Understanding Mountain Radio Signal Paths
VHF/UHF radio propagation in mountainous terrain can be difficult and sometimes may seem impossible. As you know, higher frequencies travel primarily line-of-sight. Unlike HF, they do not reflect off the ionosphere, so their range can be rather limited (although knowledgeable DXer’s know special tricks, like meteor scatter.)
However, all is not lost. It is possible to make sense out of using higher frequencies in mountain communities, and at the November WARS (Willits Amateur Radio Society) seminar Tim Hanna, WB9NJS, described some of his experimentation and research on this topic.
Tim got interested in mountain VHF propagation after being surprised that he could communicate over 2M FM from his home QTH in Willits to Wayne, W6WMV, in Finely (south of Clear Lake.) The signal path is a good distance and there are two significant mountains in the way; regardless, Tim was still able to communicate quite well.
In his presentation Tim reviewed the ways radio signals change direction:
- Reflection: wave changes direction by bouncing off other objects (a metal fence post for example).
- Refraction: wave changes direction due to variations in the air (or other medium). Examples are cold and hot spots where the density of the air differs.
- Diffraction: wave changes direction by passing over sharply defined edges — often called the “knife edge effect”.
Next, Tim introduced a handy mapping tool for analyzing signal paths. The software is from DeLorme and makes it fairly easy to go from a flat map of the path to an elevation profile view that helps you see the obstacles that are in the way.
Once that was done, an elevation profile could be generated:
Looking at this profile, you might begin to wonder how Tim’s signal was able to reach Wayne in Finely. There are a two peaks that should block the signal. Tim speculated that perhaps knife edge diffraction was helping his signal bend over one or more peaks.
Tim also noticed another possibility. It could be that signals were being reflected off Mount Konocti behind Wayne. If so, the signal map becomes:
To plot such an elevation profile, the profile is “unfolded” to show it by distance. It becomes:
To understand this profile, note that that tall peak on the right isn’t blocking the signal, it’s reflecting the signal. That’s how reflection is shown on a profile plot.
So, this signal path seems more likely. Although there may be a knife edge effect at the peak closer to Tim, the reflection of the signal from Mount Konocti behind Wayne makes sense. The middle peak becomes less of a factor because it’s no longer in the way.
Tim went on to show several other signal paths for unusual radio contacts he’s made from his QTH on the west side of Willits. Some are not such direct paths as the one to Wayne in Finely, but looking at the elevation profiles helps you understand how such paths are even possible. For example, he’s able to communicate with K6FTY who lives a considerable distance to the north. The trick again is to use a directional antenna to reflect the signal off various high-elevation peaks.
Thanks again to Tim Hanna for his educational WARS presentation. It’s good information for those of us who live in the mountainous terrain of Mendocino and Lake Counties. If you find you can’t make a VHF/UHF QSO directly, you now know that you can try reflecting or diffracting your signal off various peaks to increase the likelihood of your success.