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77-482: Medium wave ( MW ) is a part of the medium frequency (MF) radio band used mainly for AM radio broadcasting . The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band . During the daytime, reception is usually limited to more local stations, though this is dependent on the signal conditions and quality of radio receiver used. Improved signal propagation at night allows

154-1011: A dipole reception pattern with sharp nulls along the axis of the rod, so that reception is at its best when the rod is at right angles to the transmitter, but fades to nothing when the rod points exactly at the transmitter. Other types of loop antennas and random wire antennas are also used. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm North American Regional Broadcasting Agreement The initial NARBA bandplan , also known as

231-405: A loading coil at their base. Receiving antennas do not have to be as efficient as transmitting antennas since in this band the signal-to-noise ratio is determined by atmospheric noise. The noise floor in the receiver is far below the noise in the signal, so antennas small in comparison to the wavelength, which are inefficient and produce low signal strength, can be used. The weak signal from

308-563: A skywave . The medium-wave transmitter at Berlin-Britz for transmitting RIAS used a cross dipole mounted on five 30.5-metre-high guyed masts to transmit the skywave to the ionosphere at nighttime. Because at these frequencies atmospheric noise is far above the receiver signal-to-noise ratio , inefficient antennas much smaller than a wavelength can be used for receiving. For reception at frequencies below 1.6 MHz, which includes long and medium waves, loop antennas are popular because of their ability to reject locally generated noise. By far

385-411: A "Radio Moving Day", but he refused on the grounds that "My experience has been that proclamations by the mayor mean just exactly nothing and I issue as few as I can.") The frequency changes affected "about a thousand stations in seven countries". The following chart reviews the assignments before and after March 29, 1941, including information about individual U.S. and Canadian stations, and summarizes

462-527: A band from 190 to 435 kHz, which overlaps from the LF into the bottom part of the MF band. 2182 kHz is the international calling and distress frequency for SSB maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band. 500 kHz was for many years the maritime distress and emergency frequency , and there are more NDBs between 510 and 530 kHz. Navtex , which

539-516: A clear channel were known as Class I-B. The Agreement assigned six Class I-A frequencies each to Mexico and Canada, and one to Cuba. Reflecting the existence of improved radio design, the Agreement also reduced the "same market" minimum frequency separation from 50 to 40 kHz. (Mexico elected to further adopt a 30 kHz "same market" spacing, unless this was in conflict with an adjoining nation's "border zone" allocations.) This closer spacing

616-746: A few specially licensed AM broadcasting stations. These channels are called clear channels , and they are required to broadcast at higher powers of 10 to 50 kW. Initially, broadcasting in the United States was restricted to two wavelengths: "entertainment" was broadcast at 360 meters (833 kHz), with stations required to switch to 485 meters (619 kHz) when broadcasting weather forecasts, crop price reports and other government reports. This arrangement had numerous practical difficulties. Early transmitters were technically crude and virtually impossible to set accurately on their intended frequency and if (as frequently happened) two (or more) stations in

693-440: A ground system consisting of many copper cables, buried shallowly in the earth, radiating from the base of the antenna to a distance of about a quarter wavelength. In areas of rocky or sandy soil where the ground conductivity is poor, above-ground counterpoises are sometimes used. Lower power transmitters often use electrically short quarter wave monopoles such as inverted-L or T antennas , which are brought into resonance with

770-518: A lower one for omnidirectional and a higher one for directional radiation with minima in certain directions. The power limit can also be depending on daytime and it is possible that a station may not operate at nighttime, because it would then produce too much interference. Other countries may only operate low-powered transmitters on the same frequency, again subject to agreement. International medium wave broadcasting in Europe has decreased markedly with

847-404: A poor vertical radiation pattern, and 195 electrical degrees (about 400 millivolts per meter using one kilowatt at one kilometre) is generally considered ideal in these cases. Mast antennas are usually series-excited (base driven); the feedline is attached to the mast at the base. The base of the antenna is at high electrical potential and must be supported on a ceramic insulator to isolate it from

