schmave said:
Wow ... only 65 miles? WLW goes at least 100-110 before it starts to suffer from cancellation ... same for, in my experience, WBAP and WJR among others. Some of the Chicago signals make it farther than that into Indiana before cancellation becomes an issue.
I thought we had covered ground wave and Atlanta earlier in this discussion but we have not.
Science 101. The coverage you get from a radio station during the daytime is GROUND WAVE. It goes out 20, 50, 70, 100 miles depending on the transmitter power, the frequency, and GROUND CONDUCTIVITY.
At night there is the phenomena called sky wave. See my comments above about ionosphere, etc.
Technically the GROUNDWAVE goes just as far out at night as it does in the daytime, but at the edges of groundwave coverage, it gets covered up by skywave coming from every direction from multiple stations. If your small town has a 1,000 watt station on 1230, 1240, 1340 1400, 1450 or 1490, it also (normally) has 1,000 watts at night. The night time coverage is typically about 1/3 the miles from the transmitter as is the daytime coverage.
In different parts of the world, the soil is more conductive or less conductive of this ground wave. If you are in the wheat belt on the corn belt you probably have excellent ground conductivity and stations go forever and ever it seems.
NOW, let's talk WSB in Atlanta. Starting somewhere in Atlanta or maybe a bit further south, and in a belt running north-northeast there is an area of LOUSY, TERRIBLE GROUND CONDUCTIVITY. AM stations in Atlanta and all the small communities going up along I-85 get much, much less coverage area than a similar station in say Kansas, Texas, Illinois, etc. The FCC maps that show the approximate ground conductivity indicate for this area in Georgia a value of 2. My home county in Arkansas has a value of 30.
If I remember correctly, Art Sutton who owns some stations in this area of reduced ground conductivity posted recently that he had an engineering company do some testing and found the value in Northeast Georgia was ACTUALLY a 1 rather than the 2 shown on the maps. Andy why would someone have this measurement done? If you are applying for a new AM frequency, a power increase, or a station move, that changes the interference boundaries. AM goes by actual measurements. FM goes by arbitrary mileage circles.