Re: Hearing Minnesota's Classic Rock Station Instead of XTU...In NJ...What the H
Stuart Greenberg said:
Could someone please take a minute or so to explain how "Tropospheric Bending" works? also why is it very common this time of year? ???
"Tropo DX" is based on refraction, which causes waves to bend due to changes in the speed of propagation through the atmosphere. This phenomenon was first observed with light:
http://en.wikipedia.org/wiki/Refraction
Electromagnetic radiation moves at the full "speed of light" in a vacuum; gases slow down this motion. Dense (compressed) air near the earth's surface slows the wave down more than the thin air encountered at high altitudes. Under normal conditions, density gradually decreases as elevation increases, causing the top of a wavefront to move just a bit faster than the bottom, so there is usually some moderate bending over the horizon. This is what allows most Philadelphia FM stations to be heard in Atlantic City on a day-to-day basis.
The fun stuff happens when layers of air with significantly different densities and/or humidities form, one on top of the other -- for instance, a temperature inversion, where an elevated warm layer overrides a cooler layer. In the sharp transition region between warm and cold, a high degree of bending takes place, forcing the the wave to follow a "duct", which can carry signals over terrain for a few hundred miles.
This article explains more about Tropo Ducting from an amateur radio perspective:
http://www.astrosurf.com/luxorion/qsl-propa6.htm
During the summer, ducts typically form near the coast and across water in the evening, but tend to "blow apart" after the sun rises the next morning and thermal convention begins to mix up the layered air.
E-skip, on the other hand, is a type of reflection that occurs in the ionosphere. The wave doesn't "bend", but bounces off ionized particles and heads back down to earth. You might compare tropo with a lens -- and E-skip with a mirror.
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