View source for Low frequency

{{Short description|30–300 kHz range of the electromagnetic spectrum}}
{{redirect|Low band|the musical group Low|Low (band)}}
{{use dmy dates|date=March 2024}}
{{MWband
| freq = 30–300&nbsp;[[kHz]]
| wave = 10–1&nbsp;[[metre|km]]
}}
'''Low frequency''' ('''LF''') 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 | date = 3 June 1991 | pages = S-18 | url = https://books.google.com/books?id=zYLXU4fkD34C&pg=RA15-PA18}}</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 30–300&nbsp;[[kHz]]. Since its wavelengths range from 10–1&nbsp;[[kilometre|km]], respectively, it is also known as the '''kilometre band''' or '''kilometre waves'''.

LF radio waves exhibit low signal [[attenuation]]<!--or [[Attenuation distortion|signal attenuation]]?-->, making them suitable for long-distance communications. In Europe and areas of [[North Africa|Northern Africa]] and 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 beacons, navigation ([[LORAN]], mostly defunct), information, and weather systems. A number of [[#Standard time signals|time signal broadcasts]] also use this band. The main mode of transmission used in this band is [[ground wave]]s, in which LF radio waves travel just above the Earth's surface, following the terrain. LF ground waves can travel over hills, and can travel far beyond the horizon, up to several hundred kilometers from the transmitter.

==Propagation==
[[Image:Atmosphericnoise.PNG|thumb|260px|Atmospheric [[radio noise]] increases with decreasing frequency. At the LF band and below, it is far above the thermal noise floor injected by amplifier circuits in the receiver, so weak signals can be amplified in the receiver to compensate with no perceivable increase in the noise (''see'' [[signal to noise ratio|SNR]]). Consequently, ''for reception'', even inefficient antennas much smaller than the wavelength are adequate.]]

Because of their long [[wavelength]], low frequency [[radio wave]]s 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]]'', with the radio waves traveling horizontally through the atmosphere just above the surface of the Earth,  is the main mode in the LF band.<ref name=Seybold>
{{cite book
 |last  = Seybold   |first = John S.   |author-link=John S. Seybold
 |year  = 2005
 |title = Introduction to RF Propagation
 |publisher = John Wiley and Sons
 |isbn  = 0471743682
 |pages = 55–58
 |url   = https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 |access-date  = 2020-11-30  |via = Google books   |url-status   = live
 |archive-url  = https://web.archive.org/web/20210416234908/https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 |archive-date = 2021-04-16
}}
</ref> Ground waves are absorbed by the Earth as they travel, so the signal strength (power density) decreases exponentially with distance from the transmitting antenna, limiting transmission distance.  The attenuation of signal strength with distance is lower than at higher frequencies. Low frequency ground waves can be received up to {{convert|2000|km|mi}} from the transmitting antenna.  Ground waves must be [[vertical polarization|vertically polarized]] (the [[electric field]] is vertical while the [[magnetic field]] is horizontal), so vertical [[monopole antenna]]s are used for transmitting.

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 magazine |title=Understanding LF propagation |first=Alan (G3NYK) |last=Melia |magazine=[[Radcom]] |publisher=[[Radio Society of Great Britain]] |location=Bedford, UK |issue=9 |volume=85 |pages=32}}</ref>

==Uses==
===Radio broadcasting===
{{main | Longwave}}
[[AM broadcasting]] is authorized in the [[longwave]] band on frequencies between 148.5 and 283.5&nbsp;kHz in Europe and parts of Asia.

===Standard time signals===
{{further|Radio clock#List of radio time signal stations}}
[[File:Atomic clock.jpg|thumb|200px|{{center|An LF [[radio clock]]}}]]
In Europe and Japan, many low-cost consumer devices have since the late 1980s contained [[radio clock]]s with an LF receiver for these signals. Since these frequencies propagate by [[ground wave]] only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, and the receiver. In the United States, such devices became feasible for the mass market only after the output power of [[WWVB]] was increased in 1997 and 1999.

[[JJY]] transmitting broadcast on the exact same frequency, and has a similar [[timecode]].

