1. How are these maps drawn?
These maps are drawn using engineering data from the FCC. The coverage pattern for each FM station is calculated using the effective radiated power (ERP) of the station and the antenna height above average terrain (HAAT). The HAAT is calculated in all directions based upon the average ground elevation between 1.5 and 10 miles from the station in each direction.
The coverage pattern for each AM station is based on the standard groundwave field strength pattern, the frequency of the station, and the ground conductivity of the local area.
2. What do the red, purple, and blue lines mean?
The red, purple, and blue lines correspond to the "local", "distant", and "fringe" coverage areas of each radio station:
3. What criteria do you use to define the "local", "distant" and "fringe" coverage areas?
The "local", "distant" and "fringe" lines on the FM maps correspond to the predicted 60, 50, and 40 dBu field strength contours respectively.
The "local", "distant" and "fringe" lines on the AM maps corresponds to the predicted 2.5, 0.5, and 0.15 mV/m contours respectively (of the horizontal groundwave propogation only).
4. Why do the AM patterns look so funny?
There are two factors that might give an AM station an odd looking coverage area: directional antenna patterns and ground conductivity.
5. Why do the FM patterns look so funny?
FM radio stations can also have directional antenna patterns, although FM patterns are usually not as severe as AM patterns. FM radio waves, however, are called "line of sight", which means that they do not travel well through solid objects, such as mountains or hills.
6. How accurate are these maps?
These maps are generated using the same data and most of the same algorithms that the FCC uses when trying to predict coverage of radio stations and interference with other nearby radio stations.
However, there are many factors that contribute to radio reception. One of the biggest factors is your radio; some radios will perform much better than others in trying to pick up distant radio stations. Other factors include interference from radio signals bouncing off nearby buildings (multipath interference), interference from other stations on nearby frequencies, or interference from nearby electrical equipment in your area.
Very mountainous terrain, or very non-typical geology can also affect radio signals. If you're on a mountain, you may be able to pick up radio signals much farther than indicated on our maps. Likewise, if you're in a valley, you may have trouble receiving many stations.
At night, AM signals can bounce off the ionosphere and travel great distances. This means that at night you may be able to pick up an AM station from hundreds of miles away. It also means that at night, AM stations that are hundreds of miles away might interfere with stations closer to your area.
We hope that you enjoy using these maps, but we also hope you understand that they are "predictions" based upon the information that we have available to us, and that your actual reception of these stations may vary considerably.