FM VS AM Technology Help

FM is Frequency Modulation, AM is Amplitude Modulation. AM signals generally reach 150 to 200 miles during the day. Thus, AM can be used for international broadcasting, providing the target distance does not exceed the power of the transmitter. At night, an AM signal can travel 1,000 miles or more. The difference is the sun. During daytime hours, the sun creates a sponge-like atmospheric layer that absorbs AM waves, dramatically reducing their reach.Another significant factor in the distance limitation of AM stations in North America that is not encountered in other areas is the regulation on transmitter size. AM stations within the U.S. are limited to 50,000 watts of power. The answer for North American stations may then appear to be to raise the power limitation. However, with increased wattage comes the corresponding increase in cost per broadcast minute. In Europe, Asia and the Middle East, where these costs are less of an issue, AM transmitters of 1,000,000 watts or more are commonplace. A note about the path of AM waves as compared with FM or shortwave signals:AM waves follow the curvature of the earth, but do die out as they move further from the source. This makes AM broadcasting ideal for places where the signal is only required to cover a city or metropolitan area. A common limitation, however, is signal fade or interference. Frequency Modulation or FM radio signals do not follow the curvature of the earth, but travel in a straight line from the source. Thus, the best position for the antenna is as high as possible over the target area. An FM signal from a 200-foot tower is likely to cover only a 50-mile radius. Therefore, the chief limitation of FM is distance. FM is almost never used for international broadcasting unless the transmitter sits right on the border. As any listener can attest, the advantage of FM is definitely clarity over either AM or shortwave. And, since the atmosphere does not absorb FM waves, the signal is just as clear night or day.

Shortwave radio signals tend to follow the curvature of the earth bouncing between the atmosphere and the earth. Absorption over water is practically zero; over land is minimal, but over sand is substantial. This is a key factor in the placement of the transmitter relative to the target audience, since shortwave is the longest traveling type of radio broadcasting. Shortwave equipment is not as costly as FM. It covers large geographic areas and requires less power than either FM or AM transmitters. Thus, it is the best bargain for the broadcasting dollar.The chief limitation of shortwave is its susceptibility to uncontrollable interference. However, when shortwave broadcasts are digitized, they can be made every bit as clear as FM. HCJB World Radio is actively working with other international broadcasters to develop global standards and test digital shortwave technology to bring the best possible broadcast quality to listeners in the not too distant future.


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