Medium frequency

Medium frequency (MF) is the ITU designation[1] 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 hectometer (1,000 to 100 m). Frequencies immediately below MF are denoted low frequency (LF), while 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.

Medium frequency
Frequency range
0.3 to 3 MHz
Wavelength range
1000 to 100 m
Medium frequency
MF's position in the electromagnetic spectrum.


Radio waves at MF wavelengths propagate via ground waves and reflection from the ionosphere ( called skywaves).[2] Ground waves follow the contour of the Earth. At these wavelengths they can bend (diffract) over hills, and travel beyond the visual horizon, although they may be blocked by mountain ranges. Typical MF radio stations can cover a radius of several hundred miles from the transmitter, with longer distances over water and damp earth.[3] 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 the sky are reflected (actually 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, 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. 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 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.

Uses and applications

2008-07-28 Mast radiator
Mast radiator of a commercial MF AM broadcasting station, Chapel Hill, North Carolina, USA

A major use of these frequencies is AM broadcasting; AM radio stations are allocated frequencies in the medium wave broadcast band from 526.5 kHz to 1606.5 kHz[4] in Europe; in North America this extends from 525 kHz to 1705 kHz[5] 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 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.[6] RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.[7] Non-directional navigational radio beacons (NDBs) for maritime and aircraft navigation occupy 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 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.[8]

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 the Americas). Amateur operators transmit CW morse code, digital signals and SSB 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 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.[9]

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.[10]


Ferritantenne 2
Ferrite loopstick receiving antenna used in AM radios
Amateur T cage antenna 2BML 1922
Cage T antenna used by amateur radio transmitter on 1.5 MHz.

Transmitting antennas commonly used on this band include monopole mast radiators, top-loaded wire monopole antennas such as 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 antenna, the quarter wave monopole, is physically large at these frequencies, 25 to 250 metres (82 to 820 ft) requiring a tall radio mast. Usually the metal mast itself is 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 a ground system consisting of many heavy copper cables, buried a few feet 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 used.

Lower power transmitters often use electrically short quarter wave monopoles such as inverted-L or T antennas, which are brought into resonance with 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 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 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.

See also


  1. ^ "Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications" (PDF). ITU. Archived from the original (PDF) on 31 October 2013. Retrieved 20 February 2013.
  2. ^ Seybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. pp. 55–58. ISBN 0471743682.
  3. ^ "Ground wave MF and HF propagation" (PDF). Introduction to HF Propagation. IPS Radio and Space Services, Sydney Australia. Retrieved 27 September 2010.
  4. ^ "United Kingdom Frequency Allocation Table 2008" (PDF). Ofcom. p. 21. Retrieved 26 January 2010.
  5. ^ "U.S. Frequency Allocation Chart" (PDF). National Telecommunications and Information Administration, U.S. Department of Commerce. October 2003. Retrieved 11 August 2009.
  6. ^ MF/HF SSB Frequencies Archived 6 September 2007 at the Wayback Machine
  7. ^
  8. ^ U.S. Government Frequency Allocation Chart
  9. ^ "The 500 KC Amateur Radio Experimental Group". Retrieved 5 April 2018.
  10. ^ " - How to listen to cordless telephone conversations". 6 January 2009. Retrieved 5 April 2018.

Further reading

  • Charles Allen Wright and Albert Frederick Puchstein, "Telephone communication, with particular application to medium-frequency alternating currents and electro-motive forces". New York [etc.] McGraw-Hill Book Company, inc., 1st ed., 1925. LCCN 25008275

External links

Amateur radio frequency allocations

Amateur radio frequency allocation is done by national telecommunication authorities. Globally, the International Telecommunication Union (ITU) oversees how much radio spectrum is set aside for amateur radio transmissions. Individual amateur stations are free to use any frequency within authorized frequency ranges; authorized bands may vary by the class of the station license.

