Low-profile broadband dual-mode four-arm slot spiral antenna with dual Dyson balun feed

The authors propose a low profile 4-arm slot spiral for broadband commercial applications. The antenna is designed for circularly polarised dual-mode, 1st (normal) mode and 2nd (split-beam) mode, operation from 1.4-3.5GHz and 2.5-6GHz, respectively. Phase progressions at arm ports are realised through the odd and even coplanar waveguide (CPW) excitations, a dual Dyson balun, and a single broadband 180 degrees hybrid. A finite-element boundary-integral (FE-BI) based numerical tool is extensively used to study effects of various antenna parameters, including slot width, resistive taper etc. The designed antenna has return loss better than 10 dB, nominal gain of 3 dBic, wobble on wave less than 3 dB and axial ratio lower than 4 dB over realised bandwidth for both modes. When compared with classical printed 4-arm spirals, the proposed antenna has only two access ports and is about five times shallower. Thus, a much simpler mode-forming network is needed and significant reduction in the cost can be achieved. Measured and simulated results are presented and, generally, a very good agreement between the two is obtained.