Compact Wideband Circularly Polarized Loop Antenna Based on Dual Common and Differential Modes

This letter presents a novel design method of a compact wideband loop antenna for circularly polarized (CP) radiation based on two sets of common modes (CMs) and differential modes (DMs). Different CMs and DMs are excited in the loop antenna with the same amplitude of radiated electric fields by using equal-amplitude and in-phase excitation. A wideband CP radiation performance is achieved through the combination of two sets of CMs and DMs with an innate 90° phase delay. Moreover, the shared-aperture configuration of CMs and DMs achieves a size reduction. An experimental prototype was fabricated and measured to verify the design concept for CP operation. The experimental results indicate that an overlapped bandwidth of 2.30–3.40 GHz (38.6%) with |S₁₁|<–10 dB, axial ratio <3 dB, and gain variation <1 dB is obtained within compact size of 0.4 λ₀ × 0.4 λ₀ × 0.009 λ₀ (λ₀ is the wavelength in free space at the center operating frequency). Such a compact, wideband, and stable-gain CP antenna with a simple feed strategy is a promising candidate for wireless communication.