PowerPedia:AM radio

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AM broadcasting is radio broadcasting using Amplitude Modulation. Amateurs and researchers have used a method to transcieve this range and utilize the longwave signals and ultra shortwave, which is akin to Tesla's wireless power system.

Operation

AM radio technology is simpler than either FM radio or 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 153–279 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). The Longwave radio broadcasting band is the range of frequencies between 148.5 - 283.5 kHz, which correspond to wavelengths between 1000 and 2000 meters. This range is included within the low frequency band (but the low frequency band extends above and below longwave signals). Longwave signals have the property of following the curvature of the earth, making them ideal for continuous, continental communications. Unlike shortwave radio, longwave signals do not reflect or refract using the ionosphere, so there are fewer interference-caused fadeouts. Instead, the D-layer of the ionosphere and the surface of the earth serve as a waveguide directing the signal.

The earliest radio transmitters were all longwave transmitters, because propagation of radio waves of higher frequency was not yet understood. Radio alternator or spark-gap transmitters were commonly used to generate the radio frequency carrier wave. In Europe, North Africa, and Russia, (ITU Regions 1 and 3) longwave radio frequencies in the band 148.5 to 283.5 kHz are used for domestic and international broadcasting.

Carrier frequencies are multiples of 9 kHz ranging from 153 to 279 kHz. There are two exceptions in Germany, where two stations are offset by 3 kHz either side of 180 kHz. Historically LW stations operated on frequencies as high as 413 KHz (although the highest carrier frequency currently in use for LW broadcasting is 279 KHz) several countries transmitted radio programming over power lines or telephone lines using LW frequencies. These systems were known variously as Linjesender, Telefonrundspruch or Wire Broadcasting. Some stations derive the value of their carrier from an atomic clock. It can be therefore used as frequency standard.

In North America during the 1970s the longwave frequencies 167, 179 and 191 KHz were used for a short lived network of Civil defence emergency broadcasting stations with stations at Ault, Colorado and Cambridge, Kansas. Nowadays the 160-190 kHz range is used there for Part 15 Lowfers. Frequencies from 190 to 435 kHz and 510 kHz to 530 kHz are now used for navigational beacons, and 500 kHz is the maritime distress (emergency) frequency. In the Americas, frequencies between 190 and 435 kHz and between 510 and 530 kHz are used for navigational beacons called "Non-directional beacons" (NDB's).

Medium wave is 520–1,710 kHz. In the Americas (ITU region 2) 10 kHz spacing is used; elsewhere it is 9 kHz. Short wave is 2,300–26,100 kHz, divided into 15 broadcast bands. Shortwave broadcasts generally use a narrow 5 kHz channel spacing. 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.

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 by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with. Mediumwave (MW) radio, also called Medium frequency (MF), is a common band for broadcasting. The standard AM broadcast band is 530 kHz to 1710 kHz in North America, but remains only up to 1610 kHz elsewhere. Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km (248.2 Mi) away in New York City, due to groundwave propagation. The effectiveness of groundwave signals largely depends on ground conductivity—higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably.

In most of the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of ±5 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of ±4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands. In the US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts. Many North American stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range.

In Europe, each country is allocated a number of frequencies on which high power (up to 2.5 MW) can be used; the maximum power is also subject to international agreement. In most cases there are two power limits: a lower one for omnidirectional and a higher one with directional radiation with minimums showing toward certain directions. The power limit can also be depending on daytime and it is possible, that a station may not work at nighttime, because it would produce than too much interference. Other countries may only operate low-powered transmitters on the same frequency, again subject to agreement. For example, Russia operates a high-powered transmitter, located in its Kaliningrad exclave and used for external broadcasting, on 1386 kHz. The same frequency is also used by low-powered local radio stations in England; other parts of England can still receive the Russian broadcast. International mediumwave broadcasting in Europe has decreased markedly with the end of the Cold War and the increased availability of satellite and Internet TV and radio, although the cross-border reception of neighboring countries' broadcasts by expatriates and other interested listeners still takes place.

Due to the high demand for frequencies in Europe, many countries operate single frequency networks; in Britain, BBC Radio Five Live broadcasts from various transmitters on either 693 or 909 kHz. These transmitters are carefully synchronized to minimize interference from more distant transmitters on the same frequency.

Stereo transmission is possible and offered by some stations in the U.S., Canada, Australia, South Africa, and France. However, there are multiple standards for AM stereo with C-QUAM being the legally-permitted one in the United States, and receivers that implement the technologies are relatively rare but not uncommon. Another well-known system, which is no longer used, is Kahn Powerside.

