View source for Medium wave

{{short description|Radio transmission using wavelengths 200-1000 m}}
{{broader|Medium frequency}}
{{Use British English|date=August 2022}}

[[File:2008-07-28 Mast radiator.jpg|thumb|Typical [[mast radiator]] of a commercial medium wave [[AM broadcasting]] station, Chapel Hill, North Carolina, U.S.]]

'''Medium wave''' ('''MW''') is a part of the [[medium frequency]] (MF) [[radio band]] used mainly for [[AM broadcasting|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 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 the [[FM broadcast band]] but require more energy and longer antennas. Digital modes are possible but {{asof|lc=true|2020}} had not yet reached momentum.

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 is available, (however digital radio still has coverage issues in many parts of Europe).{{Citation needed|date=November 2023}} 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 radio|short wave]] (SW) radio bands.

== Spectrum and channel allocation ==
:{| class="wikitable"
|-
! area
! kHz (centre/carrier frequency)
! spacing
! channels
|-
| Europe, Asia, Africa
| 531&ndash;1602&nbsp;kHz
| 9 kHz
| 120
|-
| Australia
| 531&ndash;1701&nbsp;kHz (low power stations above 1602&nbsp;kHz)
| 9 kHz
| 131
|-
| North and South America
| 530&ndash;1700&nbsp;kHz
| 10 kHz
| 118
|}
For Europe, Africa and Asia the MW band consists of 120&nbsp;channels with [[Carrier wave|carrier frequencies]] from 531 to 1602&nbsp;kHz spaced every 9&nbsp;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&nbsp;kHz.<ref>{{cite report |title=United Kingdom Frequency Allocation Table |date=22 June 2017 |page=16 |url=https://www.ofcom.org.uk/__data/assets/pdf_file/0016/103309/uk-fat-2017.pdf |via=ofcom.org.uk |access-date=August 22, 2017}}</ref> Australia uses an [[AM expanded band|expanded band]] up to 1701&nbsp;kHz.

North and South America use 118&nbsp;channels from 530 to 1700&nbsp;kHz<ref>{{cite report |title=United States Frequency Allocations |date=January 2016 |type=spectrum wall chart |publisher=[[National Telecommunications and Information Administration]], [[U.S. Department of Commerce]] |url=https://www.ntia.doc.gov/files/ntia/publications/january_2016_spectrum_wall_chart.pdf |access-date=2017-08-22}}</ref> using 10&nbsp;kHz spaced channels. The range above 1610&nbsp;kHz is primarily only used by low-power stations; it is the preferred range for services with automated traffic, weather, and tourist information.

== Sound quality ==
The channel steps of 9 and 10&nbsp;kHz would ordinarily require limiting the audio bandwidth to 4.5 and 5&nbsp;kHz, respectively, without causing any interference to adjacent channels, because the audio spectrum is transmitted twice, once on each [[Single-sideband modulation|side band]] (i.e. ±4.5&nbsp;kHz of carrier frequency for 9&nbsp;kHz channels and ±5&nbsp;kHz of carrier frequency for 10&nbsp;kHz channels). This is adequate for talk and news but not for high-fidelity music. However, many stations use audio bandwidths up 10&nbsp;kHz,<ref>{{cite web |url=http://edocket.access.gpo.gov/cfr_2006/octqtr/47cfr73.44.htm |series=Code of Federal Regulations |title=§&nbsp;73.44 AM transmission system emission limitations |archive-url=https://web.archive.org/web/20110927113038/http://edocket.access.gpo.gov/cfr_2006/octqtr/47cfr73.44.htm |archive-date=2011-09-27 |df=dmy-all}}</ref> which is not [[High fidelity|hi-fi]] but sufficient for casual listening. In the UK, until 2024 most stations used a bandwidth of 6.3&nbsp;kHz.<ref>{{cite web |url=https://mediumwave.de |title=Medium Wave in Central Europe |date=2020-01-21 |df=dmy-all}}</ref> However in 2024, Ofcom expanded the allowed bandwidth to 9&nbsp;kHz, giving a noticeable improvement in quality. With AM, it largely depends on the frequency filters of each receiver how 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.

== Propagation characteristics ==
{{unreferenced section|date=March 2025}}
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 {{convert|200|-|300|mi|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 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 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 a few specially licensed AM broadcasting stations.  These channels are called [[Clear-channel station|clear channels]], and they are required to broadcast at higher powers of 10 to 50&nbsp;kW.

