Some radio signals come back seconds later, and nobody knows why
2026-08-11 · EA5IYR
In 1927, a Norwegian engineer named Jørgen Hals was listening to a Dutch transmitter in Eindhoven when he heard something that should not have been possible.
The signal arrived. Then, about three seconds later, it arrived again. Why three seconds is a problem
Radio travels at the speed of light. Three seconds of delay means roughly 900,000 kilometres of travel.
The Moon is 384,000 km away. A three-second echo has gone past it and come back. And long delayed echoes have been logged at up to thirty seconds, which is somewhere around ten million kilometres — well beyond anything in the neighbourhood.
Meanwhile the mundane explanations are all far too fast. A normal ionospheric hop is measured in milliseconds. Even a signal that laps the entire planet returns in 0.138 seconds, and that effect is well understood and regularly observed.
There is nothing between 0.138 seconds and three seconds that anyone can point at. It was taken seriously immediately
Hals wrote to Carl Størmer, the mathematician who had spent his career on the aurora, and to Balthasar van der Pol at Philips, who was running the Eindhoven transmitter.
Van der Pol sent deliberate test signals. Størmer logged the returns. They confirmed the effect was real and recorded delays ranging from about three to fifteen seconds, varying unpredictably. Then they failed to explain it, and said so.
That is roughly where the matter still stands. The theories
Several have been proposed over the decades, and none has been generally accepted.
Ducting in the magnetosphere, where a signal is trapped along a field line and released late. Reflection from plasma clouds far out in space. Non-linear effects in the ionosphere that store and re-radiate energy. Mode conversion into slow-moving plasma waves and back again — that last one at least has the virtue of explaining a delay without requiring absurd distances.
The difficulty is that echoes are sporadic and nobody has managed to produce them on demand. A phenomenon you cannot summon is very hard to study. The strangest chapter
In 1973 the writer Duncan Lunan went back through Størmer and van der Pol's original tables of delay times, wondering whether the variation might be a code.
He plotted delay against sequence number, swapped the axes, and got something that looked to him like a star map of the constellation Boötes — with one star, Epsilon Boötis, notably out of position. His reading was that a probe from that star had parked itself near Earth and was announcing its presence by echoing our transmissions back with meaningful delays.
It made the newspapers. Lunan later withdrew the interpretation himself.
It is easy to laugh at, and worth not laughing too hard: he was doing something defensible — looking for structure in a dataset nobody could explain — and then he published a correction when it did not hold. That is more than most. It still happens
Amateur operators still report them, occasionally, usually on the higher HF bands during disturbed conditions. If you run enough power and listen carefully to your own signal you may one day hear yourself arrive late.
If you do, log it precisely — frequency, time, delay, and what the ionosphere was doing that day. Nearly a century on, that is still the only kind of evidence anybody has, and there is still nowhere near enough of it. A time station in the background of the recording turns an anecdote into a measurement.
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Sources: Wikipedia — Long delayed echo, University of Oslo — the five most likely explanations for LDEs, arXiv — Long Delayed Echo: a new approach, TIME archive, 1973 — Message from a Star.