FMDX is the search for long distance FM broadcast stations via unusual radio propgation. Most commonly Tropospheric propgation, Sporadic-E, And Meter Scatter.
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Tropospheric propgation allows VHF signals including FM Broadcast to travel well beyond their intended line of sight range. This often incorrectly is called tropospheric ducting when there is several different kinds of tropospheric propgation including scatter, enhancement, and ducting of course.
Tropospheric enhancement and ducting are both casued by a inversion layer however ducting can allow VHF signals to travel for 1,000 miles or more.
Tropospheric enhancement can be common during the summer months however tropospheric ducting is much more rare.
Sproadic-E is a unusual form of radio progpation that uses patachy clouds that form sporadically in the E layer of the ionosphere.
A single Sproadic-E cloud can allow VHF signals to travel from 500-1600 Miles. Extremely short Sproadic-E is rarely seen but can occur down to 350 Miles with a extremely ionized cloud.
Resources and links below.
DXmaps - Real time propgation information and reports
VHF Propagation Map
William Hepburn's Worldwide Tropospheric Ducting Forecast
Space Weather Prediction Center
DxWorld TV/FM Skip Log
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