Wideband Circularly Polarized Endfire Antenna Based on Counter-Wound Normal-Mode Helix

This article presents a wideband circularly polarized (CP) endfire antenna based on a compact normal-mode helix. The counter-wound helix element is used to achieve good omnidirectional CP radiation and is further extended into an array configuration in a log-periodic fashion. It operates at the 3 lambda/2 mode and is differentially fed by a double-sided parallel strip line. A wideband balun is used to feed the whole array. For experimental verification, the helices and the balun are fabricated using the selective laser melting (SLM) and printed circuit board (PCB) techniques, respectively. The measured results agree well with the simulated ones, with a wide impedance bandwidth of 50% and an axial ratio (AR) bandwidth of 46% being obtained. In addition, the proposed structure is very compact, with the dimensions of only 2.24 lambda(0) x 0.11 lambda(0) x 0.53 lambda(0) at the lowest operating frequency. The proposed antenna successfully demonstrates that the omnidirectional normal-mode helix element has great potential for achieving wideband unidirectional CP radiation.