Radio amateurs use a variety of transmission modes, including Morse code, radioteletype, data, and voice. Specific frequency allocations vary from country to country and between ITU regions as specified in the current ITU HF frequency allocations for amateur radio. The list of frequency ranges is called a band allocation, which may be set by international agreements, and national regulations. The modes and types of allocations within each frequency band is called a bandplan; it may be determined by regulation, but most typically is set by agreements between amateur radio operators.

National authorities regulate amateur usage of radio bands. Some bands may not be available or may have restrictions on usage in certain countries or regions. International agreements assign amateur radio bands which differ by region.

Digital selective calling

Digital selective calling or DSC is a standard for transmitting pre-defined digital messages via the medium-frequency (MF), high-frequency (HF) and very-high-frequency (VHF) maritime radio systems. It is a core part of the Global Maritime Distress Safety System (GMDSS).

Grote Reber

Grote Reber (December 22, 1911 – December 20, 2002) was an American pioneer of radio astronomy, which combined his interests in amateur radio and amateur astronomy. He was instrumental in investigating and extending Karl Jansky's pioneering work, and conducted the first sky survey in the radio frequencies.His 1937 radio antenna was the second ever to be used for astronomical purposes and the first parabolic reflecting antenna to be used as a radio telescope. For nearly a decade he was the world's only radio astronomer.

High frequency

High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 to 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters (ten to one hundred metres). Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (2.31–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.

Kennelly–Heaviside layer

The Kennelly–Heaviside layer, named after Arthur E. Kennelly and Oliver Heaviside, also known as the E region or simply the Heaviside layer, is a layer of ionised gas occurring between roughly 90–150 km (56–93 mi) above the ground — one of several layers in the Earth's ionosphere. It reflects medium-frequency radio waves. Because of this reflective layer, radio waves radiated into the sky can return to Earth beyond the horizon. This "skywave" or "skip" propagation technique has been used since the 1920s for radio communication at long distances, up to transcontinental distances.

Propagation is affected by time of day. During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. Conversely, on the night (lee) side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection, called skywave. The extent of the effect is further influenced by the season, and the amount of sunspot activity.

Long Range Certificate

The Long Range Certificate is an internationally valid certificate issued to radio station operators. It entitles the holder to participate in marine communications on leisure crafts using Marine VHF, Medium Frequency, High Frequency radios and Inmarsat satellite communication as agreed in Global Maritime Distress and Safety System.

The certificate is consistent with the agreement of the Article S47 of the ITU Radio Regulations.

Medium wave

Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz, using channels spaced every 9 kHz, and in North America an extended MW broadcast band ranges from 525 kHz to 1705 kHz, using 10 kHz spaced channels. 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.


Nautel Ltd. is a Canadian manufacturer of AM and FM radio broadcast transmitters, navigational radio beacons, Differential Global Positioning System (DGPS) transmitters, NOAA weather radio transmitters, LF PNT/eLORAN transmitters, SONAR high-power low-frequency amplifiers and SONAR systems, medium frequency (MF) telegraph and NAVTEX transmitters, and high frequency (HF) amplifiers for dielectric heating applications. Nautel is best known as the first company to develop a commercially available fully solid state broadcast transmitter.


Navtex (Navigational Telex) is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent maritime safety information (MSI) to ships.

Navtex was developed to provide a low-cost, simple, and automated means of receiving this information aboard ships at sea within approximately 370 km (200 nautical miles) off shore.

There are no user fees associated with receiving navtex broadcasts, as the transmissions are typically transmitted from the National Weather Authority (Italy) or Navy or Coast Guard (as in the US) or national navigation authority (Canada).

Where the messages contain weather forecasts, an abbreviated format very similar to the shipping forecast is used.

Navtex is a component of the International Maritime Organization/International Hydrographic Organization Worldwide Navigation Warning Service (WWNWS). Navtex is also a major element of the Global Maritime Distress Safety System (GMDSS). SOLAS Convention mandated certain classes of vessels must carry navtex, beginning August 1, 1993.

