The broadband cousin of the Yagi. Unlike a Yagi — sharp but single-band — a log-periodic holds its gain, pattern, and impedance across a wide frequency range (often 10:1 or more) from one fixed structure. That is exactly why coastal and ship-shore stations use them: one antenna works the whole HF maritime spread without retuning.
1. The Classic Horizontal LPDA
A row of dipoles whose lengths and spacings scale by a constant ratio (τ, tau) from a short front end to a long back end. Two things make it work: the scaled taper, and the transposed feed that flips phase 180° between adjacent elements.
Figure 1. Horizontal LPDA. Short (high-frequency) elements at the front, long (low-frequency) at the back. The amber elements are the active region for the frequency shown; it slides along the boom as frequency changes.
How it works
The taper. Element lengths and spacings shrink by a fixed ratio τ toward the short end. At any frequency in the band, some set of elements is the right length.
The active region. Only the two or three dipoles near a half-wavelength actually radiate. As frequency changes, that region slides along the boom — the antenna behaves like a small Yagi that walks back and forth.
The transposed feed. The twin feeder is criss-crossed between elements, driving neighbors 180° out of phase. That phasing makes the short elements ahead act as directors and the long elements behind as reflectors — so the main beam fires off the short end, toward the feed.
2. The Vertical Log-Periodic — The Coast-Station Form
Stand the same antenna on end over a saltwater ground plane and you get the form that dominates ship-shore sites — and the one that, from a distance, simply looks like a tall vertical with rungs.
Figure 2. Vertical log-periodic over saltwater. Same electrical principle as Figure 1, reoriented for vertical polarization and a low takeoff angle out toward the ships.
Why vertical, and why over saltwater
Vertical polarization → low takeoff angle. Standing the elements vertical launches energy at a shallow angle instead of up toward the sky.
Saltwater is the best RF ground on Earth. A low-angle vertical signal couples into a strong groundwave and skips off at a shallow angle for long-haul skywave.
Result. The energy goes out toward the horizon and the ships, across the whole band, from one fixed antenna — exactly what a coast station needs.
Spotting it at KPH. When you watch footage of the Bolinas antenna farm, two details confirm a log-periodic rather than a plain vertical or a vertical curtain: the elements are graded in size (not all identical), and the feed line zig-zags between elements rather than running straight. That transposition is the log-periodic signature. The vertical orientation is what makes it read as "a vertical" from a distance — while electrically it is still the broadband workhorse drawn above.
Concept reference for study; antenna principles per standard HF antenna engineering (LPDA geometry, τ/σ design, active-region theory). Coast-station application context: Maritime Radio Historical Society (KPH / Bolinas) and commercial HF log-periodic practice. Prepared for n6jet.com · 73 de N6JET.