FM Broadcast DX is the search for long distance FM broadcast stations via unusual radio propagation. Most commonly Tropospheric effects, Sporadic-E, And Meteor Scatter.
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Tropospheric propagation allows VHF signals including FM Broadcast to travel well beyond their intended line of sight range. This is often incorrectly is called tropospheric ducting when there is several different kinds of tropospheric propagation including scatter, enhancement, and ducting of course.
Tropospheric enhancement and ducting both occur by an 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. See here for a in depth explanation of Tropospheric modes
Sporadic-E is an unusual form of radio propagation that uses patchy clouds that form sporadically in the E layer of the ionosphere.
A single Sporadic-E cloud can allow VHF signals to travel from 500-1600 Miles. Extremely short Sporadic-E is rarely seen but can occur down to 350 Miles with a extremely ionized cloud.
Resources and links below.
FMLIST - database of worldwide FM radio, digital radio and TV
DXmaps - Real time propgation information and reports Look at the FM DX and MUF Sp-E tabs
VHF Propagation Map
William Hepburn's Worldwide Tropospheric Ducting Forecast
FM PI Code Allocations | National Radio Systems Committee
Space Weather Prediction Center