Here is a great tool for RF Analysis which:

• Deals with diffraction effects,
• Computes the Fresnel effects,
• Handles elevation data from many different sources

RF Links have many issues that plaque all too many folks in charge of ensuring radios work as planned. A tool like this provides this kind of view…

With this tool you can understand why some RF links actually fade even when the two antennas are in complete view of each other. When the wavelengths get longer than light wild things happen. The program is free and, from what I can tell so far, works very well.

http://www.cplus.org/rmw/english1.html