT) 系统。