Efficiency Analysis of Parasitic Common and Differential Modes for Ultrathin Circularly Polarized Microstrip Antenna

Abstract

This article proposes a comprehensive analysis on the efficiency performance of an ultrathin circularly polarized (CP) microstrip antenna. For ultrathin antennas with intrinsic narrow bandwidth, traditional parasitic-loading methods for bandwidth enhancement encounter significant in-band efficiency reduction, which is named as “efficiency dip,” due to the strong differential coupling between main and parasitic resonators. To address this issue, we propose a feasible structure of parasitic resonator pair. In this case, the common and differential modes (DMs) of the resonator pair are excited simultaneously, with the parasitic common mode (CM) to eliminate the efficiency dip. Inevitably, another efficiency dip appears due to the parasitic DM, but is located out of band without bandwidth deterioration. A prototype is fabricated and tested, showing a CP bandwidth of 3.5% with an extremely low profile of $0.012~\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at the center frequency). The final efficiency is over 68% across the band. This article provides a guideline for designing wideband CP antennas with ultrathin profile and small area, for example, for mobile terminals.