Reggae Spectrum

FM reception, which travels both on the ground and through the air, is prone to interference by a variety of sources: other radio stations, lightning storms, and nightfall, when an outlying FM station's signal can reflect off the ionosphere and skip over areas that receive the signal during the day.

1. These maps are cool, how are they 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 horizontal 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 ground wave propagation only).

With their external antennas, car radios tend to receive FM radio best, but sound and signal quality varies from car to car. Cars that employ their rear window defrosters as antennas may receive FM signals better than those with traditional external antennas; some people can find improvement by replacing their factory-installed antenna with an aftermarket antenna of higher quality. Reception may be disturbed as a car moves from one location to another, passing in and out of areas where the signal is obstructed by power lines or tall buildings.

Portable and desktop radios also often work well with FM broadcasts. Many contain internal, ferrite loop antennas; sometimes the power cord is also the antenna. These antennas are relatively directional, meaning that the quality of the received signal changes depending on where you put the radio. You may improve reception simply by moving your radio around until you are able to catch more of the signal. In some cases an external FM antenna may improve reception with a portable radio as it does with a component receiver.

When it comes to FM reception, not all component FM/AM receivers are created equal. Some have built-in antennas; some do not. Many high-end receivers, however, do have an external connection for an FM loop antenna which might have come with the receiver when you purchased it. If your stereo receives FM poorly or not at all, you will need to install this loop antenna-usually a rectangular piece of plastic with two wires that connect to two screws on your receiver-and orient it appropriately for the best reception. Better external loop antennas are also available. These are usually 8 to 12 inches in diameter and can be oriented and tuned just like you tune your radio to help eliminate nighttime interference and noise. Some must be hooked up directly to the external connections on your receiver; others need only be placed in close proximity to your receiver's existing FM antenna.

All radio reception is affected by distance, physical obstacles and other broadcast signals. But often the handicaps imposed on radio signals can be lessened with relatively simple enhancements.

First, a word of caution: Make sure that any antenna you purchase is returnable if it does not give you the result you desire. That said, there are many sites both locally and on the Internet that offer antennas or the instructions necessary for building your own.

Radio Shack and similar electronics and hardware stores often stock antennas for both FM and AM reception. A good FM loop antenna sells for $10 to $50 depending on features and looks. FM antennas and combined FM/AM antennas are often available as well, for prices that can range from $20 to $200 depending on the features you want. Often it is best to call around first to find someone knowledgeable about radio antennas and there are also many online resources.