Theoretical Consideration of Axial Ratio Deterioration in CP Loop-Shaped Antennas

Abstract

Conventionally, the deterioration in the axial ratio (AR) for a circularly polarized antenna based on a traveling wave current is explained using backward currents reflected from the antenna arm end. This letter reveals the existence of a new cause for AR deterioration in a situation of no backward currents. For this, a theoretical loop antenna model of one-guided wavelength circumference is constructed, where only a forward traveling wave current (a positive s'-directed current) flows with attenuation due to the radiation. The derived AR equation finds that the AR deteriorates due to the attenuation of the current, even when backward currents do not exist. Subsequently, a current that flows in the direction normal to the positive s'-directed current (a t'directed current specified by perturbation weight p) is added to the loop. It is found that the AR in the presence of the attenuating current is improved by virtue of the positive t'-directed current.

Discussion of the Axial Ratio of a Square Loop Antenna With an Attenuating Current

“…For comparison, the corresponding axial ratio for the generalized round loop antenna model, ARs, is quoted here [11] ARs = ,…”

“…Figures 1(a) and (b) depict, respectively, natural and metamaterial loop antennas, both having a square shape. These are the counterparts of the round loop antennas in [11].…”

“…It is known that the AR for a non-resonant CP antenna, whose radiation is generated by a traveling wave current on the antenna conductor, is degraded by backward currents (currents reflected toward the feed point). Recently, an additional cause of AR degradation has been discussed using a generalized round loop antenna model [11], where the current on a loop of 1g circumference decays with 1) H. Nakano, 2) T. Abe, and 3) J. Yamauchi are all with the Department of Science and Engineering, Hosei university, Tokyo 184-8584, Japan (e-mail: 1) hymat@hosei.ac.jp, 2) tomoki.abe.5r@gmail.com, and 3) j.yma@hosei.ac.jp). A. Mehta is with the Department of Science and Engineering, Swansea University, attenuation constant  due to radiation.…”

“…A. Mehta is with the Department of Science and Engineering, Swansea University, attenuation constant  due to radiation. The discussion in [11] leads to the conclusion that the AR on the antenna axis is degraded by the attenuation of the traveling wave current, even when backward currents do not exist.…”

“…Based on the formulated radiation field, the axial ratio on the antenna axis, ARz, is derived in section III. This is compared to the axial ratio of the corresponding round loop antenna model, ARs [11]. It is clarified how ARz behaves differently from ARs.…”