![Medium_frequency](https://upload.wikimedia.org/wikipedia/commons/2/21/Medium_frequency.png)
Medium frequency (MF) is the
ITU
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
designation
for
radio frequencies
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...
(RF) in the range of 300
kilohertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...
(kHz) to 3
megahertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, me ...
(MHz). Part of this band is the
medium wave
Medium wave (MW) is the 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 ...
(MW)
AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one
hectometer
The hectometre (American and British English spelling differences#-re, -er, International spelling as used by the International Bureau of Weights and Measures; SI symbol: hm) or hectometer (American and British English spelling differences#-re, -e ...
(1000 to 100 m). Frequencies immediately below MF are denoted
low frequency
Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre wave.
LF radio waves exh ...
(LF), while the first band of higher frequencies is known as
high frequency
High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten ...
(HF). MF is mostly used for
AM radio broadcasting,
navigational radio beacons, maritime ship-to-shore communication, and transoceanic
air traffic control
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...
.
Propagation
Radio waves at MF wavelengths propagate via
ground wave
Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in rad ...
s and reflection from the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
(called
skywave
In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature of ...
s).
Ground waves follow the
curvature of Earth
Spherical Earth or Earth's curvature refers to the approximation of figure of the Earth as a sphere.
The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Ancient Greek philoso ...
. At these wavelengths they can bend (
diffract
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
) 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.
MF
broadcasting stations use ground waves to cover their listening areas.
MF waves can also travel longer distances via
skywave
In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature of ...
propagation, in which radio waves radiated at an angle into the sky are
refracted
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
back to Earth by layers of charged particles (
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s) in the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
, the
E and
F layer
The F region of the ionosphere is home to the F layer of ionization, also called the Appleton–Barnett layer, after the English physicist Edward Appleton and New Zealand physicist and meteorologist Miles Barnett. As with other ionospheric secto ...
s. 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
Solar phenomena are natural phenomena which occur within the atmosphere of the Sun. These phenomena take many forms, including solar wind, radio wave flux, solar flares, coronal mass ejections, coronal heating and sunspots.
These phenomena are ...
.
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 station
A clear-channel station is an AM broadcasting, AM radio station in North America that has the highest protection from Interference (communication), interference from other stations, particularly concerning night-time skywave propagation. The syste ...
s'', are required to broadcast at higher powers of 10 to 50 kW.
Uses and applications
![2008-07-28 Mast radiator](https://upload.wikimedia.org/wikipedia/commons/7/72/2008-07-28_Mast_radiator.jpg)
A major use of these frequencies is
AM broadcasting
AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmis ...
;
AM radio station
Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...
s are allocated frequencies in the
medium wave
Medium wave (MW) is the 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 ...
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 is generally treated as one of the
shortwave bands
Shortwave bands are frequency allocations for use within the shortwave radio spectrum (the upper medium frequency Fband and all of the high frequency Fband). Radio waves in these frequency ranges can be used for very long distance (transcontinent ...
.
There are a number of
coast guard
A coast guard or coastguard is a maritime security organization of a particular country. The term embraces wide range of responsibilities in different countries, from being a heavily armed military force with customs and security duties to ...
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.
[MF/HF SSB Frequencies](_blank)
RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.
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
NAVTEX (NAVigational TEleX), sometimes styled Navtex or NavTex, is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent maritime safety inf ...
, which is part of the current
Global Maritime Distress Safety System
The Global Maritime Distress and Safety System (GMDSS) is a worldwide system for automated emergency signal communication for ships at sea developed by the United Nations' International Maritime Organization (IMO) as part of the SOLAS Convent ...
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
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...
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
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
, digital signals and SSB and AM voice signals on this band. Following
World Radiocommunication Conference
The World Radiocommunication Conference (WRC) is a conference organized by the ITU to review and, as necessary, revise the Radio Regulations, the international treaty governing the use of the radio-frequency spectrum as well as geostationary and ...
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
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...
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.
Antennas
![Amateur T cage antenna 2BML 1922](https://upload.wikimedia.org/wikipedia/commons/7/75/Amateur_T_cage_antenna_2BML_1922.jpg)
Transmitting antennas commonly used on this band include
monopole mast radiator
Mast, MAST or MASt may refer to:
Engineering
* Mast (sailing), a vertical spar on a sailing ship
* Flagmast, a pole for flying a flag
* Guyed mast, a structure supported by guy-wires
* Mooring mast, a structure for docking an airship
* Radio mas ...
s, top-loaded wire monopole antennas such as the inverted-L and
T antenna
A T-antenna, T-aerial, flat-top antenna, or (capacitively) top-loaded antenna is a monopole radio antenna with transverse capacitive loading wires attached to its top. T-antennas are typically used in the VLF, LF, MF, and shortwave bands,
a ...
s, and wire
dipole antennas.
Ground wave
Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in rad ...
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, requiring a tall
radio mast
Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television. There are two main types: guyed and self-supporting structures. They are among the tallest human-made ...
. 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
Mast, MAST or MASt may refer to:
Engineering
* Mast (sailing), a vertical spar on a sailing ship
* Flagmast, a pole for flying a flag
* Guyed mast, a structure supported by guy-wires
* Mooring mast, a structure for docking an airship
* Radio mas ...
''. The monopole antenna, particularly if
electrically short An electrically small or electrically short antenna is an Antenna (radio), antenna much shorter than the wavelength of the signal it is intended to transmit or receive. Electrically short antennas are generally less efficient and more challenging t ...
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 An electrically small or electrically short antenna is an Antenna (radio), antenna much shorter than the wavelength of the signal it is intended to transmit or receive. Electrically short antennas are generally less efficient and more challenging t ...
quarter wave monopoles such as inverted-L or
T antenna
A T-antenna, T-aerial, flat-top antenna, or (capacitively) top-loaded antenna is a monopole radio antenna with transverse capacitive loading wires attached to its top. T-antennas are typically used in the VLF, LF, MF, and shortwave bands,
a ...
s, 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
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
is determined by atmospheric noise. The
noise floor
In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored.
In radio com ...
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
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) di ...
(also known as a ''
ferrite rod
In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coup ...
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
Direction finding (DF), or radio direction finding (RDF), isin accordance with International Telecommunication Union (ITU)defined as radio location that uses the reception of radio waves to determine the direction in which a radio station ...
(RDF) receivers. The ferrite rod antenna has a
dipole
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
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 antenna
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) d ...
s and
random wire antenna
A random wire antenna is a radio antenna consisting of a long wire suspended above the ground, whose length does not bear a particular relation to the wavelength of the radio waves used, but is typically chosen more for convenience. The wire may ...
s are also used.
See also
*
Electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
*
Global Maritime Distress Safety System
The Global Maritime Distress and Safety System (GMDSS) is a worldwide system for automated emergency signal communication for ships at sea developed by the United Nations' International Maritime Organization (IMO) as part of the SOLAS Convent ...
*
Maritime broadcast communications net
*
Navtex
NAVTEX (NAVigational TEleX), sometimes styled Navtex or NavTex, is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent maritime safety inf ...
*
Types of radio emissions
The International Telecommunication Union uses an internationally agreed system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, ...
References
*
Federal Standard 1037C
Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms, is a United States Federal Standard issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, a ...
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
tc.McGraw-Hill Book Company, inc., 1st ed., 1925. LCCN 25008275
External links
* Tomislav Stimac, "
Definition of frequency bands (VLF, ELF... etc.)'". IK1QFK Home Page (vlf.it).
{{Authority control
Radio spectrum