Also known as: log-periodic dipole array, LPDA, log periodic
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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.
Where to buy
For directional gain that stays usable across many bands, a wideband LPDA like the Antenna World COM-072708LP (700–2700 MHz, ~9 dBi, around $45) points at a distant UHF/700/800 MHz trunking site and keeps its beam as you move between systems — no retuning, no antenna swap. Sold as a cellular panel, it doubles as a scanner beam across exactly the bands where P25 and 800 MHz trunking live. Mount the elements vertical and aim the short end of the boom at the tower.
Note the band: these panels do not cover 136–174 MHz VHF — use a Yagi there. And directivity fixes weak signal, not encryption. See the best scanner antenna guide for the full antenna line-up.
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Sources
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Log-periodic antenna — Wikipedia, for the scaled-element geometry, alternating feed, active region, and the wideband constant-gain behaviour. ↩
Frequently asked questions
Which log-periodic antenna should I buy for wideband scanning?
A wideband LPDA such as the Antenna World COM-072708LP (700–2700 MHz, ~9 dBi, around $45) gives directional gain and rear rejection across many bands from one feedpoint — useful for aiming at a distant 700/800 MHz P25 or trunking site without retuning or swapping antennas. Sold for cellular use, these panels double as scanner beams; the higher UHF bands they cover are exactly where P25 Phase 1/2 and much trunking lives.
Log-periodic or Yagi for a distant site?
A Yagi gives more gain on the one frequency it is cut for; an LPDA gives a bit less gain but holds that gain and pattern across a huge band. If you chase a single known channel, a Yagi wins; if you want one directional antenna that stays useful from UHF up through 800 MHz as you move between systems, the log-periodic is the practical choice.
Will a 700–2700 MHz cellular LPDA work for VHF scanning?
No — those panels start around 700 MHz, so they cover UHF and 800/900 MHz trunking but not the 136–174 MHz VHF band. If you need VHF directivity, use a VHF/UHF Yagi instead. Match the antenna’s band to the systems you actually monitor.
How do I mount and aim a log-periodic?
Fix it to a mast with the elements vertical (to match land-mobile polarization) and point the short front end of the boom at the site — an LPDA fires off its shortest elements. Keep the coax short and low-loss, especially at 800 MHz where cable loss is high.