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924-436: A quarter- wavelength (about 310 millivolts per meter using one kilowatt at one kilometre) to 5/8 wavelength (225 electrical degrees; about 440 millivolts per meter using one kilowatt at one kilometre), while high power stations mostly use half-wavelength to 5/9 wavelength. The usage of masts taller than 5/9 wavelength (200 electrical degrees; about 410 millivolts per meter using one kilowatt at one kilometre) with high power gives

1001-456: A station's existing vertical radiator towers, an important factor for readjusting directional antenna parameters to accommodate the new frequency. Individual stations were specified to be Class I, II III or IV, with the class determining the maximum power a station could use and its interference protection standards. In all of the participating countries Class I and II stations were exclusively assigned to Clear Channel frequencies, while Class III

1078-420: A vertical radiator wire. A popular choice for lower-powered stations is the umbrella antenna , which needs only one mast one-tenth wavelength or less in height. This antenna uses a single mast insulated from ground and fed at the lower end against ground. At the top of the mast, radial top-load wires are connected (usually about six) which slope downwards at an angle of 40–45 degrees as far as about one-third of

1155-479: Is AM broadcasting ; AM radio stations are allocated frequencies in the medium wave broadcast band from 526.5 kHz to 1606.5 kHz in Europe; in North America this extends from 525 kHz to 1705 kHz Some countries also allow broadcasting in the 120-meter band from 2300 to 2495 kHz; these frequencies are mostly used in tropical areas. Although these are medium frequencies, 120 meters

1232-479: Is adequate for talk and news but not for high-fidelity music. However, many stations use audio bandwidths up 10 kHz, which is not hi-fi but sufficient for casual listening. In the UK, until 2024 most stations used a bandwidth of 6.3 kHz. However in 2024, Ofcom expanded the allowed bandwidth to 9khz, giving a noticeable improvement in quality. With AM, it largely depends on the frequency filters of each receiver how

1309-432: Is available, (however digital radio still has coverage issues in many parts of Europe). Many countries in Europe have switched off or limited their MW transmitters since the 2010s. The term is a historic one, dating from the early 20th century, when the radio spectrum was divided on the basis of the wavelength of the waves into long wave (LW), medium wave, and short wave (SW) radio bands. For Europe, Africa and Asia

1386-698: Is generally treated as one of the shortwave bands . There are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz. These include, as examples, the French MRCC on 1696 kHz and 2677 kHz, Stornoway Coastguard on 1743 kHz, the US Coastguard on 2670 kHz and Madeira on 2843 kHz. RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz. Non-directional navigational radio beacons (NDBs) for maritime and aircraft navigation occupy

1463-513: Is part of the current Global Maritime Distress Safety System occupies 518 kHz and 490 kHz for important digital text broadcasts. Lastly, there are aeronautical and other mobile SSB bands from 2850 kHz to 3500 kHz, crossing the boundary from the MF band into the HF radio band. An amateur radio band known as 160 meters or 'top-band' is between 1800 and 2000 kHz (allocation depends on country and starts at 1810 kHz outside

1540-646: Is possible and is or was offered by some stations in the U.S., Canada, Mexico, the Dominican Republic, Paraguay, Australia, The Philippines, Japan, South Korea, South Africa, Italy and France. However, there have been multiple standards for AM stereo . C-QUAM is the official standard in the United States as well as other countries, but receivers that implement the technology are no longer readily available to consumers. Used receivers with AM Stereo can be found. Names such as "FM/AM Stereo" or "AM & FM Stereo" can be misleading and usually do not signify that

1617-432: Is primarily only used by low-power stations; it is the preferred range for services with automated traffic, weather, and tourist information. The channel steps of 9 and 10 kHz require limiting the audio bandwidth to 9 and 10 kHz (at maximum without causing interference; ±4.5 kHz (9 kHz) and ±5 kHz (10 kHz) on each two sidebands) because the audio spectrum is transmitted twice on each side band . This

