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What is a helical antenna?

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A helical antenna is a conductor wound as a helix, usually over a ground plane. In axial (endfire) mode, with circumference C between about 3/4λ and 4/3λ and pitch angle around 12–14°, it radiates a circularly polarized beam along its axis, as introduced by John Kraus in 1946–47; in normal mode, with dimensions ≪ λ, it behaves like a short loaded monopole radiating broadside.[1][2]

What are the axial-mode design formulas?

Kraus gives empirical relations for an axial-mode helix of N turns, circumference C and turn spacing S: input resistance R ≈ 140·C/λ Ω; half-power beamwidth ≈ 52λ3/2/(C√(NS)) degrees; directivity D ≈ 15·N·C²S/λ³; axial ratio AR = (2N+1)/(2N).[1][2] Later measurements found the directivity formula optimistic, so treat it as an upper estimate.[3]

How is a helix matched to 50 Ω?

The roughly 140 Ω input resistance is usually transformed by tapering the first quarter turn close to the ground plane, adding a strip that forms a short low-impedance line, or a quarter-wave transformer.[1]

Where are helical antennas used?

Axial-mode helices are used for satellite telemetry, amateur satellite and space links, and as array elements; the sense of circular polarization follows the winding direction. Normal-mode helices are the “rubber duck” and stubby antennas of handheld radios, and printed helices appear in sub-GHz IoT devices.[1][4] See also the quadrifilar helix.

Frequently asked questions

Which winding gives RHCP?

A right-hand wound helix, like a standard screw thread, radiates RHCP in axial mode under the IEEE definition.

Is a rubber duck antenna efficient?

It is compact but less efficient and narrower in bandwidth than a full-size λ/4 whip.

Does the ground plane matter?

Yes; axial-mode helices typically use a ground plane of at least about λ/2 diameter.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “Antennas for All Applications, 3rd ed.”. J. D. Kraus and R. J. Marhefka, McGraw-Hill. 2002.
  2. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
  3. “Antenna Theory and Design, 3rd ed.”. W. L. Stutzman and G. A. Thiele, Wiley. 2012.
  4. “Modern Small Antennas”. K. Fujimoto and H. Morishita, Cambridge University Press. 2013.
Categories: Antenna types