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What is a spiral antenna?

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A spiral antenna is a frequency-independent antenna whose arms follow a spiral defined mainly by angles. The equiangular (log) spiral r = r0eaφ, studied by Dyson, and the Archimedean spiral r = r0 + aφ both radiate circularly polarized beams over bandwidths of 10:1 or more, limited by the outer and inner radii.[1][2][3]

Why are spirals frequency independent?

Rumsey argued that an antenna whose shape is defined entirely by angles is frequency independent; truncation is unavoidable, but if current decays quickly beyond an active region where the circumference is about one wavelength, the truncated structure behaves nearly ideally between its cutoff frequencies.[2] The lowest frequency is set by the outer circumference ≈ λ, the highest by the feed region.

What is the impedance of a spiral?

A two-arm self-complementary planar structure in free space has an input impedance of η0/2 ≈ 188 Ω regardless of frequency (Mushiake’s relation). Practical printed spirals on substrates, or with cavities, are lower and are fed through broadband baluns.[2][4]

How is a spiral made unidirectional?

A free-standing planar spiral radiates bidirectionally. An absorber-loaded cavity makes it unidirectional at the cost of about 3 dB of efficiency; conical spirals and reflector-backed designs avoid some of that loss over narrower bands.[4]

Frequently asked questions

Where are spirals used?

Electronic warfare, direction finding, wideband monitoring and some GNSS and UWB applications.

What gain does a spiral have?

Typically a few dBi, roughly constant over the band.

Is spiral polarization fixed?

The sense of CP follows the winding direction.

References

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

  1. “The Equiangular Spiral Antenna”. J. D. Dyson, IRE Transactions on Antennas and Propagation, vol. 7, no. 2, pp. 181-187. April 1959.
  2. “Frequency Independent Antennas”. V. H. Rumsey, Academic Press. 1966.
  3. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
  4. “Antenna Engineering Handbook, 4th ed.”. J. L. Volakis (ed.), McGraw-Hill. 2007.
Categories: Antenna types