We demonstrate a concept of compact and highly efficient circular traveling wave series-fed microstrip patch array working at 10 GHz. The patch elements are excited by a microstrip line through coplanar proximity coupling. A dual-port feeding allows us to select the direction of the traveling waves and, consequently, the sense of circular polarization of the patches. Simultaneous feeding to both ports with proper amplitude and phase difference can result in radiated waves of arbitrary polarization. Owing to the circular array arrangement and the strong proximity coupling, the antenna exhibits a compact structure and achieves a high antenna efficiency of 90% with seven patches. Based on reciprocity, the antenna can readily be also utilized for polarimetry applications for which the signal amplitude at each port is proportional to the amplitude of each individual sense of polarization. Measured results of an antenna prototype successfully validate the concept.
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