Compact Stacked-Layer Patch Antenna for Dual and Quad Circularly Polarized Beam Generations: Principle and Design Approach

In this work, a systematic design approach is presented to generate both dual and quad circularly polarized (CP) beams based on two proposed stacked-layer patch antennas. Specifically, two novel composite modes, namely, "TM20-odd TM01 coupled mode" and "odd TM20-odd TM02 coupled mode," are featured for the dual- and quad-beam generations, respectively. To begin with, characteristic mode analysis (CMA) is adopted to examine the coupled orthogonal modes and tune for the prerequisite quadrature phase difference. After that, by means of the cavity-model theorem, theoretical derivations based on the equivalent magnetic currents (EMCs) are conducted to shed light on the mechanism of the CP multibeam generation process. In particular, the novel idea of "supplementary EMC pair" is first proposed, together with the radiation properties provided by the "TM10-TM01 coupled mode" antenna, aiming to conduct a simplified yet accurate array factor analysis. After the adoption of this creative design approach, for the first time, the CP states over the upper half planes for the two designs are evaluated precisely; hence, the dual- and quad-beam performances can be readily appraised accordingly. Finally, two antenna prototypes are simulated, fabricated, and measured to validate the working principle. For the dual-beam antenna, a compact volume of 0.03 lambda(3)(g) is highlighted. Compared to other related designs, this work features the widest 3-dB axial ratio beamwidth (ARBW) of a total 160 degrees coverage in the elevational plane, with an attractive increment of 110 degrees pronounced. As for the proposed quad-beam antenna, the design features the most compact volume of 0.073 lambda(3)(g). Besides, the overall 3-dB ARBW accounting for the four beams has decreased to 88 degrees. In conclusion, with sound agreements between theory and practice, this work stands out prominently for its clear insight to provide the most compact dual- and quad-beam CP antennas with wide and narrow beamwidth characteristics, respectively.