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1694-448: Is the ITU designation for radio frequencies (RF) in the range of 300  kilohertz (kHz) to 3  megahertz (MHz). Part of this band is the medium wave  (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometers (1000 to 100 m). Frequencies immediately below MF are denoted as low frequency (LF), while

1771-516: Is the ITU-approved system for use outside North America and U.S. territories . Some HD Radio receivers also support C-QUAM AM stereo, although this feature is usually not advertised by the manufacturer. For broadcasting, mast radiators are the most common type of antenna used, consisting of a steel lattice guyed mast in which the mast structure itself is used as the antenna. Stations broadcasting with low power can use masts with heights of

1848-523: Is valuable in providing radio programming to sparsely settled areas using high-powered transmitters. However, it also leads to the need for international cooperation in station assignments, to avoid mutually interfering signals. In an effort to rationalize assignments, a major reallocation went into force in the U.S. on November 11, 1928, following the standards set by the Federal Radio Commission 's (FRC) General Order 40 . At that time,

1925-773: The Beverage antenna ) and the ferrite sleeve loop antenna. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Medium frequency Medium frequency ( MF )

2002-449: The FM broadcast band but require more energy and longer antennas. Digital modes are possible but have not reached momentum yet. MW was the main radio band for broadcasting from the beginnings in the 1920s into the 1950s until FM with a better sound quality took over. In Europe, digital radio is gaining popularity and offers AM stations the chance to switch over if no frequency in the FM band

2079-624: The Federal Communications Commission (FCC) to shut down, reduce power, or employ a directional antenna array at night in order to avoid interference with each other due to night-time only long-distance skywave propagation (sometimes loosely called ‘skip’). Those stations which shut down completely at night are often known as "daytimers". Similar regulations are in force for Canadian stations, administered by Industry Canada ; however, daytimers no longer exist in Canada,

2156-490: The ionosphere and return to Earth at much greater distances; this is called the skywave . At night, especially in winter months and at times of low solar activity, the lower ionospheric D layer virtually disappears. When this happens, MW radio waves can easily be received many hundreds or even thousands of miles away as the signal will be reflected by the higher F layer . This can allow very long-distance broadcasting, but can also interfere with distant local stations. Due to

2233-600: The last station having signed off in 2013, after migrating to the FM band . Many countries have switched off most of their MW transmitters in the 2010s due to cost-cutting and low usage of MW by the listeners. Among those are Germany, France, Russia, Poland, Sweden, the Benelux, Austria, Switzerland, Slovenia and most of the Balkans. Other countries that have no or few MW transmitters include Iceland, Ireland, Finland and Norway. Large networks of transmitters are remaining in

2310-697: The "Havana Treaty", was signed by the United States, Canada, Mexico, Cuba, the Dominican Republic, and Haiti on December 13, 1937, and took effect March 29, 1941. A series of modifications and adjustments followed, also under the NARBA name. NARBA's provisions were largely supplanted in 1983, with the adoption of the Regional Agreement for the Medium Frequency Broadcasting Service in Region 2 (Rio Agreement), which covered

2387-533: The AM band was defined as 96 frequencies, running in 10 kilocycle-per-second (kHz) steps from 550 to 1500 kHz, which were divided into what became known as "Local", "Regional", and "Clear Channel" frequencies. The only provision the FRC made addressing international concerns was that six frequencies — 690, 730, 840, 910, 960, and 1030 — were designated for exclusive Canadian use. On May 5, 1932, through an exchange of letters,

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2464-535: The Americas). Amateur operators transmit CW morse code , digital signals and SSB and AM voice signals on this band. Following World Radiocommunication Conference 2012 (WRC-2012), the amateur service received a new allocation between 472 and 479 kHz for narrow band modes and secondary service, after extensive propagation and compatibility studies made by the ARRL 600 meters Experiment Group and their partners around

2541-462: The Bahamas being granted use of the 1540 kHz clear channel by the U.S. The interim agreement expired on March 29, 1949, and there was great difficulty in agreeing on a replacement, in particular due to Mexican objections, which led to two failed conferences. A new NARBA agreement, to be effective for five years after ratification, was finally signed at Washington, D.C., on November 15, 1950, for

