A monopole-fed circularly polarized loop antenna

An advantage of a one-point-fed circularly polarized (CP) Yagi-Uda loop array is to change the state of polarization (right hand circular polarization, RHCP; or left hand circular polarization, LHCP) easily by changing the position of the gap on the driven element mechanically or electronically. But in practice, it is difficult to obtain symmetrical patterns for RHCP and LHCP due to the asymmetric antenna configuration and the unbalanced coaxial feed. In this paper, a CP loop antenna with a monopole feed is introduced to overcome the disadvantages mentioned above. This antenna is expected to allow good symmetrical patterns, when the switching circuit is installed to change the polarization. Also it is easy to obtain good impedance matching by adjusting the monopole length.