Medium wave

Medium wave is a part of the medium frequency 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 the reception of much longer distance signals. This can cause increased interference because on most channels multiple transmitters operate simultaneously worldwide. In addition, amplitude modulation 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 like HD Radio is used mostly in the US while DRM30 is mostly used as test transmissions by India, South Korea, and China.
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. 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, medium wave, and short wave radio bands.

Spectrum and channel allocation

For Europe, Africa and Asia 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 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 kHz would ordinarily require limiting the audio bandwidth to 4.5 and 5 kHz, respectively, without causing any interference to adjacent channels, because the audio spectrum is transmitted twice, once on each side band. This 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 9 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

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, 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 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 they are required to broadcast at higher powers of 10 to 50 kW.

Use in North America

Initially, broadcasting in the United States was restricted to two wavelengths: "entertainment" was broadcast at 360 meters, with stations required to switch to 485 meters 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 two 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, did little to solve this overcrowding.
In 1923, the 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 May 15, 1923, Commerce Secretary Herbert Hoover announced a new bandplan which set aside 81 frequencies, in 10 kHz steps, from 550 kHz to 1350 kHz. Each station would be assigned one frequency, 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 the Federal Communications Commission 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. 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 last station having signed off in 2013, after migrating to the FM band.

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, 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 low power services on the former high power frequencies. On 22 May 2017, the UK government regulator Ofcom awarded the former offshore "pirate radio" station Radio Caroline a community licence to broadcast to Suffolk and north Essex on 648 kHz with a power of 1 kW. The frequency had formerly been used by the BBC World Service. In Italy, the government closed its high power transmitters but low power private stations remain. As 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 can be used; the maximum power is also subject to international agreement by the International Telecommunication Union.
In most cases there are two power limits: 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 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.
Due to the high demand for frequencies in Europe, many countries set up single frequency networks; in the United Kingdom, BBC Radio 5 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.

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, Iran 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. However, it was closed back after few months.
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.