===Military===
{{further|Communication with submarines}}
Radio signals below 50&nbsp;kHz are capable of penetrating ocean depths to approximately {{convert|200|metres|feet}}; the longer the wavelength, the deeper they go. The British, German, Indian, Russian, Swedish, United States,<ref>
{{cite web
 | title=Very Low Frequency (VLF)
 | year = 1998
 | department = United States Nuclear Forces
 | website = fas.org
 | url=http://www.fas.org/nuke/guide/usa/c3i/vlf.htm
 | access-date = 2008-01-09  | url-status=live
 | archive-url=https://web.archive.org/web/20071227165518/http://www.fas.org/nuke/guide/usa/c3i/vlf.htm
 | archive-date = 2007-12-27
}}
</ref>
and possibly other [[navy|navies]] communicate with [[submarine]]s on these frequencies.

In addition, [[Royal Navy]] nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the [[BBC Radio 4]] transmission on 198&nbsp;kHz in waters near the UK. It is rumoured that they are to construe a sudden halt in transmission, particularly of the morning news programme [[Today (BBC Radio 4)|''Today'']], as an indicator that the UK is under attack, whereafter their [[Letters of last resort|sealed orders]] take effect.<ref>
{{cite episode
 | title=The Human Button
 | series = [[Today (BBC Radio 4)|Today]]
 | air-date=2008-12-02
 | network=[[BBC]]
 | station=[[BBC Radio 4]]
 | url=https://www.bbc.co.uk/programmes/b00fq2sy
 | access-date=2011-08-06 | url-status=live
 | archive-url=https://web.archive.org/web/20110203232901/http://www.bbc.co.uk/programmes/b00fq2sy
 | archive-date=2011-02-03 | df = dmy-all
}}
</ref>

The United States has four LF stations maintaining contact with its submarine force: [[Aguada, Puerto Rico]], [[Naval Air Station Keflavik|Keflavik, Iceland]], [[Awase Airfield|Awase, Okinawa]], and [[Naval Air Station Sigonella|Sigonella, Italy]], using AN/FRT-95 solid state transmitters.

In the U.S., the [[Ground Wave Emergency Network]] or GWEN operated between 150 and 175&nbsp;kHz, until replaced by satellite communications systems in 1999. GWEN was a land based military radio communications system which could survive and continue to operate even in the case of a nuclear attack.

===Experimental and amateur===
The 2007&nbsp;[[World Radiocommunication Conference]] (WRC-07) made a worldwide amateur radio allocation in this band. An international 2.1&nbsp;kHz allocation, the [[2200-meter band|{{nobr|{{gaps|2|200}} meter band}}]] (135.7–137.9&nbsp;kHz) is available to [[amateur radio]] operators in several countries in Europe,<ref>
{{cite report
 |title=ERC {{nobr|Recommendation 62-01 E}}
 |year=1997
 |section=Use of the band 135.7–137.8&nbsp;kHz by the Amateur Service
 |id={{nobr|ERC Rec 62-01 E}}
 |publisher=[[European Conference of Postal and Telecommunications Administrations]] (CEPT)
 |place=Mainz, DE
}}
</ref>
New Zealand, Canada, US,<ref name=ARRL-bandchart>
{{cite AV media
 |title   = Regulatory Band Chart
 |date    = 2017-09-22
 |edition = 11×17″ color
 |medium  = chart / graphic
 |publisher = [[American Radio Relay League]]
 |place   = Newington, CT
 |url     = http://www.arrl.org/files/file/Regulatory/Band%20Chart/Band%20Chart%20-%2011X17%20Color.pdf
 |access-date  = 2020-11-26 |url-status=live
 |archive-url  = https://web.archive.org/web/20201111222535/http://www.arrl.org/files/file/Regulatory/Band%20Chart/Band%20Chart%20-%2011X17%20Color.pdf
 |archive-date = 2020-11-11  |df = dmy-all
}}
</ref>
and French overseas dependencies.

The world record distance for a two-way contact is over 10,000&nbsp;km from near [[Vladivostok]] to [[New Zealand]].<ref name=ZLUA0>
{{cite news
 |title = QSO ZL / UA0 on 136&nbsp;kHz
 |department = The World of LF
 |website = wireless.org.uk
 |url = http://www.wireless.org.uk/newspic92.htm
 |access-date = 2006-06-01   |url-status=live
 |archive-url = https://web.archive.org/web/20070929025210/http://www.wireless.org.uk/newspic92.htm
 |archive-date = 2007-09-29
}}
</ref>
As well as conventional [[Morse code]] many operators use very slow computer-controlled Morse code (so-called [[QRP operation#QRSS|"QRSS"]]) or specialized digital communications modes.