== Use in North America ==
{{See also|North American Regional Broadcasting Agreement}}
Initially, broadcasting in the United States was restricted to two wavelengths: "entertainment" was broadcast at 360 meters (833&nbsp;kHz), with stations required to switch to 485 meters (619&nbsp;kHz) when broadcasting weather forecasts, crop price reports and other government reports.<ref name="erh">{{cite web|url=https://earlyradiohistory.us/buildbcb.htm |title=Building the Broadcast Band |publisher=Earlyradiohistory.us |access-date=2010-05-07}}</ref> 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 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,<ref name="erh" /> did little to solve this overcrowding.

In 1923, the [[United States Department of Commerce|Commerce Department]] realized that as more and more stations were applying for commercial licenses, it was not practical to have every station broadcast on the same three wavelengths. On 15&nbsp;May 1923, Commerce Secretary [[Herbert Hoover]] announced a new bandplan which set aside 81 frequencies, in 10&nbsp;kHz steps, from 550&nbsp;kHz to 1350&nbsp;kHz (extended to 1500, then 1600 and ultimately 1700&nbsp;kHz in later years). Each station would be assigned one frequency (albeit usually shared with stations in other parts of the country and/or abroad), no longer having to broadcast [[Weather forecasting|weather]] and government reports on a different frequency than entertainment. Class A and B stations were segregated into sub-bands.<ref>{{cite book|title=Stay tuned: a history of American broadcasting|year=2002|publisher=Psychology Press|isbn=0-8058-2624-6|author=Christopher H. Sterling |author2=John M. Kittross|page=95}}</ref>

In the US and Canada the maximum transmitter power is restricted to 50&nbsp;kilowatts, while in [[Europe]] there are medium wave stations with transmitter power up to 2&nbsp;megawatts daytime.<ref>{{cite web|url=http://www.mwlist.org/mwlist_quick_and_easy.php?area=1&kHz=540|title=MWLIST quick and easy: Europe, Africa and Middle East|access-date=11 December 2015}}</ref>

Most United States [[AM broadcasting|AM radio]] stations are required by 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]] [[Radio propagation|propagation]] (sometimes loosely called ‘skip’).{{citation needed|date=March 2025}} 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, the [[CJDL-FM|last station]] having signed off in 2013, after migrating to the [[FM broadcasting|FM band]].{{citation needed|date=March 2025}}

== Use in Europe ==
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,<ref>{{cite web |title=Fast alle ARD-Radiosender stellen Mittelwelle ein | publisher=heise.de |date=2015-01-06 |url=http://www.heise.de/newsticker/meldung/Fast-alle-ARD-Radiosender-stellen-Mittelwelle-ein-2512316.html |access-date=2015-12-31}}</ref> 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 the UK, Spain and Romania. In the Netherlands and Scandinavia, some stations have launched{{when|date=March 2025}} low power services on the former high power frequencies. On 22 May 2017, the UK government regulator [[Ofcom]] awarded the former offshore station [[Radio Caroline]] a community licence to broadcast to [[Suffolk]] and north [[Essex]] on 648&nbsp;kHz with a power of 1&nbsp;kW.<ref name="Ofcom">{{Cite web|title=Ofcom awards five new AM community radio licences|url=https://www.ofcom.org.uk/about-ofcom/latest/media/media-releases/2017/ofcom-awards-five-new-am-community-radio-licences|access-date=19 May 2017|website=Ofcom|language=en|archive-date=22 December 2017|archive-url=https://web.archive.org/web/20171222034210/https://www.ofcom.org.uk/about-ofcom/latest/media/media-releases/2017/ofcom-awards-five-new-am-community-radio-licences|url-status=dead}}</ref> The frequency had formerly been used by the [[BBC World Service]]. In Italy, the government closed its high power transmitters{{when|date=March 2025}} but low power private stations remain{{which|date=March 2025}}.  As the MW band is thinning out, many local stations{{which|date=March 2025}} 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.{{citation needed|date=March 2025}}

In Europe, each country is allocated a number of frequencies on which high power (up to 2&nbsp;MW) can be used; the maximum power is also subject to international agreement by the [[International Telecommunication Union]] (ITU).<ref>{{cite web
| url=http://www.itu.int
| title=International Telecommunication Union
| publisher=ITU
| access-date=2009-04-24
}}</ref>

In most cases there are two power limits: a lower one for [[omnidirectional antenna|omnidirectional]] and a higher one for [[directional antenna|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.{{citation needed|date=March 2025}} 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 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.{{citation needed|date=March 2025}}

In the late 20th century, overcrowding on the Medium wave band was a serious problem in parts of Europe{{where|date=March 2025}}, contributing to the early adoption of [[Very high frequency|VHF]] [[FM broadcasting]] by many stations (particularly in Germany).{{citation needed|date=March 2025}}
Due to the high demand for frequencies in Europe, many countries set up single frequency networks; in the United Kingdom, [[BBC Radio Five Live]] broadcasts from various transmitters on either 693 or 909&nbsp;kHz. These transmitters are carefully synchronized to minimize interference from more distant transmitters on the same frequency.{{citation needed|date=March 2025}}

== Use in Asia ==
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. Example of the famous medium wave radio stations in South East Asia is DZBB and DZAS in the Philippines, and Radio RASIL and RRI in Indonesia.
 
