AM broadcasting

AM broadcasting is the process of radio broadcasting using Amplitude Modulation.

History

AM was the dominant method of broadcasting during the first four-fifths of the 20th century and remains widely used into the 21st.

AM radio began with the first, experimental broadcast in 1906 by Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. The great increase in the use of AM radio came the following decade. The first licensed commercial radio services began on AM in the 1920s. XWA of Montreal, Quebec (later CFCF) was 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.

Operation

AM radio technology is simpler than FM radio and DAB. 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.

Frequency Bands

AM radio is broadcast on several frequency bands:

Long wave is 153 kHz–279 kHz; it is not available far into the Western Hemisphere, and European 9 kHz channel spacing is generally used (historically frequencies as high as 413 kHz were used but currently there are no terrestrial LW broadcasters above 279 kHz). Long wave is used for radio broadcasting in Europe, Africa, Oceania and parts of Asia (ITU regions 1 and 3). In the United States and Canada, Bermuda and U.S. territories this band is mainly reserved for aeronautics, though a small section of the band could theoretically be used for microbroadcasting under the United States Part 15 rules. Due to the propagation characteristics of long wave signals, the frequencies are used most effectively in latitudes north of 50°.
Medium wave is 520 kHz–1,610 kHz. In the Americas (ITU region 2) 10 kHz spacing is used; elsewhere it is 9 kHz. ITU region 2 also authorizes the Extended AM broadcast band between 1610 kHz and 1710 kHz. Medium wave is by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with.
Short wave is 2.3 MHz–26.1 MHz, divided into 15 broadcast bands. Shortwave broadcasts generally use a narrow 5 kHz channel spacing. Short wave is used by audio services intended to be heard at great distances from the transmitting station. The long 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 by broadcast services – 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. In many parts of the world short wave radio also carries audible, encoded messages of unknown purpose from numbers stations.

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, such as ship and aircraft mobile stations, point-to-point communications, baby monitors, walkie talkies, cordless telephones, radio control, "ham" radio, etc.; these services may use other forms of modulation such as continuous wave or sideband, and are not broadcast services intended for reception by the general public.

Limitations of AM radio

Because of its susceptibility to atmospheric and electrical interference and the generally lower sound fidelity of superheterodyne receivers, AM broadcasting has attracted mostly talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1960s and 1970s. Previously, 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.^{[citation needed]}

The limitation on AM fidelity comes from current receiver design. Most modern receivers are built with an early 20th century design which enhanced ease of tuning at the expense of not passing the full spectrum of audio contained in AM radio broadcasts. Moreover, to fit more transmitters on the AM broadcast band, in the United States maximum transmitted audio bandwidth is limited to 10 kHz by an NRSC standard adopted by the FCC. The audio bandwidth of most currently manufactured radios can be half that.

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.)

Some other radio stations are granted clear channel rights, meaning that they broadcast on frequencies whose use is more restricted and thus relatively unaffected by interference from other stations. Nowadays relatively few stations enjoy clear channel status.

The hobby of listening to long distance signals is known as DX or DX'ing, from an old telegraph abbreviation for "distance unknown". 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 signals can be severely disrupted in large urban centres by concrete bridges with metal reinforcements, other Faraday cage structures, tall buildings and sources of radio frequency interference (RFI) and electrical noise, such as electrical motors, fluorescent lights, traffic signals, 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 although 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.

Other distribution methods

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 broadcasting audio of significantly greater fidelity that that of standard AM, 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.

Amateur AM & microbroadcasting

Increasingly, amateur radio operators are giving a second life to retired AM broadcast transmitters, donated or sold cheaply to hobbyists by stations with no further need. The attraction is part of a vintage amateur radio specialty in the hobby, and it resembles old car collecting. There are often large gatherings on the shortwave amateur bands, where the transmitters are used for 2-way contacts among aficionados. Nighttime propagation is especially reminiscent of the early days of radio, as individual stations warm up their transmitters for hours of storytelling and technical discussion with other "ham" enthusiasts far and near. These amateur stations often place an emphasis on high-fidelity reception as well as striving toward good transmitted audio. Some amateurs listen on the big speaker of an antique floor console with shortwave bands such as those from Philco, GE, Crosley and Zenith. Occasionally, an amateur using AM gets reception reports from an "SWL," a shortwave listener who has picked up their signals on an antique radio, or perhaps with a modern "world-band" receiver.^[1]

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.

Notes

^ Courson, Paul (2001-06-20). "Chrome and Glass Shine Again: Hams Give Second Life to Legendary Transmitters With Names Like RCA, Collins, Gates and Raytheon". Radio World. Retrieved 2007-04-06.

External links

"Building the Broadcast Band" the development of the 520-1700 kHz MW (AM) band

[radioworldnewspaper-1] Courson, Paul (2001-06-20). "Chrome and Glass Shine Again: Hams Give Second Life to Legendary Transmitters With Names Like RCA, Collins, Gates and Raytheon". Radio World. Retrieved 2007-04-06.

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