Circularly polarized loop-line antennas: Fundamental study on dual polarization

Two types of microstrip line antennas made of loop elements are treated and a basic study of dual circular polarizations is performed. The analytical values obtained by the method of moments are used for the study. Dual polarizations are attempted with an antenna combining two conventional series loop-line antennas. Unwanted radiation near an elevation angle of 0° in the radiation pattern is recognized. The radiated field is separated into emissions from various parts of the antenna. The study results indicate that the radiation from the loop elements is weak. Next, based on the study results, a parallel loop-line antenna is proposed. After a design method for the antenna is presented, the dual polarization characteristics are demonstrated. In comparison with the case of a series loop-line antenna, the unwanted radiation at an elevation angle of 0° is decreased from -9 dB to -18 dB. Finally, the frequency characteristics of the parallel loop-line antenna are studied. It is shown that a circular polarization with an axial ratio of less than 3 dB can be obtained over a bandwidth of 1%.