Medium wave

Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz[1] and in North America an extended MW broadcast band goes from 535 kHz to 1705 kHz.[2]

Contents


Medium wave propagation characteristics

Medium wave signals have the property of following the curvature of the earth (the groundwave) at all times, and also refracting 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, 250 miles (400 km) 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 may pick up signals as far away as Mexico City and Seattle reliably, depending on atmospheric noise and man-made interference.

Some experiments and trials are planned or under way for a digital modulation such as Digital Radio Mondiale (DRM)[3].

Medium wave in the Americas

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.[4] 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,[4] 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 Hoover announced a new bandplan which set aside 81 frequencies, in 10 kHz steps, from 550 kHz to 1350 kHz (extended to 1500, then 1600kHz 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 and government reports on a different frequency than entertainment. Class A and B stations were segregated into sub-bands.[5]

Nowadays in most of the Americas, mediumwave broadcast stations are separated by 10 kHz and have two sidebands of up to ± 5 kHz in theory, although in practice stations transmit audio of up to 10 kHz [6]. 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 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 (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 ("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.

Medium wave in Europe

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 [7] . 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 work 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. 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 the United Kingdom, which has approximately 250 medium-wave transmitters of 1 kW and over [8]; other parts of the United Kingdom 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 neighbouring 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.

Overcrowding on the Medium wave band is a serious problem in parts of Europe contributing to the early adoption of VHF FM broadcasting by many stations (particularly in Germany). However in recent years several European countries (Including Ireland, Poland and, to a lesser extent Switzerland) have started moving away from Medium wave altogether with most/all services moving exclusively to other bands (usually VHF).

Stereo and digital radio transmissions on medium wave

Stereo transmission is possible and offered by some stations in the U.S., Canada, Mexico, the Dominican Republic, Paraguay, Australia, The Philippines, Japan, South Korea, South Africa, and France. However, there are multiple standards for AM stereo with C-QUAM being the most common in the United States as well as other countries, and receivers that implement the technologies are relatively rare.

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.

Antennas

For transmission, mast radiators are most commonly used. Stations broadcasting with low power can 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 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 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.

For medium-wave (AM) broadcasting, quarter-wave masts are between 153 ft and 463 ft high, depending on the frequency. Because quarter-wave masts are so large they can be unnecessarily costly and uneconomic, and other types are more commonly used on medium wave for local broadcast stations of under 5 kW, examples being T- and L-antennas. The design of these smaller antennas uses the technique commonly used on Long Wave to allow smaller masts to be used [9]. In this method the mast size is reduced, producing less radiation resistance and increased reactance, and wires are added, supported by the same mast or masts, to compensate these deficiencies adequately. The details depend on the requirement for grounded or insulated towers and masts. The dimensions can and must be calculated accurately to meet the required specifications in bandwidth, power handling and radiation efficiency.

A popular choice for lower-powered stations is the umbrella radiator, 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. In that sense it is a simple monopole, but at the top of the mast extra 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 ground anchors. Thus the umbrella antenna uses the guy wires as part of the antenna. The added umbrella-wires improve the current distribution on the vertical mast, thus increasing the radiation resistance, and they also increase the antenna capacitance and hence the bandwidth.

In some rare cases dipole antennas are used, which are slung between two masts or towers. Such antennas are intended to radiate a skywave. The medium-wave transmitter at Berlin-Britz for transmitting RIAS used a cross dipole mounted on five 30.5 meter high guyed masts to transmit the skywave to the ionosphere at nighttime.

Europe's largest antenna park [10] 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 low-noise 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. This class of antennas includes the ferrite-rod antenna, also known as a loopstick antenna, which is by far the most common antenna in use for broadcast reception throughout the world. Transistor radios contain a loopstick as the receiving antenna.

See also

References

  1. . Ofcom. p. 21. http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/UK-FAT-Table-2008/ukfat08.pdf. Retrieved 2010-01-26. 
  2. . National Telecommunications and Information Administration, U.S. Department of Commerce. October 2003. http://www.ntia.doc.gov/osmhome/allochrt.pdf. Retrieved 2009-08-11. 
  3. . . BBC. http://www.bbc.co.uk/devon/digitalmediumwave/faqs_feature.shtml. Retrieved 2009-04-24. 
  4. a b . Earlyradiohistory.us. http://earlyradiohistory.us/buildbcb.htm. Retrieved 2010-05-07. 
  5. Christopher H. Sterling; John M. Kittross (2002). . Psychology Press. p. 95. . 
  6. . . U.S. Government Printing Office. Revised as of October 1, 2006. http://edocket.access.gpo.gov/cfr_2006/octqtr/47cfr73.44.htm. Retrieved 2009-04-24. 
  7. . ITU. http://www.itu.int. Retrieved 2009-04-24. 
  8. List of medium-wave transmitters in the UK
  9. Weeks, W.L 1968, Antenna Engineering, McGraw Hill Book Company, Section 2.6
  10. . http://mediumwave.info/reports.html. Retrieved 2009-04-24. "DX 183 - EUROPE'S LARGEST ANTENNA PARK"