AM radio is radio broadcasting using amplitude modulation. This was the dominant system of radio in the first two thirds of the 20th century, and it remains important into the 21st.
Because of the inferior sound quality of AM broadcasting, the medium lends itself particularly to talk radio, news radio, and public radio, while music radio has in recent decades tended to be broadcast using FM.
AM radio technology is simpler than other types of radio, such as FM radio and DAB. An AM receiver detects the power of the radio wave and amplifies changes in the power measurement to drive a speaker or earphones. The earliest crystal radio receivers used this principle.
AM radio was used for small-scale voice and music broadcasts before World War I. The great increase in the use of AM radio came the following decade. The first commercial radio services began on AM in the 1920s. Radio programming boomed during the "Golden Age of Radio" (1920s to 1950s). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.
AM radio is broadcast in on several frequency bands:
(In the US, the allocation of these bands is managed by the FCC.)
- Medium wave is by far the most used for commercial radio broadcasting; this is the "AM radio" that most people are familiar with.
- Long wave is used in Europe, Africa, Asia, and Australasia (ITU regions 1 and 3). In the Americas, however, this band is reserved for aeronautical navigation. Due to the propagation of long wave signals, the frequencies are used most effectively in latitudes north of 50°.
- Short wave is used by radio services intended to be heard at great distances away from the transmitting station; the far range of short wave broadcasts comes at the expense of lower audio fidelity. The mode of propagation for short wave is different, see High frequency. AM is used mostly for broadcast uses – other shortwave users may use a modified version of AM such as SSB or an AM_compatible version of SSB such as SSB with carrier reinserted.
Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, amateur radio, etc.
Medium wave and short wave radio signals act in different ways during daytime and nighttime. During the day, AM signals travel by groundwave, refracting around the curve of the earth over a distance of rarely more than 162 kilometres (100 miles) from the signal's transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling many AM radio stations to be heard much farther from their point of origin than is normal during the day. This phenomenon can be easily observed by scanning an AM radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly after sunset, or even to suspend broadcasting entirely during nighttime hours. The hobby of listening to long distance signals is known as DX or DX'ing, from an old telegraph abbreviation for "distant". Several non-profit hobbyist clubs are devoted exclusively to DXing the AM broadcast band, including the National Radio Club and International Radio Club of America.
AM radio signals can also be easily disrupted in large urban centres by skyscrapers and other sources of radio frequency (RF) interference.
FM signals, however, are not affected by these types of interference. As a result, AM radio has lost its dominance as a broadcasting format, and in many cities is now restricted to news, sports and talk radio stations.
Stereo transmissions are possible, AM-Stereo.
- FM radio, History of radio
- "Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country (http://www.salestores1.com/woreltab.html)".