Also known as: log-periodic dipole array, LPDA, log periodic
A log-periodic antenna, most often a log-periodic dipole array (LPDA), is a directional antenna whose electrical properties repeat periodically with the logarithm of frequency, giving it nearly constant gain and pattern across a very wide band — often a decade (10:1) or more.1 It looks like a Yagi with many elements, but every dipole is connected to the feedline in alternating phase, and the elements grow in a fixed geometric ratio along the boom.
How it works
The design is defined by two constants: a scale factor τ (each element’s length and spacing is τ times the next) and a spacing factor σ. Because the geometry scales logarithmically, the array looks electrically the same at frequencies separated by factors of τ — hence “log-periodic.” At any given frequency only a handful of elements — those close to a half wavelength — are actually resonant. This active region does the radiating; longer elements behind it act like reflectors and shorter ones ahead act like directors, so locally the array behaves like a small Yagi.
The key trick is the alternating (transposed) feed: adjacent dipoles connect to opposite sides of the twin-boom feedline. This 180° flip, combined with the physical spacing, gives the active-region elements the phase relationship needed to fire the beam toward the short end of the array. As frequency rises or falls, the active region simply slides to a different set of elements, but its size and behaviour stay the same — so gain, beamwidth, and SWR hold steady across the whole design band.
The trade-off against a Yagi is gain for bandwidth. Because only a few elements are ever active, an LPDA of a given boom length has modest gain (typically 6–9 dBi) compared with a Yagi that dedicates all its elements to one frequency. What it buys is enormous frequency coverage from a single feedpoint with a stable pattern and match.
Relevance to SDR
Log-periodics are the natural directional antenna for wideband scanning and surveillance. A single LPDA can cover, say, 400–1000 MHz — spanning multiple public- safety and trunking bands — while giving the forward gain and rear rejection of a beam. For an SDR user who wants directivity toward a distant trunking site but does not want to retune or swap antennas for every band, an LPDA is the practical choice.
GopherTrunk is a receive-only decoder and does not steer or select elements; the antenna’s directivity is fixed by where the boom points, exactly as with a Yagi. Where an LPDA wins is that its broadband nature keeps the same beam usable across the many bands GT can decode, from low-VHF LTR up through 800 MHz P25. For maximum gain on one known frequency a Yagi still beats it; for coverage, the log-periodic wins.
Sources
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Log-periodic antenna — Wikipedia, for the scaled-element geometry, alternating feed, active region, and the wideband constant-gain behaviour. ↩