An analysis for circularly-polarized (CP) ring slot antennas fed by a coupling strip with resistive loading is presented. By treating the antenna as a two-port circuit, the amplitudes and phases of two orthogonal modes excited from the slot antenna can be found and expressed in terms of the S parameters of the circuit. The axial ratio calculated through the S parameters has a good agreement with that obtained from the pattern simulation of IE3D. The effects of varying key parameters of the coupling strip on the amplitudes and phases of the dual orthogonal modes are investigated in detail.
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