Four-arm 2nd- mode slot spiral antenna with simple single-port feed

The design and performance of a novel 4-arm equiangular slot spiral antenna operating in the 2nd mode are discussed in this letter. A straightforward feeding method consisting of a single vertical coaxial probe and no underlying mode forming network is proposed. Along with inherent simplicity, this feed enables design flexibility with respect to the slot-to-metal ratio, growth rate, and other spiral parameters. Numerical tools including an in-house finite element-boundary integral (FE-BI) code and Ansoft HFSS are used in designing the antenna. Good performance over an octave bandwidth is achieved, with axial ratio and WoW less than 3 dB, nominal gain of 3 dBic, and VSWR less than 1.6. Performance is verified with measurements. A method for increasing the bandwidth up to 3 : 1, which is the theoretical value determined by the excitation of the first higher order mode, is demonstrated computationally.