An omnidirectional circularly polarized (OCP) collinear helical antenna employing a form of self-canceling sections is proposed in this paper. It consists of three separateλ/2 (where λis the free space wavelength at the center frequency) helical sections, which serve as the radiating elements, and two λ/2 self-canceling sections to connect them. The self-canceling section is simply a wire folded in half to form two parallel sections with surface current directions opposing each other, thereby canceling out the far-field components. The current direction is reversed every half wavelength at the operating frequency. Thus, the surface current vector along the three helical wires would point in the same direction, achieving enhanced radiation.A prototype of the proposed antenna with a diameter of 0.12λand a height of 0.51λhas been fabricated and tested. Measured results demonstrate a maximum gain of 2.23 dBic, an axial ratio and impedance bandwidth of 15.8% and 2%, respectively.
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.
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