I used to, as part of my job, do Earth intercept coverage maps for antenna locations especially for sites here in Texas. So far, I have not found, on the Internet, a true Earth intercept program. At 440 MHz the signal basically stops and at 902 MHz, and higher, the signal definitely, for all practical purposes, stops. At 144 MHz and 222 MHz the signal goes slightly beyond this point. At 50 MHz, the signal definitely goes beyond this point. The Earth intercept method requires considerably more plotting points and relies on the fact that, on a 4/3rds Earth profile, when the signal comes in contact with the surface of the Earth, it stops. The higher the frequency the more accurate the predictions. The Earth intercept method produces an estimated coverage map that, depending on the frequency of the source, can be between 95% to 99% accurate. Unfortunately, most of those are really only an approximation. There are a number of computer programs to predict the coverage of base stations, repeaters, etc.
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