For anybody who's wondered what exactly causes our VHF/UHF signals to 'bend' as they move through the atmosphere, you might find this explanation helpful. The wave front has a finite diameter at any given point along its path as it moves to the far end receiving location. (The fact that the wavefront is not a pencil thin 'beam' is key) Typically, the water content per unit volume of air decreases as a function of height above the ground. In other words, it's less humid, the higher up you go. Now, because the velocity of propagation of a radio wave is slower in air that has more humidity, you can imagine how the 'higher edge' of the wavefront will move a little bit faster than the lower edge of the wave front. So as the wavefront moves forward through the air, the wavefront tends to 'arc over' since the upper portions of the wave front are moving faster than the lower parts. That's atmospheric refraction physically explained in a nut shell. I don't know what kind of nut. Taking this a bit further, I can talk about cases of so-called "ducting" outages that I have witnessed in rough propagation regions including the Gulf coast and the Imperial Valley in California. But contrary to people's claims that extended path outages are due to true "ducting", I am convinced that in many of these cases, the outage is simply due to the signal refracting to the point where it is driven into the dirt (or gator filled swamp in some cases) before it reaches the intended receive location. Sure there will be instances where the wavefront gets "trapped" between two elevated layers but I suspect in many cases the ducting outages are simpler than that, with the signal simply getting driven into the terrain, falling short of target.
73 N6AF Chuck
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Topic: Refraction explained from a 'different angle' (Read 2785 times)