Israel returns to mediumwave after the outbreak of [[Gaza war]].<ref>{{cite web | url=https://www.radioworld.com/global/israel-reactivates-am-for-war-time-communications | title=Israel Reactivates AM for War-Time Communications | date=14 January 2024 }}</ref>

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 and digital transmissions ==
{{See also|AM stereo|HD Radio|DRM30}}
[[File:Tandy AM Stereo.jpg|thumb|''Realistic'' TM-152 AM stereo tuner c. 1988]]
Stereo transmission 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 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 is the ITU-approved system for use outside [[North America]] and [[United States territory|U.S. territories]].  Some HD Radio receivers also support C-QUAM AM stereo, although this feature is usually not advertised by the manufacturer.

==Antennas==
[[Image:Wire T antenna station WBZ 1925.jpg|thumb|Multiwire [[T antenna]] of radio station WBZ, Massachusetts, USA, 1925.  T antennas were the first antennas used for medium wave broadcasting, and are still used at lower power.]] 
For broadcasting, [[mast radiator]]s 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 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 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 [[electrical insulator|insulator]] to isolate it from the ground. Shunt-excited masts, in which the base of the mast is at a [[node (physics)|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 aerial|cage]] or long-wire 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 at a certain height.

Directional aerials consist of [[tower array|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 {{convert|153|ft|m}} and {{convert|463|ft|m}} high, depending on 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.<ref>Weeks, W.L 1968, ''Antenna Engineering'', McGraw Hill Book Company, Section 2.6</ref> 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&nbsp;kW, [[T-antenna|T- and L-antennas]] are often used, which consist of one or more horizontal wires suspended between two masts, attached to 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 the total height, where they are terminated in insulators and thence outwards to [[Earth anchor|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 antenna]]s are used, which are slung between two masts or towers. Such antennas are intended to radiate a [[skywave]]. The medium-wave transmitter at [[Transmitter Berlin-Britz|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.

===Receiving antennas===
[[Image:Ferritantenne 2.jpg|thumb|Typical ferrite rod antenna used in AM radio receivers]]
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&nbsp;MHz, which includes long and medium waves, [[loop antenna]]s are popular because of their ability to reject locally generated noise.  By far the most common antenna for broadcast reception is the [[Ferrite rod|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<ref>{{Cite web|title=4: MW Aerials – Antennas – Medium Wave Circle|url=https://mwcircle.org/4-mw-aerials-antennas/|access-date=2021-11-28|language=en-GB}}</ref> include broadband untuned loops, elongated terminated loops, wave antennas (e.g. the [[Beverage antenna]]) and the ferrite sleeve loop antenna.

==See also==
* [[Digital Radio Mondiale]], a digital standard that is applicable on the medium wave
* [[DAB radio]]
* [[FM broadcasting|FM radio]]
* [[List of European medium wave transmitters]]
* [[MW DX]]
* [[Satellite radio]]
* [[Geneva Frequency Plan of 1975]]
* [[Monopole antenna]]

==References==
{{reflist}}

==External links==
* [https://earlyradiohistory.us/buildbcb.htm "Building the Broadcast Band"]—the development of the 520–1700&nbsp;kHz MW (AM) band
* [[:File:United States Effective Ground Conductivity Map.png|Map of Estimated Effective Ground Conductivity in the USA]]
* [https://web.archive.org/web/20091008063015/http://www.mwlist.org/ MWLIST]—worldwide database of MW and LW stations
* [https://mwcircle.org/ The Medium Wave Circle]- A [[UK]]-based club for Medium Wave [[DXing|DX]]'ers and enthusiasts.
* [https://web.archive.org/web/20100902152011/http://www.mwlist.org/mwlist_quick_and_easy.php?area=1&kHz=153 MWLIST quick and easy: Europe, Africa and Middle East]—List of long- and medium wave transmitters with Google Maps links to transmission sites

{{radio spectrum}}
{{EMSpectrum}}
{{Audio broadcasting}}
{{Telecommunications}}
{{Radio listening}}
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{{DEFAULTSORT:Medium wave}}
[[Category:Bandplans]]