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What is a log-periodic antenna?

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A log-periodic antenna is a frequency-independent structure whose dimensions scale geometrically, so its impedance and pattern repeat periodically with the logarithm of frequency and vary little between periods. The common log-periodic dipole array (LPDA) uses dipoles whose lengths and spacings shrink by a scale factor τ = Ln+1/Ln, fed by a transposed (crisscrossed) line from the short end; it was introduced by Isbell in 1960.[1][2]

How is an LPDA designed?

The design parameters are τ (typically 0.8–0.95) and the relative spacing σ = dn/(2Ln) (typically 0.05–0.2). Larger τ and optimum σ give higher directivity at the expense of more elements and a longer boom. Carrel’s design curves, reproduced and corrected in Balanis, give directivity of roughly 7–11 dBi versus τ and σ.[2] The longest element is about λ/2 at the lowest frequency and the shortest about λ/2 at the highest, with extra margin for the active region.

Why does it work over a wide band?

At any frequency, only the few elements near half-wave resonance (the active region) radiate strongly. The transposed feed gives successive elements 180° phase reversal, producing a backfire beam toward the short end. As frequency changes, the active region moves along the structure while keeping the same electrical shape, consistent with Rumsey’s principle of frequency independence.[3][1]

Where are log-periodics used?

EMC test antennas (often combined with a biconical for 30 MHz–1 GHz and beyond), TV reception, HF rotatable arrays, wideband monitoring and printed LPDAs for UWB and test fixtures. Compared with a Yagi of the same boom length, an LPDA has lower gain but far wider bandwidth.[2]

Frequently asked questions

What is the input impedance of an LPDA?

Set mainly by the feeder impedance and element loading; designs commonly target 50 Ω via a balun.

Is an LPDA polarization linear?

Yes, parallel to the elements.

Why does the phase center move?

The active region moves along the boom with frequency, which matters for reflector feeds and precise measurements.

References

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

  1. “Log Periodic Dipole Arrays”. D. E. Isbell, IRE Transactions on Antennas and Propagation, vol. 8, no. 3, pp. 260-267. May 1960.
  2. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
  3. “Frequency Independent Antennas”. V. H. Rumsey, Academic Press. 1966.
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