The UK allocated a 2.8&nbsp;kHz sliver of spectrum from 71.6&nbsp;kHz to 74.4&nbsp;kHz beginning in April&nbsp;1996 to UK amateurs who applied for a Notice of Variation to use the band on a noninterference basis with a maximum output power of 1&nbsp;Watt&nbsp;[[effective radiated power|ERP]]. This was withdrawn on 30&nbsp;June 2003 after a number of extensions in favor of the cross-European standard 136&nbsp;kHz band.<ref name=ofcom>
{{cite press release
 |title=UK Spectrum Strategy 2002
 |date=16 September 2016
 |publisher=[[Ofcom]]
 |url=http://www.ofcom.org.uk/static/archive/ra/topics/spectrum-strat/future/strat02/strategy02app_a.doc
 |access-date=5 June 2006 |url-status=live
 |archive-url=https://web.archive.org/web/20070930031406/http://www.ofcom.org.uk/static/archive/ra/topics/spectrum-strat/future/strat02/strategy02app_a.doc
 |archive-date=30 September 2007
}}
</ref>
Very slow Morse Code from G3AQC in the UK was received {{convert|3275|mi|km}} away, across the [[Atlantic Ocean]], by W1TAG in the US on 21-22&nbsp;November 2001 on 72.401&nbsp;kHz.{{efn|
Low-frequency experimenter Lawrence "Laurie" Mayhead, G3AQC, has added another LF accomplishment to his list – trans-Atlantic reception of his 73&nbsp;kHz signal. [...] Mayhead reports that on the night of 21-22&nbsp;November, his signal on 72.401&nbsp;kHz was received in the US. "I managed to transmit a full call sign to John Andrews, W1TAG, in Holden, Massachusetts", he said. Mayhead was using dual-frequency CW – or DFCW – featuring elements that are two minutes long; Andrews detected his signal using ARGO {{nobr|DSP software. — [[ARRL]] Nov 2001<ref>
{{cite news
 |title      = G3AQC's signal spans the Atlantic on 73&nbsp;kHz!
 |date       = 30 November 2001
 |periodical = The ARRL Letter
 |publisher  = [[American Radio Relay League]]
 |place      = Newington, CT
 |url = http://www.arrl.org/arrlletter?issue=2001-11-30
 |access-date = 12 January 2014  |url-status = live
 |archive-url=https://web.archive.org/web/20140112215834/http://www.arrl.org/arrlletter?issue=2001-11-30
 |archive-date=12 January 2014
}}
</ref>}}
}}

In the United States, there is an exemption within FCC Part&nbsp;15 regulations permitting unlicensed transmissions in the frequency range of 160–190&nbsp;kHz. Longwave radio hobbyists refer to this as the '[[LowFER]]' band, and experimenters, and their transmitters are called '[[LowFER]]s'. This frequency range between 160&nbsp;kHz and 190&nbsp;kHz is also referred to as the {{nobr|{{gaps|1|750}} meter}} band. Requirements<ref>
{{cite report
 |title = Part&nbsp;15 — Radio frequency devices
 |section = § 15.217 Operation in the band 160–190 kHz ({{nobr|47 CFR 15.217}})
 |series = The Electronic Code of Federal Regulations
 |website = ecfr.gov
 |id = {{nobr|47 CFR 15.217}} ({{nobr|47 CFR 15.206}})
 |url = http://www.ecfr.gov/cgi-bin/text-idx?SID=ea364b89f317550d594a9315deca6022&mc=true&node=pt47.1.15&rgn=div5#se47.1.15_1209
 | access-date = 17 March 2024
}}{{failed verification |date=March 2024}} Cited section {{nobr|47 CFR 15.206}} not present / removed: section id §15.205 skips to §15.207&nbsp;.
</ref> include:
* The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt.
* The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.
* All emissions below 160&nbsp;kHz or above 190&nbsp;kHz shall be attenuated at least 20&nbsp;dB below the level of the unmodulated carrier.
* As an alternative to these requirements, a field strength of 2400/F(kHz)&nbsp;microvolts/meter (measured at a distance of 300 meters) may be used (as described in 47CFR15.209).
* In all cases, operation may not cause harmful interference to licensed services.
Many experimenters in this band are amateur radio operators.<ref>
{{cite web
 |title = Part&nbsp;15 — Radio frequency devices
 |series = Federal Register 
 |url = http://www.ecfr.gov/cgi-bin/text-idx?SID=7f66d50bc733c74f45ff68ec5dda7d93&node=47:1.0.1.1.16&rgn=div5#47:1.0.1.1.16.3
 |access-date=2014-07-21  |url-status=live
 |archive-url=https://web.archive.org/web/20140726053324/http://www.ecfr.gov/cgi-bin/text-idx?SID=7f66d50bc733c74f45ff68ec5dda7d93&node=47:1.0.1.1.16&rgn=div5#47:1.0.1.1.16.3
 |archive-date=2014-07-26
}}
</ref>

