A Four-Arm Electrically Small Omnidirectional CP Helical Antenna and Its Expansion as a Compact and Highly Efficient Dual-CP Array

A novel ultracompact four-arm electrically small omnidirectional circularly polarized (OCP) helical antenna is reported that consists of a half-wavelength normal-mode helical antenna as the driven element and three parasitic helical arms. With the introduction of the three properly arranged parasitic arms besides the driven element, the in-phased current density is largely strengthened and thus the radiation resistance of the multiarm helical antenna is substantially enlarged from
$3.3~\Omega $
(driven-element only) to
$45~\Omega $
. Good impedance matching with standard transmission line becomes feasible. The entire volume of the design is only
$\pi $
(
$0.05\,\,\lambda _{0})^{2} \times 0.06\,\,\lambda _{0}$
(ka < 0.39). It exhibits excellent OCP radiation performance with the axial ratio (AR) and gains variations less than 0.5 and 0.3 dB in 360° in the azimuth plane. Thanks to its ultracompact size, the single element is feasible to be expanded into a compact and highly efficient collinear dual-polarized OCP (DOCP) array with higher directivity and dual-CP functionality. A four-element prototype was demonstrated. Measurement shows the array achieves the peak realized gain (RG) values of 7.55 and 7.26 dBic for RHCP and LHCP, respectively. The overlapped impedance and AR bandwidth covers 0.98–1.02 GHz with isolation greater than 25 dB between the two polarizations.