Compact Clock-Shaped Broadband Circularly Polarized Antenna Based on Characteristic Mode Analysis

This paper presents the design process of a clock-shaped broadband circular polarized (CP) antenna based on Characteristic Mode Analysis (CMA). It is composed of a circular ring, two microstrip lines approximately perpendicular to each other, and 50-Ω improved coplanar waveguide (CPW). The two microstrip lines are introduced in the X and Y directions, respectively, to break the symmetry for CP operation. Different from the conventional method in which the operation is verified by the simulated surface current distribution, CMA provides physical insight into different modes of the antenna and reveals the mechanism of radiating CP wave. Mode currents and characteristic fields of the selected modes are studied for AR bandwidth enhancement. An improved double-sided printed CPW is introduced to excite the desired modes and achieve wideband impedance matching. To validate the proposed concept, a prototype with a compact size of 0.29λ L × 0.29λ L × 0.013λ L (λ L corresponding to the maximum operation frequency) is fabricated and measured. The measured -10 dB impedance bandwidth and 3 dB AR bandwidth are 84.6% from 2.38 GHz to 5.8 GHz and 43.4% from 2.4 GHz to 3.73 GHz, respectively. The overlapping operating bandwidth can cover the ISM band, the TD-LTE band, and the WiMAX band. Besides, the proposed antenna has a stable radiation pattern over the operating band.