View source for Longwave

{{short description|Radio transmission using wavelengths above 1000 m}}
{{other uses}}

{{Use dmy dates|date=January 2020}}

[[File:Harumphy.radio dial.jpg|thumb|Tuning dial on 1946 [[Dynatron Radio|Dynatron]] Merlin T.69 console radio receiver, showing LW wavelengths between 800 and 2000&nbsp;metres (375–150&nbsp;kHz)]]

In [[radio]], '''longwave''' (also spelled '''long wave''' or '''long-wave'''<ref name="Graf-p23">{{cite book |last=Graf |first=Rudolf F. |title=Modern Dictionary of Electronics, 7th Ed. |url= https://books.google.com/books?id=AYEKAQAAQBAJ&q=longwave |publisher=Newnes |year=1999 |location=Boston MA |pages=23 |isbn=0750698667}}</ref> and commonly abbreviated '''LW'''<ref name=Macmillan>{{cite web |title=long wave |url= https://www.macmillandictionary.com/dictionary/british/long-wave |work=Macmillan Online Dictionary |publisher=Macmillan Publishers |access-date=20 June 2016 |url-status=live |archive-url= https://web.archive.org/web/20160811005154/https://www.macmillandictionary.com/dictionary/british/long-wave |archive-date=11 August 2016}}</ref>) is the part of the [[radio spectrum]] with [[wavelength]]s longer than what was originally called the [[medium-wave]] (MW) [[Broadcast band|broadcasting band]]. The term is historic, dating from the early 20th century, when the radio spectrum was considered to consist of LW, MW, and [[short wave radio|short-wave]] (SW) radio bands. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short' (i.e. [[very high frequency|VHF]], [[ultra high frequency|UHF]], and [[microwave]]).

In contemporary usage, the term ''longwave'' is not defined precisely, and its intended meaning varies. It may be used for radio wavelengths longer than 1,000&nbsp;m<ref name=Macmillan/> i.e. [[Frequency|frequencies]]{{NoteTag|Wave length and frequency are inversely related, with lower frequencies corresponding to longer wavelengths; 300&nbsp;kHz corresponds to 1,000&nbsp;m.}} smaller than 300&nbsp;[[kilohertz]] (kHz),<ref name=CambridgeDictionary>{{cite web |title=long wave |url = https://dictionary.cambridge.org/dictionary/english/long-wave |work=Cambridge Online Dictionary |publisher = [[Cambridge University Press]] |via = Cambridge.org |access-date=20 June 2016 |url-status=live |archive-url = https://web.archive.org/web/20160820044538/https://dictionary.cambridge.org/dictionary/english/long-wave |archive-date=20 August 2016  }}</ref><ref name="Graf-p437">{{cite book |last=Graf |first=Rudolf F. |title=Modern Dictionary of Electronics |edition = 7th |publisher=[[Newnes]] |year=1999 |page = 437 |url = https://books.google.com/books?id=uah1PkxWeKYC&q=longwave++%221000+meters&pg=PA437 |isbn = 0750698667 }}</ref> including the [[International Telecommunication Union]] (ITU) [[low frequency]] (LF, 30–300&nbsp;kHz) and [[very low frequency]] (VLF, 3–30&nbsp;kHz) bands. Sometimes the upper limit is taken to be higher than 300&nbsp;kHz, but not above the start of the medium wave broadcast band at 520&nbsp;kHz.<ref name=lwca/>

In Europe, Africa, and large parts of Asia ([[International Telecommunication Union region|International Telecommunication Union Region 1]]), where a range of frequencies between 148.5 and 283.5&nbsp;[[kHz]] is used for [[AM broadcasting]]<ref>{{cite book |author=Barun Roy |title=Enter The World of Mass Media |url=https://books.google.com/books?id=up7rxtHN_2oC&pg=PT21 |date=September 2009 |publisher=Pustak Mahal |isbn=978-81-223-1080-1 |page=21}}</ref> in addition to the MW band, the term ''longwave'' usually refers specifically to this broadcasting band, which falls wholly within the LF band of the radio spectrum (30–300&nbsp;kHz). The "Longwave Club of America" ([[United States]]) is interested in "frequencies below the AM broadcast band"<ref name=lwca>{{cite web |url = http://www.lwca.org/about_lwca.html |publisher=[[Longwave Club of America]] |title = About LWCA |access-date=20 June 2016 |url-status=live |archive-url = https://web.archive.org/web/20160627211810/http://lwca.org/about_lwca.html |archive-date=27 June 2016  }}</ref> (i.e., all frequencies below 520&nbsp;kHz).