===Meteorological information broadcasts===
{{Expand section|date=June 2008}}
A regular service transmitting [[radioteletype|RTTY]] marine meteorological information in [[SYNOP]] code on LF is the German Meteorological Service ([[Deutscher Wetterdienst]] or [http://www.dwd.de DWD]). The DWD operates station DDH47 on 147.3&nbsp;kHz using standard ITA-2 alphabet with a transmission speed of 50&nbsp;[[baud]] and FSK modulation with 85&nbsp;Hz shift.<ref>
{{cite web
 |title = DWD Sendeplan
 |access-date = 2008-01-08  |url-status = dead
 |url = http://www.dwd.de/de/wir/Geschaeftsfelder/Seeschifffahrt/Sendeplaene/Sendeplaene.htm
 |archive-url = https://archive.today/20120730093823/http://www.dwd.de/de/wir/Geschaeftsfelder/Seeschifffahrt/Sendeplaene/Sendeplaene.htm
 |archive-date = 2012-07-30
}}
</ref>

===Radio navigation signals===
{{Expand section|date=June 2008}}
In parts of the world where there is no longwave broadcasting service, [[Non-directional beacon]]s used for aeronavigation operate on 190–300&nbsp;kHz (and beyond into the MW band). In Europe, Asia and Africa, the NDB allocation starts on 283.5&nbsp;kHz.

The [[LORAN]]-C radio navigation system operated on 100&nbsp;kHz.

In the past, the [[Decca Navigator System]] operated between 70&nbsp;kHz and 129&nbsp;kHz. The last Decca chains were closed down in 2000.

[[Differential GPS]] telemetry transmitters operate between 283.5 and 325&nbsp;kHz.<ref>
{{cite report
 |first = Alan (G4TMV) |last = Gale
 |year = 2011
 |title = World DGPS database for DXers
 |version = 4.6
 |url = http://www.ndblist.info/datamodes/worldDGPSfreqorder.pdf
 |access-date = 2008-01-14   |url-status = dead   |via = ndblist.info
 |archive-url = https://web.archive.org/web/20110721225343/http://www.ndblist.info/datamodes/worldDGPSfreqorder.pdf
 |archive-date = 2011-07-21
}}
</ref>

The commercial "[[Datatrak]]" radio navigation system operates on a number of frequencies, varying by country, between 120–148&nbsp;kHz.

===Other applications===
Some radio frequency identification ([[RFID]]) tags utilize LF. These tags are commonly known as LFIDs or LowFIDs (low frequency identification). The LF RFID tags are [[near field and far field|near-field]] devices, interacting with the [[resonant inductive coupling|inductive near field]], rather than with radiated waves (radio waves) that are the only part of the electromagnetic field that persists into the far field. As such, they are technically ''not'' radio devices nor radio antennas, even though they do operate at radio frequencies, and are ''called'' "antennas" in the RFID trade, but not in [[radio engineering]]. It is more proper, and technically more informative to think of them as secondary coils of very loosely coupled [[transformer]]s.

==Antennas==
Since the ground waves used in this band require [[vertical polarization]], vertical antennas are used for transmission. [[Mast radiator]]s are most common, either insulated from the ground and fed at the bottom, or occasionally fed through guy-wires. [[T-antenna]]s and [[T-antenna#L-antenna|inverted L-antennas]] are used when antenna height is an issue.

LF transmitting antennas for high power transmitters require large amounts of space, and have been the cause of controversy in Europe and the United States, due to concerns about possible health hazards associated with [[electromagnetic radiation and health|human exposure to radio waves]].

===Longwave receiving antennas===
[[Image:Low cost DCF77 receiver.jpg|thumb|Low cost LF [[time signal]] [[Radio receiver design|''crystal receiver'']] using ferrite [[loop antenna]].]]
Antenna requirements for LF reception are much more modest than for transmission. Although non-resonant long wire antennas are sometimes used, ferrite [[loop antenna]]s are far more popular because of their small size.

Amateur radio operators have achieved good LF reception using [[active antenna]]s: A short whip with a built-in [[pre-amplifier]].

