Helical antenna

A helical antenna is a conducting wire wound in the form of a helix (screw), usually fed by a coaxial line against a ground plane. It is the standard antenna for circular polarization at VHF/UHF.

        ____
       /    \   <- turn, circumference C = πD
       \____/
       /    \   spacing S between turns
       \____/   pitch angle α = tan⁻¹(S/C)
       /    \   N turns, axial length L = NS
  =====\____/=====  ground plane (≥ λ/2)
          |
        coax feed

Parameters: diameter D, circumference C = πD, spacing S, pitch angle α, number of turns N.

Modes of operation

• Normal (broadside) mode: helix dimensions ≪ λ (total wire length NL₀ ≪ λ, where L₀ = √(C² + S²) is the length of one turn). Maximum radiation is normal to the axis, like a short dipole plus a small loop; polarization is elliptical, circular when C = √(2Sλ). Low efficiency, narrow band. Used for small whips in handsets.
• Axial (end-fire) mode: C ≈ λ (0.75λ < C < 1.33λ), S ≈ λ/4, α ≈ 12°–14°. A single main beam along the axis, circularly polarized, wide band.

Axial-mode formulas (Kraus)

HPBW ≈ 52 λ^(3/2) / (C √(NS))  degrees
D    ≈ 15 N C² S / λ³
Zin  ≈ 140 (C/λ)  Ω

Advantages: circular polarization, wide bandwidth (about 1.7:1), moderate gain (10–15 dBi), simple. Applications: satellite and space telemetry, GPS, radio astronomy, feeds for reflectors.