== Propagation ==
Because of their long [[wavelength]], [[radio wave]]s in this frequency range can [[Diffraction|diffract]] over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ''[[ground wave]]'', is the main mode in the longwave band.<ref name="Seybold">{{cite book
 | last = Seybold
 | first = John S. 
 | title  = Introduction to RF Propagation
 | publisher = John Wiley and Sons
 | year  = 2005
 | pages  = 55–58
 | url    = https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 | isbn   = 0471743682
 }}</ref> The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies, and falls with frequency. [[Low frequency]] ground waves can be received up to {{convert|2000|km|mi}} from the transmitting antenna.  [[Very low frequency]] waves below 30&nbsp;kHz can be used to communicate at transcontinental distances, can penetrate saltwater to depths of hundreds of feet, and are used by the military to [[Communication with submarines|communicate with submerged submarines]].

Low frequency waves can also occasionally travel long distances by reflecting from the [[ionosphere]] (the actual mechanism is one of [[refraction]]), although this method, called ''[[skywave]]'' or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionospheric [[Kennelly–Heaviside layer|E layer]] or [[F Region|F layers]].  Skywave signals can be detected at distances exceeding {{convert|300|km|mi}} from the transmitting antenna.<ref>{{cite journal |title=Understanding LF Propagation |author=Alan Melia, G3NYK |journal=Radcom |publisher=[[Radio Society of Great Britain]]|location=Bedford, UK |issue=9 |volume=85 |pages=32}}</ref>

==Non-broadcast use==
===Non-directional beacons===
{{Main|Non-directional beacon}}

[[Non-directional beacon]]s transmit continuously for the benefit of [[radio direction finder]]s in marine and aeronautical navigation.  They identify themselves by a [[callsign]] in [[Morse code]].  They can occupy any frequency in the range 190–1750&nbsp;kHz.  In North America, they occupy 190–535&nbsp;kHz.  In ITU Region 1 the lower limit is 280&nbsp;kHz.

===Time signals===
There are institutional broadcast stations in the range that transmit coded time signals to radio clocks. For example:
*[[WWVB]] in [[Colorado]], United States, on 60&nbsp;kHz, 70 kW [[Effective radiated power|ERP]]
*[[DCF77]] in [[Frankfurt]], [[Germany]], on 77.5&nbsp;kHz, 50 kW
*[[JJY]] in Japan, on 40 & 60&nbsp;kHz, 50 kW
*[[RBU_(radio_station)|RBU]] 66.66&nbsp;kHz in [[Taldom transmitter]], Russia, 10 kW
*[[BPC (time signal)|BPC]] in [[Shangqiu]], [[China]], 68.5&nbsp;kHz, 90 kW
*[[Time from NPL|MSF]] time and 60&nbsp;kHz frequency standard transmitted from [[Anthorn radio station|Anthorn]] in the UK, 17 kW ERP.
*[[ALS162_time_signal|ALS162]] from [[Allouis]], [[France]], on 162&nbsp;kHz, 800 kW

[[Radio clock|Radio-controlled clocks]] receive their time calibration signals with built-in long-wave receivers. They use long-wave, rather than [[shortwave|short-wave]] or [[mediumwave|medium-wave]], because long-wave signals from the transmitter to the receiver always travel along the same direct path across the surface of the [[Earth]], so the time delay correction for the signal travel time from the transmitting station to the receiver is always the same for any one receiving location.

Longwaves travel by [[groundwave]]s that hug the surface of the Earth, unlike [[mediumwave]]s and [[shortwave radio|shortwaves]]. Those higher-frequency signals do not follow the surface of the [[Earth]] beyond a few kilometers, but can travel as [[skywave]]s, '[[refraction|bouncing]]' off different layers of the [[ionosphere]] at different times of day. These different [[radio propagation|propagation]] paths can make the time lag different for every signal received. The delay between when the long-wave signal was sent from the [[transmitter]] (when the coded time was correct) and when the signal is received by the clock (when the coded time is slightly late) depends on the overland distance between the clock and the transmitter and the [[speed of light#In a medium|speed of light through the air]], which is also very nearly constant. Since the time lag is essentially the same, a single constant shift forward from the time coded in the signal can compensate for all long-wave signals received at any one location from the same time signal station.

===Submarine communication===
The militaries of the United Kingdom, Russian Federation, United States, Germany, India and Sweden use frequencies below 50&nbsp;kHz to communicate with submerged [[submarine]]s.

===Amateur radio===
In the ITU Radio Regulations the band 135.7–137.8&nbsp;kHz is allocated (on a secondary basis) to [[Amateur radio]] worldwide, subject to a power limit of 1 watt EIRP. Many countries' regulators license amateurs to use it.

