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:

• Local Coverage: Within this area, you should be able to receive the radio station on almost any radio with moderately good to very good reception.
• Distant Coverage: Within this area, the signal of the radio station may be weak unless you have a good car radio or a good stereo with a good antenna. You may not be able to receive the station at all on walkmans or other portable radios.
• Fringe Coverage: Within this area, the station's signal will be very weak. You may be able to receive this station if you have a very good radio with a good antenna, but it's possible that interference from other stations may prevent you from picking up these stations at all.

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.

• AM Antenna Patterns: AM radio stations with one tower transmit their signal with equal strength in all directions. These stations have a circular, or "non-directional" antenna pattern. AM stations that have more than one tower can precisely tune their antenna patterns to a very specific shape. The number of towers, the spacing and orientation of each tower, and the phase and ratio of the signal that is sent to each tower all combine to create an antenna pattern that might look heart-shaped (cardioid), peanut-shaped, or shaped like a 3, 4, or multi-leaf clover.
• Ground Conductivity To a large degree, AM radio signals travel through the ground. This means that the electrical conductivity of the ground greatly affects how far the radio signal will travel. The geology of the United States varies greatly from one area to another, and this affects the conductivity of the earth. Sea water, however, has a much higher conductivity than the ground, so AM radio signals will always travel much farther over sea water than over land.

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.