Circularly polarized Yagi array with small size and symmetric pattern by using counter‐wound helix

Abstract

A circularly polarized Yagi array with a compact structure, small size, and symmetric pattern is reported in this paper. It consists of four 3-segment counter-wound helices, one driven element, one director, and two reflectors, for a moderate gain and front-to-back ratio. Similar to conventional Yagi with linear polarization, all helical elements have different but close resonant frequencies. The driven element has a resonant frequency higher than that of the reflectors but lower than that of the director. Owing to the uniform pattern of the counter-wound helical element, a very symmetric pattern with circular polarization at the endfire direction is achieved. A prototype of the 4-element array is fabricated and tested. Measured results show that an impedance bandwidth with jS 11 j < À10 dB from 1.345 to 1.42 GHz (5.42%) and a 3-dB AR band from 1.34 to 1.43 GHz (6.5%) are obtained, showing an overlapped bandwidth of 5.42%. A realized gain up to 7.03 dBi and an offset angle within 3 are also realized. The overall size of the array is as small as 0.66λ 0 Â 0.107λ 0 Â 0.40λ 0 , showing possibility of further being formed to large scanning or multi-beam arrays.