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Axial-Mode Helical

Helical Antennas UHF Satellite Circular Polarization

A coil of wire about a wavelength around that radiates a circularly polarized beam.

Band: VHF to SHF
Gain: ~8-15 dBi (turn-count dependent)
Polarization: Circular

Photos

Real-world photo of a Axial-Mode Helical in use — Real-world example. Source: Wikimedia Commons (Public domain; NOAA).

Radiation / wave patterns

New to these diagrams? How to read a radiation pattern →

Radiation pattern of the Axial-Mode Helical — Fires straight off the end of the coil (endfire), circularly polarized — point the open end at the satellite.

How & why it works

When a helix is wound so its circumference is roughly one wavelength and it is mounted over a ground plane, it operates in axial mode: a travelling wave runs along the coil and launches a beam straight off the end. Because the current rotates around the turns, the radiated field is circularly polarized, which is ideal for links where the two ends may be rotated relative to each other or where Faraday rotation scrambles linear polarization. Gain grows simply by adding more turns.

Real-world uses

Satellite uplinks/downlinks, space telemetry, and GPS reference antennas.