For VHF/UHF propagation, there's a neat tool out there online called SPLAT!. You sign up and provide your coordinates, then SPLAT! uses that information to plot your coverage based on terrain. I've found that it's remarkably accurate, showing distant hilltops where people can work me direct on simplex. We've also plotted our local repeaters that way, and SPLAT! does a great job of showing shaded areas and holes in the coverage. You can also map the SPLAT! results on Google Earth maps, making it even more interesting.
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