An Ultrawideband Circularly Polarized Monopole Antenna Based on Characteristic Mode Analysis

This letter presents an ultra-wideband (UWB) circularly polarized (CP) monopole antenna with a defected ground structure, consisting of a monopole and a coupled patch with opposite openings, different sizes, the same rotational senses. Characteristic Mode Analysis (CMA) is used to investigate CP generation mechanism, providing physical insight into different resonant modes and field distributions. Measured results exhibit a relative impedance bandwidth of 138.03% (3.30-18.00 GHz) and a relative 3-dB axial ratio (AR) bandwidth of 123.60% (4.25-18.00 GHz). The maximum of CP realized gain at the main beam direction is 8.32 dBic at 15 GHz. The overall dimension of the proposed antenna is 0.44λL×0.50λL×0.0165λL, where λLis the free-space wavelength at the lowest operating frequency. This antenna offers considerable application value in unmanned aerial vehicle (UAV) detection, wireless sensor signal transmission for its UWB CP radiation and high gain.