Self-phased Quadrifilar helix (S-P QHA)

Image of the Self-phased Quadrifilar helix (S-P QHA).

The self-phased quadrifilar helix antenna (S-P QHA), sometimes referred to as a volute antenna, consists of four helical windings each of nominal length λ/4 oriented 90° about the axis with respect to one another. The S-P QHA consists of two bifilar loops with slightly different resonant lengths, fed in parallel. To obtain the 90° current phase shift, the element of one bifilar helix is adjusted to be longer than the resonant length so that the current has a phase lead of 45°, and the other is adjusted to be shorter to obtain a phase lag of 45°.

The individual impedances of the two bifilar loops generally have unequal amplitudes and a phase difference close to 90° at resonance. This causes the degradation of radiation performance, particularly off resonance, as the two bifilar loops radiate unequal power with incorrect phasing. A cardioid-shaped beam which is uniform in the azimuth plane is only possible at the center frequency, and even there, orthogonal plane 3dB beamwidths are not exactly equal.

While beamwidths from 100° to 210° are obtainable, the cross-polarisation far-field component is much higher than for the quadrature fed QHAs and a good axial ratio is only achievable over a much narrower beamwidth. Because of the double resonance of the structure, a wide impedance bandwidth is achieved.

Typical applications of the antenna include satellite, ground station, GPS, etc.