North American Regional Broadcasting Agreement

The North American Regional Broadcasting Agreement (NARBA, Spanish: Convenio Regional Norteamericano de Radiodifusión) refers to a series of international treaties that defined technical standards for AM band (mediumwave) radio stations. These agreements also addressed how frequency assignments were distributed among the signatories, with a special emphasis on high-powered clear channel allocations.

The initial NARBA bandplan, also known as 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 the entire Western hemisphere. However, current AM band assignments in North America largely reflect the standards first established by the NARBA agreements.

Ozone layer

The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometers (9.3 to 21.7 mi) above Earth, although its thickness varies seasonally and geographically.The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Measurements of the sun showed that the radiation sent out from its surface and reaching the ground on Earth is usually consistent with the spectrum of a black body with a temperature in the range of 5,500–6,000 K (5,227 to 5,727 °C), except that there was no radiation below a wavelength of about 310 nm at the ultraviolet end of the spectrum. It was deduced that the missing radiation was being absorbed by something in the atmosphere. Eventually the spectrum of the missing radiation was matched to only one known chemical, ozone. Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer (the Dobsonmeter) that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958, Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The "Dobson unit", a convenient measure of the amount of ozone overhead, is named in his honor.

The ozone layer absorbs 97 to 99 percent of the Sun's medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which otherwise would potentially damage exposed life forms near the surface.In 1976 atmospheric research revealed that the ozone layer was being depleted by chemicals released by industry, mainly chlorofluorocarbons (CFCs). Concerns that increased UV radiation due to ozone depletion threatened life on Earth, including increased skin cancer in humans and other ecological problems, led to bans on the chemicals, and the latest evidence is that ozone depletion has slowed or stopped. The United Nations General Assembly has designated September 16 as the International Day for the Preservation of the Ozone Layer.

Venus also has a thin ozone layer at an altitude of 100 kilometers from the planet's surface.

Power-line communication

Power-line communication (PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. It is also known as power-line carrier, power-line digital subscriber line (PDSL), mains communication, power-line telecommunications, or power-line networking (PLN).

A wide range of power-line communication technologies are needed for different applications, ranging from home automation to Internet access which is often called broadband over power lines (BPL). Most PLC technologies limit themselves to one type of wires (such as premises wiring within a single building), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically transformers prevent propagating the signal, which requires multiple technologies to form very large networks. Various data rates and frequencies are used in different situations.

A number of difficult technical problems are common between wireless and power-line communication, notably those of spread spectrum radio signals operating in a crowded environment. Radio interference, for example, has long been a concern of amateur radio groups.

RF and microwave filter

Radio frequency (RF) and microwave filters represent a class of electronic filter, designed to operate on signals in the megahertz to gigahertz frequency ranges (medium frequency to extremely high frequency). This frequency range is the range used by most broadcast radio, television, wireless communication (cellphones, Wi-Fi, etc.), and thus most RF and microwave devices will include some kind of filtering on the signals transmitted or received. Such filters are commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.

Radiofrequency ablation

Radiofrequency ablation (RFA) is a medical procedure in which part of the electrical conduction system of the heart, tumor or other dysfunctional tissue is ablated using the heat generated from medium frequency alternating current (in the range of 350–500 kHz). RFA is generally conducted in the outpatient setting, using either local anesthetics or conscious sedation anesthesia. When it is delivered via catheter, it is called radiofrequency catheter ablation.

Two important advantages of radio frequency current (over previously used low frequency AC or pulses of DC) are that it does not directly stimulate nerves or heart muscle and therefore can often be used without the need for general anesthetic, and that it is very specific for treating the desired tissue without significant collateral damage.Documented benefits have led to RFA becoming widely used during the 21st century. RFA procedures are performed under image guidance (such as X-ray screening, CT scan or ultrasound) by an interventional pain specialist (such as an anesthesiologist), interventional radiologist, otolaryngologists, a gastrointestinal or surgical endoscopist, or a cardiac electrophysiologist, a subspecialty of cardiologists.