In September 2002, the United States Federal Communications Commission approved the proprietary iBiquity in-band on-channel (IBOC) HD Radio system of digital audio broadcasting, which is meant to improve the audio quality of signals. The Digital Radio Mondiale (DRM) IBOC system has been approved by the ITU for use outside North America and U.S. territories.

As aerials mostly mast radiators are used. Stations broadcasting with low power commonly use masts with heights of a quarter wavelength, while high power stations mostly use half wavelength. The usage of masts longer than 5/8 of radiated wavelength gives a bad radiation pattern. Usually mast antennas are insulated against ground and show a high voltage against ground during transmission, which complicates maintenance, installation of air safety warning lights or using the mast as a tower for UHF/VHF-radio, but there are several ways to use grounded masts or towers.

If grounded masts or towers are required, then cage aerials or longwire aerials are used. Another possibility consists of feeding the mast or the tower by cables running from the tuning unit to the guys or crossbars in a certain height. Directional aerials consist of multiple masts, which need not to be from the same height. It is also possible to realize directional aerials for mediumwave with cage aerials where some parts of the cage are fed with a certain phase difference. Other type of aerials sometimes used for mediumwave are T- and L-aerials. The kind used depends on the need for grounded or insulated towers.

In some cases dipole aerials are used, which are spun between two masts or towers. Such aerials radiate toward the sky. The mediumwave transmitter at Berlin-Britz for transmitting RIAS used a cross dipole mounted on five 30.5 meter high guyed masts to transmit the skywave up to the ionosphere at nighttime. Europe's largest antenna park DX 183 (http://mediumwave.info/reports.html) is placed in Northern Jutland, Denmark. The well-known German DX'er Wilhelm Herbst has constructed and built the antennas. DXers are welcome to use the facilities.

For most of the 20th century, the radio frequency 500 kHz was reserved world wide as the Morse code international calling and distress frequency for ships on the high seas. The frequency 2182 kHz is still used for this purpose, but employing voice transmission. Other services that operate in medium wave include Navtex and the amateur radio 160-meter band. The obsolete LORAN-A system used medium wave.

Short wave is used by radio 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.

Short wave is used by radio 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. Shortwave radio operates between the frequencies of 2,310 kHz and 30 MHz (30,000 kHz) <ref> Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).</ref> and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at that time. An alternate name is HF or high frequency radio. Short wavelengths are associated with high frequencies because there is an inverse relationship between frequency and wavelength.

Shortwave frequencies are capable of reaching the other side of the planet because they can be refracted by the ionosphere (a phenomenon known as Skywave propagation). The selection of a frequency to use to reach a target area depends on several factors; [1] The distance from the transmitter to the target receiver, and, [2] Time of day. During the day, higher shortwave frequencies (> 12 MHz) can travel longer distances than lower ones; at night, this property is reversed. The dependence to the time of the day is due to a particular transient atmosphere ionized layer forming only during day when atoms are broken up into ions by sun photons. This layer is responsible for partial or total absorption of particular frequences.

Seasons during the winter months the AM broadcast band tends to be more favorable because of longer hours of darkness. Solar conditions including the number of sunspots, solar flares, and overall solar activity. Solar flares can prevent the ionosphere from reflecting or refracting radio waves. Type of modulation. Independent from the frequency, the receiver must be capable to receive the same modulation type of the transmitter. USB, LSB, AM, CW are all modes of modulation.

Types of modulation frequently used in the shortwave frequency range are:

• AM: amplitude modulation. Usually used for shortwave broadcasting, and some aeronautical communications.
• NFM: Narrow-band frequency modulation. Because of the bandwidth required this is normally used for VHF communication, but some NFM transmissions occur in the higher HF frequencies.
• SSB: Single sideband(USB/LSB): This is used for long-range communications by ships and aircraft, for voice transmissions by amateur radio operators, and for broadcasting. LSB is generally used below 10MHz and USB above 10MHz.
• CW: Continuous/Carrier wave, which is used for Morse code communications.
• DRM: Digital Radio Mondiale: digital modulation for use on bands below 30 MHz.
• Various radioteletype, fax, digital, or other systems, which require software or special equipment to decode.

