Long delayed echoes refer to a radio phenomena where an echo of a transmission is heard that is longer than can readily be explained by most known origins, most typically 3-40 seconds.

This phenomena was first experienced by a Norwegian radio engineer, Jorgen Hals while listening to a Dutch shortwave station.

With shortwave broadcasts it is not unusual to hear an echo with a delay of about 1/7th of a second. This represents the time it takes the signal to go around the earth once using multiple hops between the ground and the ionosphere. However, in addition to this regular echo, Jorgen Hals also heard an echo of about three seconds in duration.

The 1/7th second echo corresponds well with the earth’s 24,902 mile diameter since light travels at 186,000 miles per second in free space, very slightly less than that in air, and the path taken around the world involved multiple hops between the ground and ionosphere so it was not entirely straight.

But that does not explain the three second echo. In three seconds, a radio signal would cover a distance of 558,000 miles, or could make a round trip of 279,000 miles. This corresponds fairly well to mean distance between the earth and moon which is 238,855 miles.

There are problems with moon bounce as an explanation even for this specific incidence of long delayed echo. Shortwave frequencies that are used do not penetrate the earth’s ionosphere, they are reflected back to earth which is what makes them useful, and if this had not been the case Jorgen Hals would not have been able to hear the primary signal or the 1/7th second echo. Signals which are nearly vertical to the earth can penetrate the ionosphere because the ionosphere actually ‘refracts’ rather than ‘reflects’ the signal and at a vertical angle the signal can go through.

The signal reported by Jorgen Hals was about 1/10th to 1/20th that of the primary echo. This is inconsistent with moon bounce because the huge distances involved and the relatively small area of the sky the moon occupies would result in severe attenuation. Amateur radio operators intentionally work moon bounce and to do so they use high power (maximum legal power of 1kw) combined with very high gain antennas and UHF or microwave frequencies so that a high degree of directivity can be accomplished with a reasonable antenna. Even with these optimizations, moon bounced signals are very marginal.

This however does not explain longer echo’s of up to 40 seconds or in some rare cases even more. It is also interesting to note that a shift in frequency has been reported in the reception of some echoes suggesting that the reflector, whatever it is, is moving relative to the transmission / reception site.

I also have had an experience with radio echoes but not of a long duration. The interesting aspect is that this was when I was operating a pirate radio station on 1200 Khz and at the end of the night when I powered the transmitter off I would hear occasionally hear a fraction of a second of the tail end of the broadcast on a receiver I used to monitor our transmission.

The 1/7th second echo of the shortwave signal could be explained by a trip around the earth, our signal was scarcely receivable twenty miles away so the source of this echo remains a complete mystery to me.

This phenomena has high strangeness to it. The reports are quite varied. Occasionally there are explanations but more often than not it is a complete mystery. Take for example, this report by “Dave S.” in which he reports something resembling a moving weather front that reflects signals from the east back to the east with a delay. The reports following it are also interesting.

The alien probe hypothesis suggests that an alien probe lurking somewhere in the solar system is picking up our signals and transmitting them back to us. The signals are often reported to be far too strong to be accounted for by reflection from a natural object.

There is a theory that suggests that acoustic propagation in the ionosphere may be responsible. The idea that ions, being charged, respond to electromagnetic radiation by physically moving, essentially setting up a radio frequency acoustic wave which travels at the speed of sound in the ionosphere rather than at the speed of light.

Some people believe there may be an ionized refractive region outside the earth’s own ionosphere say in the region of the Ort cloud, however I doubt this hypothesis because the earth’s orbital velocity would result in substantial Doppler shift. While long delayed echoes are often frequency shifted, the value of shift is small.

In short this is an intriguing phenomena for which no real adequate explanation exists. The echoes are observed all the way from long wave frequencies up to microwave frequencies. I have seen claims that they peak near the MUF, this is the highest frequency that the ionosphere can refract back to earth under the conditions present at the time, but reports exist both far in excess of the MUF and far below it.

This would seem to dismiss the alien probe theory as well because reports are not limited to frequencies which penetrate the earth’s ionosphere.

There are some indication that the Sun somehow plays a role as LDE’s stop just prior to a solar eclipse and resume approximately half-way through.

Some people believe there is a correspondence between auroral activity and LDE’s.

All in all it’s a great mystery. I’d be interested in hearing theories others may have regarding this phenomena as well as incident reports.