===LowFER===
In North America during the 1970s, the frequencies 167, 179, and 191&nbsp;kHz were assigned to the short-lived [[Public Emergency Radio of the United States]].

Nowadays, in the United States, [[Part 15]] of FCC regulations allow unlicensed use of the 160–190&nbsp;kHz band a transmitter / amplifier output power to the antenna of at most 1&nbsp;watt, with an antenna at most 15&nbsp;meters (49&nbsp;feet) high; this is called [[LowFER|Low Frequency Experimental Radio]] (LowFER). The 190–435&nbsp;kHz band is used for [[Non-directional beacon|navigational beacons]].

Frequencies from 472–479&nbsp;kHz are available to licensed amateurs as the new [[630-meter band|630&nbsp;m band]], part of the now-defunct [[500 kHz|maritime band]], but this is often considered a [[medium wave]] sub-band.

===Historic===
Swedish station SAQ, located at the [[Varberg Radio Station]] facility in Grimeton, is the last remaining operational [[Alexanderson alternator]] long-wave transmitter. Although the station ended regular service in 1996, it has been maintained as a [[World Heritage Site]], and makes at least two demonstration transmissions yearly, on 17.2&nbsp;kHz.<ref>[http://alexander.n.se/in-english/saq-transmission SAQ Transmission.] {{webarchive|url=http://archive.wikiwix.com/cache/20150407073704/http://alexander.n.se/in-english/saq-transmission |date=7 April 2015 }} Radiostation Grimeton SAQ. Retrieved 5 April 2015.</ref>

==Broadcasting <span class="anchor" id="longwave_broadcasting_anchor"></span>==
Longwave is used for broadcasting only within ITU Region 1. The long-wave broadcasters are located in Europe, North Africa and [[Mongolia]]. 

Typically, a larger geographic area can be covered by a long-wave broadcast transmitter compared to a [[medium-wave]] one. This is because ground-wave propagation suffers less [[attenuation]] due to [[ground conductivity]] at lower frequencies.<ref>[http://www.itu.int/rec/R-REC-P.368-9-200702-I/en ''Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.''] {{webarchive|url=https://web.archive.org/web/20120824233616/http://www.itu.int/rec/R-REC-P.368-9-200702-I/en |date=24 August 2012 }} ITU-R Recommendation P.368-9</ref>

Many countries have stopped using LW for broadcasting because of low audience figures, a lack of LW on new consumer receivers, increasing interference levels, the energy inefficiency of [[amplitude modulation]] and high electricity costs at transmitters. 

Between 2014 and 2015 Russia closed all of its LW broadcast transmitters.<ref>{{cite news |date=7 May 2018 |title=Russia says 'So long, long-wave' |work=BBC News |url=https://www.bbc.co.uk/news/blogs-news-from-elsewhere-25683656 |url-status=live |archive-url=https://web.archive.org/web/20170223232422/http://www.bbc.co.uk/news/blogs-news-from-elsewhere-25683656 |archive-date=23 February 2017 |access-date=7 May 2018 }}</ref> 

As of 2024 more than half of LW frequencies are unoccupied and some of the remaining services are scheduled for closure. [[BBC Radio 4]] (UK) announced that it will stop distinct programming for LW broadcasts in 2024 in an effort to transition listeners to other means of listening. A closure date for LW broadcasts has not yet been announced.<ref>[https://www.bbc.co.uk/mediacentre/articles/2023/bbc-radio-4-long-wave-transition BBC Radio 4 begins information campaign to transition listeners from Long Wave]</ref> The [[radio teleswitch]] service for electricity meters is broadcast with the long wave signal.  In October 2024 the Radio Teleswitch Taskforce said that it will end on 30 June 2025.<ref>{{cite web |title=End of the Radio Teleswitch Service (RTS): what we’re doing to help {{!}} Ofgem |url=https://www.ofgem.gov.uk/publications/end-radio-teleswitch-service-rts-what-were-doing-help |website=www.ofgem.gov.uk |access-date=6 December 2024 |language=en |date=3 October 2024}}</ref>

===Carrier frequencies===
With the adoption of the [[Geneva Frequency Plan of 1975]], long-wave carrier frequencies are exact multiples of 9&nbsp;kHz; ranging from 153 to 279&nbsp;kHz. One exception was a French-language station, [[Europe 1]] in Germany, which retained its prior channel spacing until the long-wave service was terminated in 2019. Other exceptions are all Mongolian transmitters, which are 2&nbsp;kHz above the internationally recognized channels.{{Clarify|date=December 2016}}

Until the 1970s, some long-wave stations in northern and eastern Europe and the Soviet Union operated on frequencies as high as 433&nbsp;kHz.<ref>{{cite book|title=Guide to Broadcasting Stations|publisher=[[Butterworth-Heinemann|Butterworth]]|edition=17th|isbn=0-592-00081-8|pages=18|year=1973}}</ref>

Some radio broadcasters, for instance [[Droitwich transmitting station]] in the UK, derive their carrier frequencies from an [[atomic clock]], allowing their use as [[frequency standard]]s.  Droitwich also broadcasts a low bit-rate data channel, using narrow-shift phase-shift keying of the carrier, for [[Radio teleswitch|Radio Teleswitch Services]].