2618-411: The Bahamas, Canada, Cuba, the Dominican Republic, Jamaica and the United States. Mexico, which had withdrawn from the conference, and Haiti, which did not participate, were to be given a chance to subscribe. (The United States and Mexico made a bilateral agreement in 1957.) This agreement formally added 540 kHz as a clear channel frequency, and also provided for Cuba to share six, and Jamaica two, of

2695-615: The Bahamas, the Dominican Republic, and United States because those countries have not formally abrogated NARBA. The United States also has active bilateral agreements with Canada ("Agreement Between the Government of the United States of America and the Government of Canada Relating to the AM Broadcasting Service in the Medium Frequency Band" (1984) and Mexico ("Agreement Between the Government of

2772-531: The MW band consists of 120 channels with carrier frequencies from 531 to 1602 kHz spaced every 9 kHz. Frequency coordination avoids the use of adjacent channels in one area. The total allocated spectrum including the modulated audio ranges from 526.5 to 1606.5 kHz. Australia uses an expanded band up to 1701 kHz. North and South America use 118 channels from 530 to 1700 kHz using 10 kHz spaced channels. The range above 1610 kHz

2849-635: The MW band is thinning out, many local stations from the remaining countries as well as from North Africa and the Middle East can now be received all over Europe, but often only weak with much interference. In Europe, each country is allocated a number of frequencies on which high power (up to 2 MW) can be used; the maximum power is also subject to international agreement by the International Telecommunication Union (ITU). In most cases there are two power limits:

2926-469: The U.S. and Canada informally endorsed and expanded the 1928 standards, including recognition of Canadian use of 540 kHz. During the 1930s, Canada also began using 1510 kHz, while in 1934 the U.S. authorized two experimental high-fidelity stations on each of 1530 and 1550 kHz. By 1939, Cuban stations existed on frequencies as high as 1600 kHz. As other countries, especially Mexico and Cuba, developed their own radio broadcasting services,

3003-436: The U.S. clear channel allocations. Some provisions remained controversial, and this version of the treaty wasn't ratified by the United States until early 1960. In 1980, Cuba gave the required one year notification that it was withdrawing from the NARBA treaty. The NARBA treaties have been substantially superseded by the "Regional Agreement for the Medium Frequency Broadcasting Service in Region 2" (Rio Agreement), which covers

3080-478: The U.S., Canada, Cuba, the Dominican Republic and Mexico, was held from January 14–30, 1941 in Washington, D.C., in order to coordinate the upcoming changes. With a few exceptions the frequency shifts were scheduled to be implemented at 0800 Greenwich Mean Time (3 a.m. E.S.T. ) on March 29, 1941, which was informally known as "moving day". (Philadelphia stations petitioned mayor Robert Lamberton to declare

3157-560: The UK, Spain and Romania. In the Netherlands and Scandinavia, some new idealistically driven stations have launched low power services on the former high power frequencies. This also applies to the ex-offshore pioneer Radio Caroline that now has a licence to use 648 kHz, which was used by the BBC World Service over decades. In Italy, the government closed its high power transmitters but low power private stations remain. As

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3234-500: The antenna can be amplified in the receiver without introducing significant noise. The most common receiving antenna is the ferrite loopstick antenna (also known as a ferrite rod aerial ), made from a ferrite rod with a coil of fine wire wound around it. This antenna is small enough that it is usually enclosed inside the radio case. In addition to their use in AM radios, ferrite antennas are also used in portable radio direction finder (RDF) receivers. The ferrite rod antenna has

3311-807: The audio is reproduced. This is a major disadvantage compared to FM and digital modes where the demodulated audio is more objective. Extended audio bandwidths cause interference on adjacent channels. Wavelengths in this band are long enough that radio waves are not blocked by buildings and hills and can propagate beyond the horizon following the curvature of the Earth; this is called the groundwave . Practical groundwave reception of strong transmitters typically extends to 200–300 miles (320–480 km), with greater distances over terrain with higher ground conductivity , and greatest distances over salt water. The groundwave reaches further on lower medium wave frequencies. Medium waves can also reflect off charged particle layers in