===Antenna heights===
Due to the long wavelengths in the band, nearly all LF antennas are [[electrically short]], shorter than one quarter of the radiated wavelength, so their low radiation resistance makes them inefficient, requiring very low resistance grounds and conductors to avoid dissipating transmitter power.
These electrically short antennas need [[loading coil]]s at the base of the antenna to bring them into resonance. Many antenna types, such as the [[umbrella antenna]] and L- and T-antenna, use capacitive top-loading (a "top hat"), in the form of a network of horizontal wires attached to the top of the vertical radiator. The [[capacitance]] improves the efficiency of the antenna by increasing the current, without increasing its height.

The height of antennas differ by usage. For some [[non-directional beacon]]s (NDBs) the height can be as low as 10&nbsp;meters, while for more powerful navigation transmitters such as [[Decca Navigator System|DECCA]], masts with a height around 100&nbsp;meters are used. [[T-antenna]]s have a height between 50–200&nbsp;meters, while mast aerials are usually taller than 150&nbsp;meters.

The height of mast antennas for [[LORAN-C]] is around 190&nbsp;meters for transmitters with radiated power below 500&nbsp;kW, and around 400&nbsp;meters for transmitters greater than {{nobr|{{gaps|1|000}} kilowatts.}} The main type of LORAN-C antenna is insulated from ground.

LF [[Longwave|(longwave) broadcasting stations]] use mast antennas with heights of more than 150&nbsp;meters or [[T-antenna|T-aerials]]. The mast antennas can be ground-fed insulated masts or upper-fed grounded masts. It is also possible to use cage antennas on grounded masts.

===Directional array antennas===
For broadcasting stations, directional antennas are often required. They consist of multiple masts, which often have the same height. Some longwave antennas consist of multiple mast antennas arranged in a circle with or without a mast antenna in the center. Such antennas focus the transmitted power toward ground and give a large zone of fade-free reception. This type of antenna is rarely used, because they are very expensive and require much space and because fading occurs on longwave much more rarely than in the medium wave range. One antenna of this kind was used by [[transmitter Orlunda]] in Sweden.

==Footnotes==
{{notelist}}

==See also==
{{div col begin|colwidth=12em}}
* [[2200-meter band]]
* [[Ground Wave Emergency Network]] (GWEN)
* [[Longwave]]
* [[LowFER]]
* [[Passive RFID]]  <!-- * [[LowFID passive RFID tags]] -->
* [[Time signal#Radio time sources|Time signal]]
* [[WGU-20]]
{{div col end}}

==References==
{{reflist|25em}}

== Further reading ==
{{refbegin|colwidth=25em|small=yes}}
* {{cite book
 |first1=G. |last1=Klawitter
 |first2=M. |last2=Oexner
 |first3=K. |last3=Herold
 |year=2000
 |title=Langwelle und Längstwelle  |lang=de
 |trans-title=Longwave and Longest Wave
 |place=Meckenheim, DE
 |publisher=Siebel Verlag GmbH
 |isbn=3-89632-043-2
}}
* {{cite book
 |last=Marten |first=M.
 |year=2007
 |title=Spezial-Frequenzliste 2007/2008  |lang=de
 |trans-title=Special Frequency List 2007–2008
 |place=Meckenheim, DE
 |publisher=Siebel Verlag GmbH
 |isbn=978-3-88180-665-7
 |pages=36–39
}}
* {{cite book
 |last1=Dennison  |first1=Mike (G3XDV)
 |last2=Moritz    |first2=Jim (M0BMU)
 |year=2007
 |title=LF Today
 |place = Potters Bar, UK
 |publisher=[[Radio Society of Great Britain]]
 |isbn=978-1-905086-36-8
}}
{{refend}}

==External links==
{{refbegin|colwidth=25em|small=yes}}
* {{cite web
 |last=Stimac |first=Tomislav (IK1QFK)
 |title=Definition of frequency bands (VLF, ELF... ''etc''.)
 |website=vlf.it
 |url=http://www.vlf.it/frequency/bands.html
}}
* {{cite web
 |title=IK1QFK
 |type=home page
 |website=vlf.it
 |url=http://www.vlf.it
}}
{{refend}}

{{Navboxes
| list =
{{Radio spectrum}}
{{Electromagnetic spectrum}}
{{Audio broadcasting}}
{{Time signal stations}}
{{Time measurement and standards}}
{{Time topics}}
}}

{{DEFAULTSORT:Low Frequency}}
[[Category:Radio spectrum]]