What is FM DX?
FM DX is a pastime in which listeners (known as Dx’ers) chase distant signals between 87.5 and 108 megahertz which they ordinarily would not be able to receive at their location. Usually an FM station will not be heard any more than about 120 km from its transmitter site.
There are several atmospheric modes which can support signals travelling further at these frequencies than they normally would. The one I am most interested in is called Sporadic E.
What happens is that clouds of gas form in the atmosphere's E layer which then reflect signals back to earth that would normally head out into space. In the northern hemisphere this is most common between late April and late August with a peak mid June. Sporadic E can also occur at other times of the year notably in October and December.
Sporadic E is mostly a day time phenomenon and occurrences can last anywhere between five minutes and many hours.
During a Sporadic E event or “opening” stations will commonly be heard ranging at distances anywhere between 1000 and 2300 km. On rare occasions shorter and greater distances will be involved. More than one E cloud can also combine along a reception path. In such rare cases signals may travel vast distances.
I have received Sporadic E on the FM band at distances up to 6456 km, setting two world records for such reception in 2003 and 2009.