3388-416: The band by "stretching out" the existing assignments, achieved by following a table which in most cases moved all the stations on a common frequency to a new, higher, dial position. This provided gaps of unassigned frequencies, most of which became clear channels allocated to Mexico and Canada. A majority of the frequency shifts were limited to between 10 and 30 kHz, which conserved the electrical height of

3465-540: The country and/or abroad), no longer having to broadcast weather and government reports on a different frequency than entertainment. Class A and B stations were segregated into sub-bands. In the US and Canada the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2 megawatts daytime. Most United States AM radio stations are required by

3542-618: The end of the Cold War and the increased availability of satellite and Internet TV and radio, although the cross-border reception of neighbouring countries' broadcasts by expatriates and other interested listeners still takes place. In the late 20th century, overcrowding on the Medium wave band was a serious problem in parts of Europe contributing to the early adoption of VHF FM broadcasting by many stations (particularly in Germany). Due to

3619-475: The entire Western hemisphere, and was signed at Rio de Janeiro, Brazil in 1981, taking effect on July 1, 1983 at 08:00 UTC . The interference protection criteria in the Rio Agreement are significantly different from NARBA's, and the concept of clear channel stations is eliminated. In adopting this agreement, the Bahamas and Canada declared their intent to renounce their adherence to NARBA. However, much of

3696-576: The entire Western hemisphere. However, current AM band assignments in North America largely reflect the standards first established by the NARBA agreements. Organized AM (mediumwave) radio broadcasting began in the early 1920s, and the United States soon dominated the North American airwaves, with more than 500 stations by the end of 1922. Due to a change in the ionosphere after the sun sets, nighttime signals from AM band stations are reflected for distances extending for hundreds of kilometers. This

3773-471: The first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting , navigational radio beacons , maritime ship-to-shore communication, and transoceanic air traffic control . Radio waves at MF wavelengths propagate via ground waves and reflection from the ionosphere (called skywaves ). Ground waves follow the curvature of Earth . At these wavelengths, they can bend ( diffract ) over hills, and travel beyond

3850-577: The frequency. Because such tall masts can be costly and uneconomic, other types of antennas are often used, which employ capacitive top-loading ( electrical lengthening ) to achieve equivalent signal strength with vertical masts shorter than a quarter wavelength. A "top hat" of radial wires is occasionally added to the top of mast radiators, to allow the mast to be made shorter. For local broadcast stations and amateur stations of under 5 kW, T- and L-antennas are often used, which consist of one or more horizontal wires suspended between two masts, attached to

3927-423: The ground. Shunt-excited masts, in which the base of the mast is at a node of the standing wave at ground potential and so does not need to be insulated from the ground, have fallen into disuse, except in cases of exceptionally high power, 1 MW or more, where series excitation might be impractical. If grounded masts or towers are required, cage or long-wire aerials are used. Another possibility consists of feeding

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4004-678: The high demand for frequencies in Europe, many countries set up 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. In Asia and the Middle East, many high-powered transmitters remain in operation. China , Indonesia , South Korea , North Korea , Japan , Thailand , Vietnam , Philippines , Saudi Arabia , Egypt , India , Pakistan and Bangladesh still use medium wave. Israel returns to mediumwave after

4081-511: The high-powered English-language " border blaster " stations that had been directing their programming toward the U.S. and causing significant interference to U.S. and Canadian stations. However, an initial international meeting held in Mexico City in the summer of 1933 failed, primarily due to a lack of agreement over how many clear channel frequencies would be assigned to Mexico. In 1937, a series of radio conferences, this time successful,

