Medium frequency
Encyclopedia
Medium frequency refers to radio frequencies
(RF) in the range of 300 kHz
to 3 MHz. Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten down to one hectometers (1,000 to 100 m). Frequencies immediately below MF are denoted low frequency
(LF), and the next higher frequencies are known as high frequency
(HF).
(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.
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. 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.
Medium wave radio stations are allocated an AM
broadcast band from 526.5 kHz to 1606.5 kHz
in Europe; in North America this extends
from 535 kHz to 1705 kHz.
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.
There is an amateur radio band known as 160 meters
or 'top-band' 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.
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. RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.
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.
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.
Conversely, MF Skywave
propagation depends on the various layers in the Ionosphere
as the signal is refracted back down to earth by the E and F layers. When heavily ionised, such as during the day, in summer and especially at times of high solar activity
, the D layer (At a lower altitude than the refractive E and F layers) can be electronically noisy and absorptive of MF waves.
Late 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 on a quiet frequency, but can have the opposite effect in many other cases. For example, due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasters provided they transmit several hundred miles apart. On nights of good MF propagation, distant stations may appear superimposed onto local ones causing interference.
will be twice that size. Given the requirements for gaining an adequate height and for a good earth, this can make demands on establishing an efficient antenna system for an MF transmitter.
On the other hand, ferrite
is very efficient at MF and so a compact and efficient reception antenna can be made from a ferrite rod with a coil of fine wire wound around it. These are common in AM radios and are also used in portable radio direction finder
(RDF) receivers. The reception pattern of ferrite rod antennas has 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.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) in the range of 300 kHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to 3 MHz. Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten down to one hectometers (1,000 to 100 m). Frequencies immediately below MF are denoted low frequency
Low frequency
Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...
(LF), and the next higher frequencies are known as high frequency
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
(HF).
Uses and applications
Non-directional navigational radio beaconsNon-directional beacon
A non-directional beacon is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF...
(NDBs) for maritime and aircraft navigation occupy a band from 190 to 435 kHz, which overlaps from the LF
Low frequency
Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...
into the bottom part of the MF band.
500 kHz was for many years the maritime distress and emergency frequency
International distress frequency
Since early in the 20th century, the radio frequency of 500 kilohertz has been an international calling and distress frequency for Morse code maritime communication. The unit kilohertz was not introduced until 1960...
, and there are more NDBs between 510 and 530 kHz. Navtex
Navtex
NAVTEX is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships...
, which is part of the current Global Maritime Distress Safety System
Global Maritime Distress Safety System
The Global Maritime Distress and Safety System is an internationally agreed-upon set of safety procedures, types of equipment, and communication protocols used to increase safety and make it easier to rescue distressed ships, boats and aircraft....
occupies 518 kHz and 490 kHz for important digital text broadcasts. In recent years, some limited amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden.
Medium wave radio stations are allocated an AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
broadcast band from 526.5 kHz to 1606.5 kHz
in Europe; in North America this extends
Extended AM broadcast band
The extended mediumwave broadcast band, commonly known as the [AM] expanded band, is a broadcast frequency allocation which moved the upper limit of the AM bandplan from 1610 to 1700 kHz.- History :...
from 535 kHz to 1705 kHz.
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.
There is an amateur radio band known as 160 meters
160 meters
Just above the mediumwave broadcast band, 160 meters is the lowest radio frequency band allocation available to amateur radio operators in most countries. Seasoned operators often refer to 160 meters as the Top Band...
or 'top-band' 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
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
, digital signals and SSB
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
voice signals on this band.
There are a number of coast guard
Coast guard
A coast guard or coastguard is a national organization responsible for various services at sea. However the term implies widely different responsibilities in different countries, from being a heavily armed military force with customs and security duties to being a volunteer organization tasked with...
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. RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.
2182 kHz
2182 kHz
The radio frequency of 2182 kHz is the international calling and distress frequency for maritime radiotelephone communications on the marine MF bands.- Transmission modes :...
is the international calling and distress frequency for SSB maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band.
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
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
radio band.
Propagation
Propagation at MF is often via ground waves. Ground wave propagation at these frequencies follows the curvature of the Earth over conductive surfaces such as the sea and damp earth. At sea, MF communications can typically be heard over several hundred miles.Conversely, MF Skywave
Skywave
Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...
propagation depends on the various layers in the Ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
as the signal is refracted back down to earth by the E and F layers. When heavily ionised, such as during the day, in summer and especially at times of high solar activity
Solar variation
Solar variation is the change in the amount of radiation emitted by the Sun and in its spectral distribution over years to millennia. These variations have periodic components, the main one being the approximately 11-year solar cycle . The changes also have aperiodic fluctuations...
, the D layer (At a lower altitude than the refractive E and F layers) can be electronically noisy and absorptive of MF waves.
Late 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 on a quiet frequency, but can have the opposite effect in many other cases. For example, due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasters provided they transmit several hundred miles apart. On nights of good MF propagation, distant stations may appear superimposed onto local ones causing interference.
Transmission and reception
Even a quarter-wave antenna at MF can be physically large (25 to 250 m (82 to 820.2 ft), depending for which part of the band), and a half-wave dipoleDipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
will be twice that size. Given the requirements for gaining an adequate height and for a good earth, this can make demands on establishing an efficient antenna system for an MF transmitter.
On the other hand, ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
is very efficient at MF and so a compact and efficient reception antenna can be made from a ferrite rod with a coil of fine wire wound around it. These are common in AM radios and are also used in portable radio direction finder
Radio direction finder
A radio direction finder is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance...
(RDF) receivers. The reception pattern of ferrite rod antennas has sharp nulls
Null (physics)
In physics a null is a point in a field where the field quantity is zero as the result of two or more opposing quantities completely cancelling each other. The field may be scalar, vector or tensor in nature...
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.
See also
- Electromagnetic spectrumElectromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
- Maritime broadcast communications netMaritime broadcast communications netIn telecommunication, a maritime broadcast communications net is a communications net that is used for international distress calling, including international lifeboat, lifecraft, and survival-craft high frequency ; aeronautical emergency very high frequency ; survival ultra high frequency ;...
- Types of radio emissionsTypes of radio emissionsThe 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, and type of information transmitted on the carrier...
- Global Maritime Distress Safety SystemGlobal Maritime Distress Safety SystemThe Global Maritime Distress and Safety System is an internationally agreed-upon set of safety procedures, types of equipment, and communication protocols used to increase safety and make it easier to rescue distressed ships, boats and aircraft....
- NavtexNavtexNAVTEX is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships...
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 articles
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).