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Log-Periodic Dipole Array (LPDA)

Directional Antennas

A directional, extremely broadband array of progressively scaled dipoles.

Band
HF to UHF (wideband)
Gain
~6-9 dBi
Polarization
Linear (element orientation)

Photos

Real-world photo of a Log-Periodic Dipole Array (LPDA) in use
Real-world example. Source: Wikimedia Commons (CC BY-SA 3.0; BAZ Antennas).

Radiation / wave patterns

Idealized radiation pattern of the Log-Periodic Dipole Array (LPDA)
Idealized azimuth radiation pattern (illustrative, generated). Radial scale in dB.

How & why it works

An LPDA is a row of dipoles whose lengths and spacings scale by a constant ratio, fed by a crisscrossed (phase-reversing) transmission line along the boom. At any frequency in its range only the dipoles near a half-wavelength form an 'active region' and radiate, while the rest are effectively idle; as frequency changes that active region simply slides along the array. This gives directional gain that stays nearly constant across a very wide bandwidth—often more than 10:1—unlike a Yagi, which is sharply tuned.

Real-world uses

TV reception, EMC/antenna test ranges, signal intelligence, and wideband monitoring.