Regional Agreement for the Medium Frequency Broadcasting Service in Region 2

The Regional Agreement for the Medium Frequency Broadcasting Service in Region 2, commonly known as the Rio Agreement, is an international treaty that defines standards for AM band (mediumwave) radio stations in the western hemisphere. Covering North America, South America, and the Caribbean, it was signed in Rio de Janeiro, Brazil on 19 December 1981 and took effect on 1 July 1983 at 08:00 UTC.

A major feature of the Agreement was the division of stations into three main classifications: Class A, normally limited to 100 kilowatts (kW) daytime and 50 kW at night; Class B, limited to 50 kW, and Class C, limited to a nighttime power of 1 kW. Individual nations can make further refinements for their particular circumstances.


Sabamobil was a magnetic tape audio cartridge format made by SABA that came to the market in 1964. It used already-available four-track ¼ inch tape on 3-inch reels (7.62 cm), with two mono channels per side, using a tape speed of 3¾ ips (9.5 cm/s), and was compatible with reel-to-reel audio tape recording except the against remove secured ends of the tape in the reel. The cartridge could be opened without the need of any tools by removing two holding clamps. Tape head and capstan were placed between the reels.In the US, the player was offered for US$136 (equivalent to $1099 in 2018), a cassette was US$14 (equivalent to $113 in 2018), and the adapter for installation in car was US$45 (equivalent to $364 in 2018). The model TK-R12 also had an builtin medium frequency AM-broadcast receiver and could also be operated portable with five D-type batteries. The drive assembly had no drive belts. It appeared in the following year of the introduction of the Compact Cassette and lost its market shares soon to 8-track and Compact Cassette, which both came in smaller cartridges.

A similar technique to reuse standard 3-inch reels was the design of the dictation machine Philips Norelco EL3581, but with rearranged tracks and slower tape speed.

Shortwave radio

Shortwave radio is radio transmission using shortwave radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), and generally extends from 1.7–30 MHz (176.3–10.0 m); from the high end of the medium frequency band (MF) just above the mediumwave AM broadcast band, to the end of the HF band.

Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is called skywave or "skip" propagation. Thus shortwave radio can be used for very long distance communication, in contrast to radio waves of higher frequency which travel in straight lines (line-of-sight propagation) and are limited by the visual horizon, about 64 km (40 miles). Shortwave radio is used for broadcasting of voice and music to shortwave listeners over very large areas; sometimes entire continents or beyond. It is also used for military over-the-horizon radar, diplomatic communication, and two-way international communication by amateur radio enthusiasts for hobby, educational and emergency purposes, as well as for long distance aviation and marine communications.

Sudden ionospheric disturbance

A sudden ionospheric disturbance (SID) is an abnormally high ionization/plasma density in the D region of the ionosphere caused by a solar flare and/or solar particle event (SPE). The SID results in a sudden increase in radio-wave absorption that is most severe in the upper medium frequency (MF) and lower high frequency (HF) ranges, and as a result often interrupts or interferes with telecommunications systems.


WRHC (branded as Cadena Azul, meaning "Blue Network") is an AM radio station broadcasting on 1550 kHz with a Spanish variety radio format. Licensed to serve Doral, Florida, United States, the station reaches the Miami metropolitan area. The station is currently owned by WRHC Broadcasting Corp.The station's name and broadcast callsign are an homage to the former Cuban radio network, RHC-Cadena Azul, which operated from 1939 until 1954.Previously known as WRIZ until 1985, the station had a phased array of four radio towers in Stiltsville south of Key Biscayne from 1967 to 1990. Because salt water is highly conductive, it makes an excellent ground plane for signals in the mediumwave radio band, allowing the station to travel farther on the same power, although this station's purpose was to put a strong signal across Miami while minimizing its signal toward the Bahamas Islands and a station on 1540.. As with all medium-frequency stations, the towers themselves were mast radiators, connected to the transmitter shack via transmission lines, held in this case a few feet above the water line by pilings.

Visible (optical)
Wavelength types
Subcarrier signals

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