4158-407: The inverted-L and T antennas , and wire dipole antennas . Ground wave propagation, the most widely used type at these frequencies, requires vertically polarized antennas like monopoles. The most common transmitting antennas, monopoles of one-quarter to five-eighths wavelength, are physically large at these frequencies, 25 to 250 metres (82 to 820 ft) requiring a tall radio mast . Usually

4235-524: The ionospheric D layer can virtually disappear. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer. This can be very useful for long-distance communication, but can also interfere with local stations. Because of the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasting stations several hundred miles apart. On nights of good skywave propagation,

4312-596: The limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasting stations several hundred miles apart. On nights of good skywave propagation, the skywave signals of a distant station may interfere with the signals of local stations on the same frequency. In North America, the North American Regional Broadcasting Agreement (NARBA) sets aside certain channels for nighttime use over extended service areas via skywave by

4389-486: The mast or the tower by cables running from the tuning unit to the guys or crossbars at a certain height. Directional aerials consist of multiple masts , which need not to be of 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. For medium-wave (AM) broadcasting, quarter-wave masts are between 153 feet (47 m) and 463 feet (141 m) high, depending on

4466-427: The metal mast itself is energized and used as the antenna, and is mounted on a large porcelain insulator to isolate it from the ground; this is called a mast radiator . The monopole antenna, particularly if electrically short requires a good, low resistance Earth ground connection for efficiency since the ground resistance is in series with the antenna and consumes transmitter power. Commercial radio stations use

4543-612: The most common antenna for broadcast reception is the ferrite-rod antenna , also known as a loopstick antenna. The high permeability ferrite core allows it to be compact enough to be enclosed inside the radio's case and still have adequate sensitivity. For weak signal reception or to discriminate between different signals sharing a common frequency directional antennas are used. For best signal-to-noise ratio these are best located outdoors away from sources of electrical interference. Examples of such medium wave antennas include broadband untuned loops, elongated terminated loops, wave antennas (e.g.

4620-410: The most significant changes: A series of modifications would follow the initial treaty, which was scheduled to expire on March 29, 1946. In early 1946, a three-year interim agreement gave Cuba expanded allocations, including the right to share five U.S., three Canadian, and two Mexican clear channel allocations, plus operate high-powered stations on some regional frequencies. The changes also resulted in

4697-478: The need arose to standardize engineering practices, reduce interference, and more fairly distribute clear channel assignments. Moreover, the development of better frequency control, and especially directional antennas, made it possible for additional stations to operate on the same or close by frequencies without significantly increasing interference. A key objective for the United States was that, in exchange for receiving clear channel assignments, Mexico would eliminate

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4774-415: The outbreak of Israel-Hamas war . China operates many single-frequency networks across the country. As of May 2023, many Japanese broadcasters like NHK broadcast in medium wave, with many high power transmitters operating across Japan. There are also some low power relay transmitters for rural areas. Some countries have stopped using mediumwave, including Malaysia and Singapore. Stereo transmission

4851-563: The radio will decode C-QUAM AM stereo, whereas a set labelled "FM Stereo/AM Stereo" or "AMAX Stereo" will support AM stereo. 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) system standardised by ETSI supports stereo and

4928-411: The reception of much longer distance signals (within a range of about 2,000 km or 1,200 miles). This can cause increased interference because on most channels multiple transmitters operate simultaneously worldwide. In addition, amplitude modulation (AM) is often more prone to interference by various electronic devices, especially power supplies and computers. Strong transmitters cover larger areas than on

5005-574: The same part of the country broadcast simultaneously the resultant interference meant that usually neither could be heard clearly. The Commerce Department rarely intervened in such cases but left it up to stations to enter into voluntary timesharing agreements amongst themselves. The addition of a third "entertainment" wavelength, 400 meters, did little to solve this overcrowding. In 1923, the Commerce Department realized that as more and more stations were applying for commercial licenses, it

5082-531: The signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency. The North American Regional Broadcasting Agreement (NARBA) sets aside certain channels for nighttime use over extended service areas via skywave by a few specially licensed AM broadcasting stations. These channels are called clear channels , and the stations, called clear-channel stations , are required to broadcast at higher powers of 10 to 50 kW. A major use of these frequencies

