AM was the dominant method of broadcasting during the first eighty years of the 20th century and remains widely used into the 21st.
AM radio began with the first, experimental broadcast on Christmas Eve of 1906 by Canadian experimenter Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. San Francisco, California radio station KCBS claims to be the direct descendant of KQW, founded by radio experimenter Charles "Doc" Herrold, who made regular weekly broadcasts in San Jose, California as early as June of 1909. On that basis KCBS has claimed to be the world's oldest broadcast station and celebrated its 100th anniversary in the summer of 2009. The great increase in the use of AM radio came late in the following decade as radio experimentation increased worldwide following World War I. The first licensed commercial radio services began on AM in the 1920s. XWA of Montreal, Quebec (later CFCF, now CINW) claims status as the first commercial broadcaster in the world, with regular broadcasts commencing on May 20, 1920. The first licensed American radio station was started by Frank Conrad, KDKA in Pittsburgh, Pennsylvania. Radio programming boomed during the "Golden Age of Radio" (1920s–1950s). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.
AM radio technology is simpler than FM radio, DAB, Satellite Radio and HD Radio. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification.
In North American broadcasting practice, transmitter power input to the antenna for commercial AM stations ranges from about 250 watts to 50,000 watts. Experimental licenses were issued for up to 500,000 watts radiated power, for stations intended for wide-area communication during disasters including Cincinnati station WLW, which used such power on occasion before World War II. WLW's superpower transmitter still exists at the station's suburban transmitter site, but it was decommissioned in the early 1940s and no current commercial broadcaster in the US or Canada is authorized for such power levels. Some other countries do authorize higher power operation (for example the Mexican station XERF formerly operated at 250,000 watts). Antenna design must consider the coverage desired and must direct the transmitted signal so as not to interfere with other stations operating on the same or adjacent frequencies.
Medium-wave and short-wave radio signals act differently during daytime and nighttime. During the day, AM signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred miles (or kilometers) from the signal transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling 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 (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. Such stations are commonly referred to as daytimers.
In the United States and Canada, some AM radio stations are granted clear channel status, meaning that they broadcast on frequencies with few other stations allocated, allowing an extended coverage area. Relatively few stations enjoy clear-channel status. Commercial broadcasters generally rely on the ground-wave coverage only as their target market for advertising.
The hobby of listening to long distance signals is known as DX or DX'ing, from an old telegraph abbreviation for "distance". 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. Similarly, people listening to short wave transmissions are SWLing.
AM radio is broadcast on several frequency bands. The allocation of these bands is governed by the ITU's Radio Regulations and, on the national level, by each country's telecommunications administration (the FCC in the U.S., for example) subject to international agreements.
Frequencies between the broadcast bands are used for other forms of radio communication, and are not broadcast services intended for reception by the general public.
Because of its susceptibility to atmospheric and electrical interference, AM broadcasting now attracts mainly talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1970s. However, in the late 1960s and 1970s, top 40 rock and roll stations in the US and Canada such as WABC and CHUM transmitted highly processed and extended audio to 11 kHz, successfully attracting huge audiences. In the UK during the 1980s, BBC Radio 4 (a largely speech channel) had an FM location, whereas BBC Radio 1, a music channel, was confined to AM broadcasts over much of the UK. Frequency response is typically 40 Hz–5 kHz with a 50 dB S/N ratio.
The limitation on AM fidelity comes from current receiver design. Moreover, to fit more transmitters on the AM broadcast band, in the United States maximum transmitted audio bandwidth is limited to 10.2 kHz by an NRSC standard adopted by the FCC in June of 1989, resulting in a channel occupied bandwidth of 20.4 kHz. The former audio limitation was 15 kHz resulting in a channel occupied bandwidth of 30 kHz.
AM radio signals can be severely disrupted in large urban centres by metal structures, tall buildings and sources of radio frequency interference (RFI) and electrical noise, such as electrical motors, fluorescent lights, or lightning. As a result, AM radio in many countries has lost its dominance as a music broadcasting service, and in many cities is now relegated to news, sports, religious and talk radio stations. Some musical genres – particularly country, oldies, nostalgia and ethnic/world music – survive on AM, especially in areas where FM frequencies are in short supply or in thinly populated or mountainous areas where FM coverage is poor.
Stereo transmissions are possible (see AM stereo), and hybrid digital broadcast systems are now being used around the world. In the United States, iBiquity's proprietary HD Radio has been adopted and approved by the FCC for medium wave transmissions, while Digital Radio Mondiale is a more open effort often used on the shortwave bands, and can be used alongside many AM broadcasts. Both of these standards are capable of broadcasting audio of significantly greater fidelity than that of standard AM with current bandwidth limitations, and a theoretical frequency response of 0-16 kHz, in addition to stereo sound and text data.
While FM radio can also be received by cable, AM radio generally cannot, although an AM station can be converted into an FM cable signal. In Canada, cable operators that offer FM cable services are required by the CRTC to distribute all locally available AM stations in this manner. In Switzerland a system known as "wire broadcasting" transmits AM signals over telephone lines in the longwave band.
Some microbroadcasters and pirate radio broadcasters, especially those in the United States under the FCC's Part 15 rules, broadcast on AM to achieve greater range than is possible on the FM band. On mediumwave (AM), such radio stations are often found between 1610 kHz and 1710 kHz. Hobbyists also use low-power AM transmitters to provide local programming for antique radio equipment in areas where AM programming is not widely available or is of questionable quality; in such cases the transmitter, which is designed to cover only the immediate property and perhaps nearby areas, is hooked up to a computer or music player.