Rectangular loop antenna for circular polarization

It is shown here that a rectangular unloaded one wavelength loop antenna works as a circularly polarized antenna. Its operating principle is presented. The current distribution on the loop of this antenna can be controlled by the ratio of the major and minor sides. It is a single-feed antenna with the simplest configuration. The current distribution of this antenna can be decomposed to the traveling wave mode currents for the forward and backward waves. Based on this fact, the condition for the circular polarization is derived in terms of the current value at the center of each side of the loop and the effective length. Based on the investigation of the realization method for this condition, a circularly polarized antenna off-center-fed at a corner of the loop was actually designed and the various characteristics, such as axial ratio and the directivity, were derived. From this study, it is found that a good circular polarization can be obtained with the antenna height within a range near 0.13λ from the reflector. Validity of the theoretical results is confirmed by experiment.