===Long-distance reception===
Because long-wave signals can travel very long distances, some [[radio amateur]]s and [[Shortwave listening|shortwave listeners]] engage in an activity called [[DXing]]. DXers attempt to listen in to far away transmissions, and they will often send a reception report to the sending station to let them know where they were heard.  After receiving a report, the sending station may mail the listener a [[QSL card]] to acknowledge this reception.

Reception of long-wave signals at distances in excess of {{convert|17000|km|mi}} have been verified.<ref>http://www.classaxe.com/dx/ndb/rww/stats#top {{webarchive|url=https://web.archive.org/web/20160216221734/http://www.classaxe.com/dx/ndb/rww/stats|date=16 February 2016}}<!--From table for all types of signal: 2016-02-18 17487[km] 10865 81[kHz] GYN2 ENG IO84nr  Edgar J Twining (TA AUS)--></ref>

===List of long-wave broadcasting transmitters===
{{See also|List of longwave radio broadcasters}}
{{wide image|Height diagram1.gif|7500px|Height diagram of the antenna towers and antenna masts of long-wave broadcasting stations}}

== See also ==
{{Portal|Radio}}
* '''[[Low frequency]]''': for other uses (military, commercial and amateur) of this part of the radio spectrum (30–300&nbsp;kHz)
* '''[[Electromagnetic spectrum]]''': [[Very low frequency]], [[Shortwave]], [[Ground wave]], [[Skywave]], [[Medium wave]]
* '''[[Radio]] broadcasting''': [[AM broadcasting]], [[BBC Radio 4]], [[BBC Light Programme]], [[Radio clock]], Office de [[Radiodiffusion-Télévision Française]], [[Warsaw radio mast]], [[Digital Radio Mondiale]], [[International broadcasting]],
* '''Shipping''': [[Global navigation satellite system]], [[Navigation]], [[Shipping Forecast]]
* '''Lists''': [[Index of wave articles]]
* '''Other''': [[1 kilometre]], [[National Institute of Standards and Technology]], [[WGU-20]]

== Notes ==
{{NoteFoot}}

== References ==
{{Reflist}}

== External links ==
<!-- Please replace any deadlinks with archive.org links, instead of removing them -->
<!-- alphabetical order -->
* Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''".  IK1QFK Home Page.
* [http://www.mwcircle.org The Medium Wave Circle] – The premier club for MW/LW enthusiasts
* [https://web.archive.org/web/20070422221857/http://www.mwcircle.org/freemag.htm Medium Wave News] – Published regularly since 1954
* [http://www.emwg.info Euro-African Medium Wave Guide]
* [http://www.lwca.org/ Longwave Club of America]
* [http://www.drmradio.dk How to receive DRM from Kalundborg longwave station]
* [https://web.archive.org/web/20130510123728/http://www.magnet-ferritantennen.de/assets/plugindata/poola/funkamateur012006.pdf Reception of long wave and very long wave with ferrite antennas 5–50 kHz]
* {{cite book | author=Klawitter, G. |author2=Oexner, M. |author3=Herold, K.  | title=Langwelle und Längstwelle | isbn=3-89632-043-2 | publisher=Siebel Verlag GmbH |location=Meckenheim | year=2000 | language = de| pages=116–131 | chapter=8.2 Langwellenrundfunk }}
* {{cite web
 | last=Busch
 | first=Heinrich
 | date=14 November 2001
 | title=Luftschiff Graf Zeppelin LZ127
 | url = http://www.seefunknetz.de/lz127.htm
 }} (German)
* [http://www.mediumwaveradio.com/longwave.php European and Asian Longwave Stations – Medium Wave Radio]
* [https://web.archive.org/web/20100902152011/http://www.mwlist.org/mwlist_quick_and_easy.php?area=1&kHz=153 List of long- and mediumwave transmitters with GoogleMap-Links to transmission sites]

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{{Audio broadcasting}}
{{radio spectrum}}
{{EMSpectrum}}
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