Field Guide · term

Also known as: helix antenna, axial-mode helix

A helical antenna is a conductor wound into a helix, usually mounted over a ground plane and fed at one end.1 Its most useful form, the axial mode helix, radiates a beam along the axis of the coil that is circularly polarized — the electric field rotates once per turn, tracing a corkscrew through space. Circular polarization is exactly what satellite links need, because a spinning or tumbling spacecraft, and the Faraday rotation the ionosphere imposes, would fade a linearly polarized signal in and out; a circularly polarized helix stays connected regardless of orientation. That, plus its useful gain and broad bandwidth, makes the helix a staple of VHF/UHF satellite ground stations.

ground plane helix circularly polarized beam
An axial-mode helix launches an end-fire beam along its axis whose field rotates once per turn, giving circular polarization.

How it works

The behaviour of a helix depends on the size of one turn compared to the wavelength.

  • Axial (beam) mode. When the circumference of a turn is about one wavelength, the current travels around each loop and, turn to turn, sets up a wave that reinforces along the axis — an end-fire beam. Because the current is always a quarter-turn ahead of itself as it climbs the helix, the radiated field rotates, producing circular polarization whose handedness follows the winding direction. Gain grows with the number of turns (a long helix of many turns gives 10–15 dBi and a tightening beamwidth), and the match to the feed line stays good over a wide band — roughly a 1.7:1 frequency range — which is unusual for a resonant antenna.
  • Normal (broadside) mode. When the whole helix is small compared to a wavelength, it radiates broadside like a shortened, loaded vertical. This is the “rubber duck” you see on handhelds: coiling the wire fits a resonant length into a stubby package, at the cost of efficiency and bandwidth.

The axial mode is the one people mean by “helical antenna” as a directional element; the normal mode is a packaging trick for compact whips.

Variants

The quadrifilar helix (QFH) winds four helical elements together and feeds them in phase quadrature. It produces a broad, near-hemispherical circularly polarized pattern rather than a narrow beam, so it does not need to be pointed — the reason it is the favourite antenna for receiving low-earth-orbit weather satellites (NOAA APT, Meteor) and other passes that cross the whole sky. Ordinary single-wire axial helices, by contrast, are pointed at geostationary satellites or used in arrays.

Relevance to SDR

For SDR satellite work the helix is a go-to build: a few turns of wire over a reflector disk gives a cheap, high-gain, circularly polarized antenna for uplinks and downlinks in the 137 MHz, 400 MHz, and higher amateur/weather-satellite bands, and the QFH variant is the classic homebrew antenna for NOAA and Meteor image reception. The end-fire helix also complements the patch antenna as the other common way to get circular polarization, trading the patch’s flat profile for more gain.

GopherTrunk decodes terrestrial land-mobile trunking (P25, DMR, NXDN, TETRA), which uses vertically polarized omnidirectional antennas, not helices, so a helical antenna is not part of a GopherTrunk setup. It is included here as the standard circularly polarized antenna and a useful contrast to linear radiators.

Sources

  1. Helical antenna — Wikipedia, for axial versus normal mode, circular polarization, and gain versus turns. ↩

See also