Some major users of the shortwave radio band include

• Domestic broadcasting in countries with a widely dispersed population with few longwave, mediumwave, or FM stations serving them
• International broadcasting to foreign audiences (which explains why shortwave is also known as "world band radio")
• Speciality political, religious, and conspiracy theory radio networks, individual commercial and non-commercial paid broadcasts for the north American and other markets.
• Utility stations transmitting messages not intended for a general public, such as aircraft flying between continents, encoded or ciphered diplomatic messages, weather reporting, or ships at sea
• Numbers stations
• Amateur radio operators
• Time signal stations. WWV operates on these frequencies: 2500kHz, 5000kHz, 10000kHz, 15000kHz, and 20000kHz. CHU Canada operates on these frequencies: 3335kHz, 7335kHz, and 14670kHz.

The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave broadcast radio receivers in use in 2002. The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunication Union, allocates bands for various services in conferences every few years. The next WRC is scheduled to take place in 2007.

At WRC-97 in 1997, the following bands were allocated to international broadcasters (listed in the table):

Analog AM shortwave broadcasting channels are allocated with a 5 kHz separation. International broadcasters, however, may operate outside the normal WRC-allocated bands or use off-channel frequencies to attract attention in crowded bands. The new digital audio broadcasting format for shortwave DRM operates in 5khz, 10khz or 20 khz channels -- so there are some ongoing discussions with respect to specific band allocation for DRM.

The power used by shortwave transmitters ranges from less than one watt for some experimental transmissions to 500 kilowatts and higher for intercontinental broadcasters. Shortwave transmitting centers often use specialized antenna designs to concentrate radio energy on a bearing aimed at the target area.

Shortwave propagation can currently be modeled by Ioncap (for point to point calculations) and VOACAP (for area coverage calculations). Ioncap is propagation prediction software, available for free from the U.S. Department of Commerce (NTIA/ITS) Institute for Telecommunication Sciences, originally developed for Voice of America (VOA). VOACAP is an improved version of IONCAP. VOACAP retains all of the theory as put forth by John Lloyd, George Haydon, Donald Lucas and Larry Teters in the 1975–1985 time-frame. Major improvements in the IONCAP program were made by Franklin Rhoads of the U.S. Navy Research Laboratory under the sponsorship of the Voice of America (1985–1996).

International broadcasting contains details on the history and practice of broadcasting to foreign audiences. In the U.S. and Canada, no license is required to own or operate a shortwave receiver. The privilege of operating a shortwave radio transmitter, for non-commercial two way communications known as amateur radio, is granted through a licensing process by the authorized ITU government body. In the USA, this licensing agency is the Federal Communications Commission (FCC).

In Canada, this licensing agency is Industry Canada. In February 1990, the FCC removed the morse code proficiency requirement for the Technician class amateur radio license, which allows one to transmit on frequencies above 30 MHz. Technician licensees who passed a 5 word per minute morse code test were allowed limited HF privileges, similar to those of Novice licensees. This has made it easier for beginners to get involved in the hobby; however, a working knowledge of Morse code is required to operate on shortwave bands. Amateur radio operators have made numerous technical advancements in the field of radio and make themselves available to transmit emergency communications when normal communications channels fail. Some amateurs practice operating off the power grid so as to be prepared for power loss. It should be noted that many amateur radio operators started out as Shortwave Listeners (SWLs) and actively encourage SWLs to become amateur radio operators.

The 2003 World Radiocommunication Conference (WRC) removed the global requirement for Morse code proficiency needed to access most shortwave frequencies for the amateur radio service, but left the decision to each administrative body (e.g. Federal Communications Commission in the United States; Industry Canada in Canada). 20 countries (largely Western Europe, Canada & Australia) have phased out this requirement from their licenses and giving access to operators who previously couldn't operate in HF. On the other hand, this trend is not global. Over 200 countries (e.g. Russia, Eastern Europe, Middle East, Africa, South America & Asia) have decided to keep the Morse Code requirement for the foreseeable future. In July 2005, the Federal Communications Commission recommended the removal of the Morse Code requirement for amateur radio licenses the United States, as part of a Notice of Proposed Rulemaking in WT Docket 05-235. This docket was released after 18 proposals, including one from the ARRL for widespread changes in the Amateur Radio Service rules were received and considered by the FCC. These proposals had attracted 6200 comments from the Amateur Radio Service community. On December 19, 2006 the FCC released its Report and Order on December 19, 2006, eliminating the morse code requirement for amateur radio licensing in the United States. The rule changes will not be effective until 30 days after being published in the Federal Register.

