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Author Topic: propagation over ridges  (Read 201 times)

Posts: 107

« on: December 12, 2017, 11:04:09 AM »

I am working on getting the best signal possible between two sites with a 3500+' ridgeline between them.  In trying to understand VHF propagation, I've come across the principles of refraction, reflection, and diffraction.  In my particular case, I believe diffraction is happening off the ridgeline so the signal is able to be received in the basin on the other side.  In reading about diffraction, I came across this article: which showed a technique using DeLorme mapping software to show elevation profiles.  I wanted to try this to see if it could give me a better picture of my signal path than the rudimentary triangles I had been diagramming, but I did not have the DeLorme software.  Instead, I found I could use a feature in Keyhole (aka. Google Earth).

If you click the "Ruler" toolbar button, then click on the "path" tab of the Ruler tool's window, check the box for "show elevation profile" and click on two (or more) points to create a path on the map, the software will show the elevation profile in a pane at the bottom.  For an example, I have plotted the elevation profile between a spot somewhere near Driggs, Idaho and the Snake River Overlook that traverses near the summit of the Grand Teton.  If you ever try this path with HT's, be sure you're the one at the Snake River Overlook.  At least you'll have a nice quiet view.


Posts: 107

« Reply #1 on: December 12, 2017, 11:19:30 AM »

Note that the X and Y axis of the elevation profile on the screen are not on the same scale.  This becomes important to understand when you are thinking about the angles of radiation needed.  Because with VHF/UHF for local communications, we're primarily interested in groundwave propagation, angles of radiation over distances of miles are usually less than 5 degrees even in the mountains whether we're trying to diffract over the ridge or just hit a repeater.

Here I have squashed the Y axis to a similar scale as X. 

Even from the Snake River Overlook on the right-side of my elevation profile drawing, the angle needed to hit the summit of the Grand Teton is only about 9 degrees, and this is only an absurd example for fun.

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