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 / 75 miles from its transmitter site.
There are several atmospheric modes which can allow signals to travel further at these frequencies than they normally would. The one I am most interested in is called Sporadic E.
What happens is that ionized (simply stated, electrified) clouds of gas form in the atmospheres E layer (90 km / 56 miles to 120 km / 75 miles up) and reflect signals back to earth which would normally head out into outer 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 “opening” stations will commonly be heard ranging at distances anywhere between 500 and 1500 miles. On rare occasions greater distances will be involved. More than one E cloud will form in a straight line along a reception path and the distance between them will be bridged either by a sea or ground reflection of the signal or by the signal moving directly from cloud to cloud through the atmosphere.
In such rare cases signals may travel vast distances.
I have received Sporadic E on the FM band at distances up to 4011 miles / 6456 km, holding the world record for such receptions between June 26th 2003 and May 31st 2010.
For an excellent explanation of Sporadic E and other propagation modes that deliver long distance FM signals please follow these links: