Long Delayed Echoes on HF: Signals That Return Seconds Later
Now and then a radio amateur transmits a CQ, and a few seconds later hears the same call coming back from the receiver, as if somebody had recorded it and played it again. It is not a feedback from the audio chain, and it is not a joke from a neighbour. It is a long delayed echo, one of the oldest unsolved puzzles of radio propagation. First reported in the 1920s, these echoes have been heard on HF many times since, and science still has no agreed explanation. This guide explains how an LDE differs from the normal echoes that we know, what has been proposed, how to rule out ordinary causes and how to record a possible event so that it has value for those who study it.
Ordinary Echoes First
To understand what is unusual, start with what is normal. Radio waves travel at the speed of light, about 300,000 kilometres per second. The circumference of the Earth is about 40,000 kilometres, so a signal that goes once around the world takes roughly 0.13 seconds. On the HF bands, with a favourable ionosphere, a strong signal can circle the globe and come back to your receiver, and you can hear it as a faint echo that follows the original by a seventh of a second. This is the round-the-world echo, and it is a normal and well understood effect, explained by the multi-hop paths between the ionosphere and the ground. The background on how the radio wave travels in space and time is in the article on the speed of light.
The short and the long path of a station can also produce two signals with a small difference in delay, as described in the article about long path and short path DX. If the two arrive almost together, the effect is a hollow, flanging sound. A third kind of echo is the Moon: a signal sent to the Moon returns after about 2.5 seconds, but it is extremely weak, and hearing it needs high power and a large antenna, as explained in our beginner’s guide to EME.
What Makes an Echo “Long Delayed”
An echo is called a long delayed echo when it comes back much later than a trip around the Earth can explain. The usual limit is more than about 0.3 seconds, and reports go from a few seconds to tens of seconds, in rare cases even longer. The signal is a clear and often undistorted copy of the original, which is another reason for surprise. The table compares the different kinds of delayed signals.
| Type | Typical delay | Explanation |
|---|---|---|
| Round-the-world echo | About 0.13 s per circuit | The signal circles the Earth by multiple ionospheric hops |
| Moonbounce | About 2.5 s | Reflection from the surface of the Moon |
| Long delayed echo | Several seconds to tens of seconds | Not agreed; several theories |
| Equipment or software delay | Constant, depends on the chain | Audio processing, latency, feedback |
The First Reports
The earliest well-known report is from 1927. A Norwegian engineer, Jørgen Hals, living near Oslo, was listening to the Dutch short-wave station of Philips at Eindhoven, and he heard the signals repeated after a few seconds. He told a mathematician and physicist, Carl Størmer, who studied the aurora, and Størmer and the Dutch physicist Balthasar van der Pol began systematic tests. They sent signals and listened for the echoes, and recorded delays of a few seconds up to about fifteen. The reports were taken seriously enough to be published, and the question of their origin started a debate that has lasted for almost a hundred years. The history is summarised in the article on the long delayed echo.
Since then, radio amateurs, shortwave listeners and professional stations have reported LDEs from time to time. Many reports come from HF around 10 to 30 MHz, but not exclusively. The events are sporadic and unpredictable, often appearing and then disappearing within minutes, so they are difficult to study. Most amateurs never hear one.
Theories: What Could Cause an Echo of Seconds
A delay of three seconds corresponds to a path of about 900,000 kilometres, more than twice the distance to the Moon. Any explanation must either send the wave on a very long path or hold it for a time. The ideas suggested fall in a few groups.
- Reflection in space. Early suggestions were that waves are reflected by plasma clouds in space, at distances of hundreds of thousands of kilometres. The echo would then be a true reflection, like a radar echo from a distant object.
- Slow propagation in plasma. In a magnetised plasma, waves can travel slowly along the field lines, with a speed far below that of light. A signal that enters such a region, travels along the magnetic field and returns would be delayed. This idea relates to the whistlers heard on very low frequencies.
- Ducting in the ionosphere. A signal might be trapped in a duct and travel round the Earth many times before it leaves the duct and reaches the receiver. The echo would then be a multiple of the round-the-world delay.
- Non-linear effects. Some suggestions involve interactions between waves in the ionosphere, in which a strong signal modifies the medium and produces a delayed response.
None of these theories has been confirmed, and each has difficulties. The reflection in space needs plasma structures that are rarely seen. The slow wave idea suits very low frequencies better than HF. The duct explanation does not fit some reports, in which the delay does not match a multiple of 0.13 seconds. For this reason, the honest answer is that the origin of LDEs is not known.
First, Rule Out the Boring Causes
Most apparent echoes turn out to be equipment, so be critical. Sound that returns a short time after you speak may come from a delay in the audio chain. Check the following before you call it an LDE.
- Your own receiver. A DSP function, such as a noise reduction, a voice recorder or a speech compressor, can introduce a delay. Switch these off and test again.
