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Long delayed echo
Long delayed echoes (LDEs) are radio echoes which return to the sender several seconds after a radio transmission has occurred. Delays of longer than 2.7 seconds are considered LDEs.〔() ARRL: Stan Horzepa, ''Radio Ghosts'' (dead link. use http://web.archive.org/web/20031105155129/http://www.arrl.org/news/features/2003/10/31/1/)〕〔(ARRL: Stan Horzepa,''Long-Delayed Echoes Again'' ) (dead link. use http://web.archive.org/web/20091112202151/http://www.arrl.org/news/features/2007/07/06/01/) 〕 LDEs have a number of proposed scientific origins.
==History==
These echoes were first observed in 1927 by civil engineer Jørgen Hals from his home near Oslo, Norway. Hals had repeatedly observed an unexpected second radio echo with a significant time delay after the primary radio echo ended. Unable to account for this strange phenomenon, he wrote a letter to Norwegian physicist Carl Størmer, explaining the event:
At the end of the summer of 1927 I repeatedly heard signals from the Dutch short-wave transmitting station PCJJ at Eindhoven. At the same time as I heard these I also heard echoes. I heard the usual echo which goes round the Earth with an interval of about 1/7 of a second as well as a weaker echo about three seconds after the principal echo had gone. When the principal signal was especially strong, I suppose the amplitude for the last echo three seconds later, lay between 1/10 and 1/20 of the principal signal in strength. From where this echo comes I cannot say for the present, I can only confirm that I really heard it.〔Carl Stormer, "Short Wave Echoes and the Aurora Borealis," Nature, 122, 681, (1928)〕
Physicist Balthasar van der Pol〔Balthus van der Pol, "Short Wave Echoes and the Aurora Borealis," Nature, 122, 878-879 (1928)〕 helped Hals and Stormer investigate the echoes, but due to the sporadic nature of the echo events and variations in time-delay, did not find a suitable explanation.〔() V. Grassmann, ''Long-delayed radio echoes, Observations and interpretations,'' VHF Communications vol 2, pp. 109-116, 1993.〕
Long delayed echoes have been heard sporadically from the first observations in 1927 and up to our time.
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