| Summary: | In this paper, a novel circularly polarized (CP) antenna that achieves high gain by maximizing the radiation aperture utilization is proposed for vehicle applications. The realization of increasing the radiation aperture is based on the application environment, making full utilization of the vehicle roof, a large metal reflector beneath the antenna, which is indispensable in the vehicle application. Developed from the structure of typical microstrip patch antenna, the design extends the radiation patch to cover the side of the dielectric substrate, which increases the size of the radiating patch without increasing the antenna's outline size. To make CP antennas more suitable for special working environments with large reflecting surface, a sequential rotation feeding structure with 90° progressive phase difference is uniquely designed based on the active port impedance of the four-ended radiation structure. This co-design method introducing the active port impedance of a multi-port radiator into the design of feeding network as the loads can effectively guarantee the CP performance of the antenna, and can greatly simplify the optimization work in the subsequent stage. Based on this design idea, a prototype working at 1.48 GHz with a 50-MHz bandwidth is designed for vehicle satellite broadcasting application. Experimental results show that the antenna achieves a high CP gain greater than 7 dBic with a compact size of $50 \times 50 \times 10\,\,mm^{3}$ ($0.247\lambda _{0} \times 0.247\lambda _{0} \times 0.05\lambda _{0}$ , $\lambda _{0} $ is the wavelength of working frequency in free space).
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