Omnidirectional circularly polarized antenna based on counter-wound helices

An innovative counter-wound helical antenna with omnidirectional circular polarization (OCP) is reported in this communication, with a significantly enhanced radiation resistance and matching bandwidth compared with the conventional normal-mode CP helix counterpart. The antenna is realized by combining several helical radiators with opposite winding directions. A three-segment OCP counter-wound helix is studied, featuring one right-handed helix in the middle and two left-handed helices sandwiching it from the top and the bottom. Besides achieving a stable OCP pattern with gain variations of less than 0.2 dB, the radiation resistance at the operating frequency f0 is increased to 26.7 Ω compared with 3.3 Ω of the traditional uniform-wound helix. Theoretical analysis and simulations are presented. To further enhance the radiation resistance, a five-segment helical antenna prototype is demonstrated and tested. The radiation resistance rises further to 80.9 Ω at f0 and the measured matching bandwidth reaches 2.8%, surpassing that of the conventional OCP helix by four times. Stable OCP radiation with a realized gain of up to 2.5 dBic is observed. Being compact and simple, the developed helical antenna could represent an ideal candidate for various wireless applications, for example, device-to-device (D2D) communications that require OCP radiations