Many hobbyists listen to shortwave broadcasters without operating transmitters. In some cases, the goal is to hear as many stations from as many countries as possible (DXing); others listen to specialized shortwave utility, or "ute", transmissions such as maritime, naval, aviation, or military signals. Others focus on intelligence signals. Many though tune the shortwave bands for the programmes of stations broadcasting to a general audience (such as the Voice of America, BBC World Service, Radio Australia, etc.). Some even listen to two way communications by amateur radio operators. Nowadays, as the Internet evolves, the hobbyist can listen to shortwave signals via remotely controlled shortwave receivers around the world, even without owning a shortwave radio. (See for example http://www.dxtuners.com) Alternatively, many international broadcasters (such as the BBC) offer live streaming audio on their websites. Shortwave listeners, or SWLs, can obtain QSL cards from broadcasters, utility stations or amateur radio operators as trophies of the hobby. Some stations even give out special certificates, pennants, stickers and other tokens and promotional materials to shortwave listeners.

Numbers stations are shortwave radio stations of uncertain origin that broadcast streams of numbers, words, or phonetic sounds. Although officially there is no indication of their origin, radio hobbyists have determined that many of them are used by intelligence services as one-way communication to agents in other countries. From 1976 to 1989, the Russian Woodpecker blotted out countless shortwave broadcasts daily; at first it was thought to be a secret submarine communication system, but it was quickly found to be an early-warning over the horizon radar system.

The development of direct broadcasts from satellites has reduced the demand for shortwave receivers, but there are still a great number of shortwave broadcasters. A new digital radio technology, Digital Radio Mondiale, is expected to improve the quality of shortwave audio from very poor to standards comparable to the FM broadcast band. The future of shortwave radio is threatened by the uprise of power line communication (PLC), also known as Broadband over Power Lines (BPL), where a data stream is transmitted over unshielded power lines. As the frequencies used overlap with shortwave bands, severe distortions make listening to shortwave radio near power lines difficult or impossible.

Some musicians have been attracted to the unique aural qualities of shortwave radio, employing shortwave radios as live instruments in a number of pieces, and or using sampled broadcasts, used tape loops of broadcasts, or drawn inspiration from the unusual sounds on some frequencies. John Cage used radios live on several occasions, while Karlheinz Stockhausen used shortwave radio in works including Telemusik (1966), Hymnen (1966-67) and Spiral (1968). Holger Czukay, a Stockhauzen student, was one of the first to use shortwave in a rock music context. Among others, John Duncan, Orchestral Manoeuvres in the Dark (on their Dazzle Ships album), Pat Metheny, Aphex Twin, Boards of Canada, John Duncan, Rush, Able Tasmans, Team Sleep, Meat Beat Manifesto, Daybrokenroses, Jonny Greenwood of Radiohead, and Wilco have also used or been inspired by shortwave broadcasts.

Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, "ham" radio, etc.

Limitations of AM radio

Because of its susceptibility to atmospheric interference and generally lower-fidelity sound, AM broadcasting is better suited to talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1960s and 1970s. Perversely then, in the UK during the 1980's, 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–7 kHz with a 50 dB S/N ratio.

Part of the limitation on AM fidelity, however, comes from current receiver design. Most modern receivers do not pass the full spectrum of audio that AM radio is capable of transmitting, while older receivers designed at the peak of AM's popularity may sound quite good by comparison.

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. 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 there is work underway to add digital radio services to currently existing AM transmissions. In the United States, the iBiquity company is developing a proprietary standard 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. While FM radio can also be received by cable, AM radio generally cannot be, 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.

History

AM was the dominant method of broadcasting in the history of radio during the first two thirds of the 20th century and remains widely used into the 21st. The Central Intelligence Agency World Factbook lists approximately 16,265 AM stations worldwide.

AM radio of shows 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.

Related

• Amplitude Modulation Signalling System, a digital system for adding low bitrate information to an AM broadcast signal.
• MW DXing, the hobby of receiving distant AM radio stations on the mediumwave band.
• FM broadcasting
• Extended AM broadcast band
• CAM-D, a proposed hybrid digital radio format for AM broadcasting
• Lists of radio stations in North and Central America
• List of radio stations
• Oldest radio station

External articles and references

• "Building the Broadcast Band" the development of the 520-1700 kHz MW (AM) band. <http://www.oldradio.com>
• Listen to live AM radio transmissions.
• Wikipedia contributors, Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

See also