A low-profile omnidirectional circularly polarized antenna using integrated patch and CRLH loop structures

A low-profile omnidirectional circularly polarized antenna is proposed by integrating a patch antenna with a loop antenna. Omnidirectional vertically polarized radiation is achieved by modifying the resonant mode within the cavity of the patch antenna, whereas omnidirectional horizontally polarized radiation is realized through the incorporation of composite right/left-handed (CRLH) line segments into the loop antenna. The loop antenna is fed by a microstrip balun, and a common-mode suppression filter is inserted between the loop antenna and the balun to mitigate common-mode interference induced by the excitation of the patch antenna. An unequal Wilkinson power divider, together with transmission lines, is employed to control the power ratio and phase difference between the two antennas, thereby generating circular polarization. Simulated results demonstrate good impedance matching at the center frequency of 2.4 GHz. The axial ratio in the xy-plane remains below 3 dB, with an omnidirectionality of approximately 1 dB. Although the measured results exhibit slight degradation due to environmental effects, they are in good agreement with the simulations, thereby validating the feasibility of the proposed design.