I. Introduction
A circularly-polarized (CP) antenna has the advantage that polarization alignment between transmission and reception antennas is not needed, unlike linearly-polarized antennas. A patch antenna and a loop antenna are representative resonant antennas with linear polarization and can be transformed into CP antennas using perturbation elements, as shown in Figs. 1(a) and (b). However, the bandwidth as a CP antenna is not wide because the perturbation elements are also resonant elements that only work across a narrow frequency range.
Resonant CP antennas. (a) Patch antenna. (b) Loop antenna.