A sinuous antenna offers a wide range of advantages for ultra wideband (UWB) applications, especially for direction finding (DF) systems. It is the first antenna that is planar, broadband, and dual polarized with a single aperture and is considered to be the most recent frequency independent antenna. In this paper, the feed contact point of the sinuous antenna was modified to allow a smooth transition from the feed network to the first radiating element at the upper frequency band to enhance the 3dB AR beamwidth for circular polarization to 90°. For the first time the 3dB AR beamwidth for the dual circular polarized sinuous antenna will be analyzed in detail. This analysis will illustrate improvements over the traditional four arm dual circular polarized sinuous antenna which is designed for bidirectional performance. In addition, unidirectional performance is also investigated with a cavity-backed design. All the designs of multi-arm planar sinuous antennas are presented in this paper for operation frequency from 2-18GHz.
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