- The sound card and software. A computer in the loop adds latency, and a loopback between the output and the input can create an echo. Digital mode programs and virtual audio cables should be checked, as we describe in the guide to the virtual audio cable.
- Monitoring and sidetone. The transmit monitor, the sidetone and the headphone mix may return your own signal late.
- The other station. A station that uses a remote system, an Internet link or a repeater with a delay will produce a late signal on the air, which you may hear twice by different paths.
If the delay is constant and exactly the same on every band and every day, the cause is almost surely in the equipment. A real LDE comes and goes, depends on the frequency and varies in delay.
How to Log a Possible LDE
If you are lucky, the recording of the event is valuable, because the evidence from the past consists mostly of notes from memory. Prepare for it before it happens. Keep a recorder of the receiver audio running when you test, and keep an accurate UTC clock. When you suspect an echo, do the following.
- Record the audio of the receiver and, if possible, your transmitted audio on a second channel, so that the delay can be measured later.
- Note the date and time in UTC, the frequency, the mode, the antenna, the power and the beam direction.
- Send a short, clear test signal, such as a series of dits or a call sign, and measure the delay between the transmitted and the received signal.
- Repeat on another frequency and with a different power, to see whether the echo follows.
- Note the space weather conditions at the time, such as the solar flux and the K index, which you can find as described in our guide to solar flux, A index and K index.
Afterwards, open the recording in a program that displays a waveform, and measure the time between the original and the echo. Compare the shape of the two. An echo that has the same pitch and the same content as your transmission, only delayed, suggests a true propagation effect. A distorted or different signal suggests an equipment problem.
Echoes You May Meet That Are Not LDEs
Several other effects are sometimes confused with long delayed echoes, and knowing them saves a false alarm. A repeater with a long hang time or a delayed audio path can return your voice a moment after you finish, and a linked system, like an Internet gateway, can add a delay of a second or more. Some digital voice modes use a buffer that delays the audio by a fraction of a second. In the case of satellites, the path itself is long: a signal that goes up to a high orbit and comes down again takes a fraction of a second, and a relay through a geostationary transponder takes about a quarter of a second. All of these are ordinary and well understood.
Another well-known effect is the fluttering sound of signals that pass near the auroral zone, where the signal reaches the receiver by many scattered paths with slightly different delays and appears as a rough, hoarse tone. That is a different phenomenon from an echo, because the paths differ by milliseconds and the sound is blurred, not repeated. If you can distinguish a clear second copy of your own call with a gap of silence in between, you have something different. In that case, the careful steps above are what to follow.
Finally, in the cold language of the researcher, an unusual observation without a record is only an anecdote. A short notebook entry with the time, band and delay is better than nothing, and an audio file is best of all. If you hear one, do not be discouraged by sceptical friends. Many experienced operators have heard the effect once in a lifetime, and the record you produce is the only way for others to judge it.
Why It Is Worth Your Attention
Few scientific questions can be explored with a radio and a recorder, and the LDE is one of them. The records made by amateurs over the decades are a significant part of the information that exists. Even if no one can say what causes the effect, your recording might help to see in what conditions it appears, for example on certain bands, at a certain time of the day, or during quiet or disturbed space weather. You can send your notes to a local society or to a research group that collects such data.
It is also a good lesson in method. The mystery shows how easy it is to believe the first explanation, and how carefully a claim must be checked before it is accepted. A good observer rules out the simple cause first, records everything and leaves the question open if it cannot be answered. These habits serve you well in every other part of the hobby.
The Bottom Line
A long delayed echo is a signal that returns seconds after it was sent, much later than the 0.13 seconds of a trip round the world or the 2.5 seconds of a bounce from the Moon. It was first reported in 1927, and its cause is still not agreed, with theories ranging from reflection in space to slow waves in plasma and ducting. Rule out equipment delays first, then record the audio of both signals with UTC time, frequency, mode and conditions, and measure the delay. Real LDEs are rare, but good records are valuable.
Frequently Asked Questions
What is a long delayed echo?
It is an echo of a radio signal that returns several seconds or more after it was sent, a delay far longer than a trip round the Earth can explain.
How is it different from a round-the-world echo?
A round-the-world echo returns after about 0.13 seconds, the time to circle the globe. A long delayed echo comes back after seconds or even tens of seconds.
Who discovered the long delayed echo?
It was first reported in 1927 by Jørgen Hals in Norway, who heard Dutch short-wave signals repeated after a few seconds. Carl Størmer and Balthasar van der Pol investigated.
Is the cause of LDEs known?
No. Several theories exist, such as reflection in space, slow waves in plasma and ducting, but none has been confirmed.
How can I tell an LDE from an equipment echo?
An equipment delay is constant and the same on every band and day. A real LDE is sporadic and varies in delay. Switch off DSP functions and check audio loops first.
How should I record a suspected LDE?
Record the audio, note UTC time, frequency, mode, antenna and power, send a short test signal and measure the delay. Repeat on another frequency to see whether the echo follows.