Simple Design Procedure of a Broadband Circularly Polarized Slot Monopole Antenna Assisted by Characteristic Mode Analysis

In this paper, a systematic design procedure of a compact and broadband circularly polarized (CP) antenna is proposed. The antenna is first designed with a C-shaped monopole, and then a C-shaped slotted patch is utilized to produce CP radiation in the lower frequency band. Thus, the antenna's overall CP bandwidth (BW) is significantly increased. In addition, unlike the conventional method in which the operation is verified by the simulated surface current distribution, characteristic mode analysis (CMA) is carried out to give more insight into different modes on the antenna and generation of circular polarization. The CMA also helps to realize a structural optimization and identify an efficient feed location. The optimized antenna prototype with a compact size of 0.29λ
₀
× 0.29λ
₀
× 0.01λ
₀
(λ
₀
is the minimum wavelength at the minimum operation frequency) achieves a usable 10-dB reflection coefficient and 3-dB axial ratio fractional BW of 91% (2.1-5.6 GHz). The antenna realized broadside gain varies from 1.5 to 3.2 dBic within this measured overlapped BW.