Wideband single-fed proximity coupled circularly polarised annular slot antenna

A wide impedance and axial ratio bandwidths single-fed method for circularly polarised (CP) annular slot antenna is proposed. Impedance and axial ratio bandwidths of approximately 18% have been achieved. The annular slot radiator operating at its fundamental TM(11) mode is fed by a simple U-shaped microstrip line structure through proximity coupling at the annular section in the middle of the line. A pair of 100-ohm impedance transformers is connected to transform and align the impedance bandwidth with the axial ratio bandwidth. Simulations show that the source current, which flows in the annular section of the U-shaped feed line, induces a circulating current in the annular slot radiator which contributes the CP radiation. Both currents on the U-shaped feed line and the annular slot radiator flow in the same direction. The antenna radiates at both broadside and backside directions of opposite type of circular polarisation. The type of circular polarisation can be selected by feeding signal into the appropriate input terminal. Moreover, this article has further demonstrated that the bandwidths of the antenna are mainly limited by the radiating elements, not the proposed U-shaped feed line proximity coupled method. A parametric study of the antenna has also been reported. Experimental and simulated results of the antennas are presented and discussed.