5159-523: The sky are refracted back to Earth by layers of charged particles ( ions ) in the ionosphere , the E and F layers . However, at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is heavily ionised, such as during the day, in summer and especially at times of high solar activity . At night, especially in winter months and at times of low solar activity,

5236-427: The structure introduced by that treaty remained intact. On June 8, 1988 another conference held at Rio de Janeiro, this time under the auspices of the International Telecommunication Union , adopted provisions effective July 1, 1990 to add ten AM band frequencies within Region 2, commonly known as the " expanded band ", and running from 1610 kHz to 1700 kHz. The 1950 NARBA provisions are still in effect for

5313-482: The total height, where they are terminated in insulators and thence outwards to ground anchors . Thus the umbrella antenna uses the guy wires as the top-load part of the antenna. In all these antennas the smaller radiation resistance of the short radiator is increased by the capacitance added by the wires attached to the top of the antenna. In some rare cases dipole antennas are used, which are slung between two masts or towers. Such antennas are intended to radiate

5390-415: The visual horizon, although they may be blocked by mountain ranges. Typical MF radio stations can cover a radius of several hundred kilometres/miles from the transmitter, with longer distances over water and damp earth. MF broadcasting stations use ground waves to cover their listening areas. MF waves can also travel longer distances via skywave propagation, in which radio waves radiated at an angle into

5467-516: The world. In recent years, some limited amateur radio operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden. Many home-portable or cordless telephones, especially those that were designed in the 1980s, transmit low power FM audio signals between the table-top base unit and the handset on frequencies in the range 1600 to 1800 kHz. Transmitting antennas commonly used on this band include monopole mast radiators , top-loaded wire monopole antennas such as

5544-535: Was held in Havana, Cuba, and the initial NARBA agreement was signed on December 13, 1937 by representatives from the United States, Canada, Cuba, Mexico, the Dominican Republic and Haiti. The most significant change was the formal addition of ten broadcasting frequencies, from 1510 to 1600 kHz, with the 106 available frequencies divided into Clear Channel (59 frequencies), Regional (41) and Local (6) designations. The official lower limit remained at 550 kHz, as it

5621-475: Was not possible to add stations at the bottom of the broadcast band due to the need to protect 500 kHz — a maritime international distress frequency — from interference. (Although operation on 540 kHz was not covered by the Agreement, unofficially it became an additional Canadian clear channel frequency.) Under the Agreement, most existing stations operating on 740 kHz or higher would have to change frequencies. Open frequencies were created throughout

5698-426: Was not practical to have every station broadcast on the same three wavelengths. On 15 May 1923, Commerce Secretary Herbert Hoover announced a new bandplan which set aside 81 frequencies, in 10 kHz steps, from 550 kHz to 1350 kHz (extended to 1500, then 1600 and ultimately 1700 kHz in later years). Each station would be assigned one frequency (albeit usually shared with stations in other parts of

5775-406: Was particularly important in the case of the two highest Local frequencies, 1420 and 1500 kHz, as stations on these frequencies were being moved to 1450 and 1490 kHz, a 40 kHz separation. According to the Agreement's provisions, its implementation was to take place within one year after its adoption by the pact's four main signatories — the United States, Canada, Cuba and Mexico. Cuba

5852-426: Was synonymous with a Regional frequency assignment. In the United States, Class IV stations were only assigned to Local frequencies, although in other countries they were assigned to both Local and Regional ones. A major change was the provision that some clear channels were allocated to be used simultaneously by two stations — those maintaining sole use of a frequency were classified as Class I-A, while stations sharing

5929-414: Was the first to ratify, on December 22, 1937, and was followed by the U.S. on June 15, 1938 and Canada on November 29, 1938. While waiting on Mexico, in 1939 the U.S. and Canada completed a frequency agreement based on the treaty standards. Mexico finally approved the treaty on December 29, 1939, and work commenced on adopting its wide-ranging provisions. An engineering conference, with representatives from

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