Editing Medium frequency

{{Short description|The range 300–3000 kHz of the electromagnetic spectrum}}
{{Use dmy dates|date=December 2020}}
{{MWband
| freq = 0.3 to 3 [[Megahertz|MHz]]
| wave = 1000 to 100 m
| bands = 
}}
[[File:Medium_frequency.png|thumb|MF's position in the [[electromagnetic spectrum]].]]

'''Medium frequency''' ('''MF''') is the [[International Telecommunication Union|ITU]] designation<ref name="1037B">{{cite book
  | title = US Federal Standard 1037B: Telecommunications, Glossary of Telecommunications Terms 
  | publisher = Office of Technology Standards, General Services Administration
  | series = 
  | volume = 
  | edition = 
  | date = 3 June 1991
  | location = 
  | pages = S-18
  | language = 
  | url = https://books.google.com/books?id=zYLXU4fkD34C&pg=RA15-PA18
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref><ref name="itu-2015-acts">{{cite conference |date=2015 |title=Final Acts WRC-15 |url=https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.12-2015-PDF-E.pdf |conference=World Radiocommunication Conference |location=Geneva, Switzerland |publisher=International Telecommunications Union |pages = 4|access-date=2025-01-12}}</ref> for [[Radio frequency|radio frequencies]] (RF) in the range of 300&nbsp;[[kilohertz]] (kHz) to 3&nbsp;[[megahertz]] (MHz). Part of this band is the [[medium wave]]&nbsp;(MW) [[AM broadcast]] band. The MF band is also known as the '''hectometer band''' as the wavelengths range from ten to one [[hectometer]]s (1000 to 100&nbsp;m). Frequencies immediately below MF are denoted as [[low frequency]] (LF), while the first band of higher frequencies is known as [[high frequency]] (HF). MF is mostly used for [[AM broadcasting|AM radio broadcasting]], [[Radio beacon|navigational radio beacons]], maritime ship-to-shore communication, and transoceanic [[air traffic control]].

==Propagation==
Radio waves at MF wavelengths propagate via [[ground wave]]s and reflection from the [[ionosphere]] (called [[skywave]]s).<ref name="Seybold">{{cite book |last1=Seybold |first1=John S. |title=Introduction to RF Propagation |publisher=John Wiley and Sons |date=2005 |pages=55–58 |url=https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57 |isbn=0471743682}}</ref> Ground waves travel just above the earth's surface, following the terrain. At these wavelengths, they can bend ([[diffraction|diffract]]) over hills, and travel beyond the [[visual horizon]], although they may be blocked by mountain ranges. Ground waves are progressively absorbed by the Earth, so the signal strength decreases exponentially with distance from the transmitting antenna. Typical MF radio stations can cover a radius of several hundred kilometres/miles from the transmitter, with longer distances over water and damp earth.<ref name="ips.gov.au">{{cite web|title=Ground wave MF and HF propagation |url=https://www.sws.bom.gov.au/Category/Educational/Other%20Topics/Radio%20Communication/Intro%20to%20HF%20Radio.pdf|work=Introduction to HF Propagation|publisher=IPS Radio and Space Services, Sydney Australia|access-date=27 September 2010}}</ref> MF [[Radio station|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 [[Refraction|refracted]] back to Earth by layers of charged particles ([[ion]]s) in the [[ionosphere]], the [[E layer|E]] and [[F layer]]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 variation|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. Because of 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]]s'', are required to broadcast at higher powers of 10 to 50&nbsp;kW.

==Uses and applications==
[[File:2008-07-28 Mast radiator.jpg|thumb|[[Mast radiator]] of a commercial MF [[AM broadcasting]] station, Chapel Hill, North Carolina, USA]]

A major use of these frequencies is [[AM broadcasting]]; [[Amplitude modulation|AM]] [[radio station]]s are allocated frequencies in the [[medium wave]] broadcast band from 526.5&nbsp;kHz to 1606.5&nbsp;kHz<ref>{{cite web
|url=http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/UK-FAT-Table-2008/ukfat08.pdf
|title=United Kingdom Frequency Allocation Table 2008
|publisher=[[Ofcom]]
|access-date=2010-01-26
|page=21
}}</ref>
in Europe; in North America this [[Extended AM broadcast band|extends]] from 525&nbsp;kHz to 1705&nbsp;kHz<ref>{{cite web
|url=http://www.ntia.doc.gov/osmhome/allochrt.pdf
|title=U.S. Frequency Allocation Chart
|publisher=National Telecommunications and Information Administration, U.S. Department of Commerce
|date=October 2003
|access-date=2009-08-11
}}</ref> Some countries also allow broadcasting in the 120-meter band from 2300 to 2495&nbsp;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&nbsp;kHz. These include, as examples, the French MRCC on 1696&nbsp;kHz and 2677&nbsp;kHz, Stornoway Coastguard on 1743&nbsp;kHz, the US Coastguard on 2670&nbsp;kHz and Madeira on 2843&nbsp;kHz.<ref name="yachtcom">[http://www.yachtcom.info/Frequencies.htm MF/HF SSB Frequencies<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20070906110613/http://www.yachtcom.info/Frequencies.htm |date=6 September 2007 }}</ref> RN Northwood in England broadcasts Weather Fax data on 2618.5&nbsp;kHz.<ref>[http://www.hffax.de/Northwood-95.txt Amended Radiofax schedule]</ref> [[Non-directional beacon|Non-directional navigational radio beacons]] (NDBs) for maritime and aircraft navigation occupy a band from 190 to 435&nbsp;kHz, which overlaps from the [[Low frequency|LF]] into the bottom part of the MF band.

[[2182 kHz|2182&nbsp;kHz]] is the international calling and distress frequency for [[Single-sideband modulation|SSB]] maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band. [[500 kHz|500&nbsp;kHz]] was for many years the maritime [[International distress frequency|distress and emergency frequency]], and there are more NDBs between 510 and 530&nbsp;kHz. [[Navtex]], which is part of the current [[Global Maritime Distress Safety System]] occupies 518&nbsp;kHz and 490&nbsp;kHz for important digital text broadcasts.  Lastly, there are aeronautical and other mobile SSB bands from 2850&nbsp;kHz to 3500&nbsp;kHz, crossing the boundary from the MF band into the [[High frequency|HF]] radio band.<ref>[http://www.ntia.doc.gov/osmhome/allochrt.pdf U.S. Government Frequency Allocation Chart]</ref>

An [[amateur radio]] band known as [[160 meters]] or 'top-band' is between 1800 and 2000&nbsp;kHz (allocation depends on country and starts at 1810&nbsp;kHz outside the Americas). Amateur operators transmit CW [[morse code]], digital signals and SSB and AM voice signals on this band.  Following [[World Radiocommunication Conference]] 2012 (WRC-2012), the amateur service received a new allocation between 472 and 479&nbsp;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&nbsp;kHz in the US, UK, Germany and Sweden.<ref>{{cite web|url=http://www.500kc.com|title=The 500 KC Amateur Radio Experimental Group|website=500kc.com|access-date=5 April 2018}}</ref>

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&nbsp;kHz.<ref>{{cite web|url=http://totse.com/en/phreak/bugs_and_taps/tapphon.html|archive-url=https://web.archive.org/web/20090106001803/http://totse.com/en/phreak/bugs_and_taps/tapphon.html|url-status=dead|archive-date=6 January 2009|title=totse.com - How to listen to cordless telephone conversations|date=6 January 2009|access-date=5 April 2018}}</ref>

==Antennas==
[[Image:Ferritantenne 2.jpg|thumb|Ferrite [[loop antenna|loopstick]] receiving antenna used in AM radios]] 
[[File:Amateur T cage antenna 2BML 1922.jpg|thumb|Cage [[T antenna]] used by amateur radio transmitter on 1.5 MHz. ]]

Transmitting antennas commonly used on this band include [[Monopole antenna|monopole]] [[mast radiator]]s, top-loaded wire monopole antennas such as the inverted-L and [[T antenna]]s, and wire [[dipole antenna]]s.    [[Ground wave]] propagation, the most widely used type at these frequencies, requires vertically polarized antennas like monopoles.

The most common transmitting antennas, monopoles of one-quarter to five-eighths wavelength, are physically large at these frequencies, {{convert|25|to|250|m|ft|0}} requiring a tall [[radio mast]].  Usually the metal mast itself is energized and 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 [[electrical length|electrically short]] requires a good, low resistance Earth [[ground (electricity)|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 copper cables, buried shallowly 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 [[counterpoise (ground system)|counterpoise]]s are sometimes used.

Lower power transmitters often use [[electrical length|electrically short]] quarter wave monopoles such as inverted-L or [[T antenna]]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]] 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 weak signal from the antenna can be [[amplifier|amplified]] in the receiver without introducing significant noise.  The most common receiving antenna is the [[Ferrite (magnet)|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 antenna|dipole]] [[Radiation pattern|reception pattern]] with sharp [[Null (physics)|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]]s and [[random wire antenna]]s are also used.

==See also==
* [[Electromagnetic spectrum]]
* [[Global Maritime Distress Safety System]]
* [[Maritime broadcast communications net]]
* [[Navtex]]
* [[Types of radio emissions]]

==References==
<references/>
* [[Federal Standard 1037C]]

==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==
* Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''".  IK1QFK Home Page (vlf.it).

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[[Category:Radio spectrum]]