View source for Very low frequency

{{Short description|The range 3–30 kHz of the electromagnetic spectrum}}
{{Redirect|VLF|the car company|VLF Automotive}}
{{MWband
| freq = 3–30&nbsp;[[kHz]]
| wave = 100-10 km
}}
[[File:VLFatPalmer.JPG|236px|thumb|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by [[Stanford University]] ]]

'''Very low frequency''' or '''VLF''' 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 3–30&nbsp;[[kHz]], corresponding to [[wavelength]]s from 100 to 10&nbsp;km, respectively. The band is also known as the '''myriameter band''' or '''myriameter wave''' as the wavelengths range from one to ten [[myriameter]]s (an obsolete metric unit equal to 10&nbsp;kilometers). Due to its limited [[Bandwidth (signal processing)|bandwidth]], [[Audio signal|audio]] (voice) transmission is highly impractical in this band, and therefore only low-[[Data signaling rate|data-rate]] coded signals are used. The VLF band is used for a few [[radio navigation]] services, government [[Time signal|time radio stations]] (broadcasting time signals to set [[radio clock]]s) and secure military communication. Since VLF waves can penetrate at least {{convert|40|m|ft|sp=us}} into saltwater, they are used for [[military communication]] with [[submarine]]s.

==Propagation characteristics==
Because of their long wavelengths, VLF radio waves can [[Diffraction|diffract]] around large obstacles and so are not blocked by mountain ranges, and they can propagate as [[ground wave]]s following the curvature of the Earth and so are not limited by the horizon. Ground waves are absorbed by the resistance of the Earth and are less important beyond several hundred to a thousand kilometres/miles, and the main mode of long-distance propagation is an [[Earth–ionosphere waveguide]] mechanism.<ref name="Hunsucker">{{Cite book |last=Hunsucker |first=R.D. |url=https://books.google.com/books?id=IQWHj2bgcxcC&pg=PA419 |title=The high-latitude ionosphere and its effects on radio propagation |last2=Hargreaves |first2=John Keith |publisher=Cambridge University Press |year=2002 |isbn=978-0-521-33083-1 |page=419}}</ref> The Earth is surrounded by a conductive layer of [[electron]]s and [[ion]]s in the upper atmosphere at the bottom of the [[ionosphere]] called the [[D layer]] at {{convert|60|–|90|km|mi|abbr=on}} altitude,<ref name="Ghosh">{{Cite book |last=Ghosh |first=S.N. |url=https://books.google.com/books?id=6Mvf4-gsVycC&pg=PA89 |title=Electromagnetic theory and wave propagation |publisher=CRC Press |year=2002 |isbn=978-0-8493-2430-7 |page=89}}</ref> which reflects VLF radio waves. The conductive ionosphere and the conductive Earth form a horizontal "duct" a few VLF wavelengths high, which acts as a [[waveguide]] confining the waves so they don't escape into space. The waves travel in a zig-zag path around the Earth, reflected alternately by the Earth and the ionosphere, in [[transverse magnetic]] (TM) mode.

VLF waves have very low path attenuation, 2–3&nbsp;dB per 1,000&nbsp;km,<ref name="Hunsucker" /> with little of the "[[fading]]" experienced at higher frequencies.<ref name="Ghosh" />  This is because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the [[F1 layer|F1]] and [[F2 layer|F2]] layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long-distance communication. Propagation distances of 5,000–20,000&nbsp;km have been realized.<ref name="Hunsucker" /> However, atmospheric noise ("[[sferics]]") is high in the band,<ref name="Ghosh" /> including such phenomena as "[[whistler (radio)|whistler]]s", caused by [[lightning]].
* VLF waves can penetrate [[seawater]] to a depth of at least {{convert|10|–|40|m|ft|abbr=off|sp=us}}, depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines.
* VLF waves at certain frequencies have been found to cause [[electron precipitation]].
* VLF waves used to communicate with submarines have created an artificial bubble around the Earth that can protect it from [[solar flare]]s and [[coronal mass ejection]]s; this occurred through interaction with high-energy radiation particles.<ref>{{Cite web |last=Susannah Darling |date=17 May 2017 |title=NASA’s Van Allen Probes Spot Man-Made Barrier Shrouding Earth |url=https://www.nasa.gov/missions/van-allen-probes/nasas-van-allen-probes-spot-man-made-barrier-shrouding-earth/ |access-date=11 June 2024 |publisher=NASA}}</ref>
{{Clear}}

==Antennas==
{{multiple image
| align             = center
| direction         = horizontal
| image1            = Cutler VLF antenna array.png
| caption1          = "Trideco" antenna tower array at the US Navy's [[VLF Transmitter Cutler|Naval Radio Station Cutler]] in Cutler, Maine, USA. The central mast is the radiating element, while the star-shaped horizontal wire array is the capacitive top load. About {{cvt|1.2|mi|km|0|order=flip}} in diameter, it communicates with submerged submarines at 24&nbsp;kHz (12,500&nbsp;meter wavelength) at a power of 1.8&nbsp;megawatts, one of the most powerful radio stations in the world.
| width1            = 370
| image2            = VLF umbrella antenna - Anthorn Radio Station UK - central mast.jpg
| caption2          = Central mast of a similar "trideco" antenna of the NATO VLF transmitter at [[Anthorn radio station]], UK, showing six insulator strings attaching the toploads to the six vertical radiator wires
| width2            = 223
| image3            = Jim Creek VLF antenna.png
| caption3          = Another type of large VLF antenna: the "valley-span" antenna, consisting of one or more long horizontal topload cables spanning a valley, fed in the center by vertical radiator cables. This example is at the US Navy [[Jim Creek Naval Radio Station|Jim Creek station]] near [[Seattle]], which transmits on 24.8&nbsp;kHz at a power of 1.2&nbsp;MW.
| width3            = 265
| image4            = Tsushima Omega Tower 1977 2.jpg
| caption4          = [[Umbrella antenna]] of the [[Omega (navigation system)|Omega navigation system]] beacon on [[Tsushima Island]], Japan, which transmitted at 10–14&nbsp;kHz; 389&nbsp;meters high, [[:ja:対馬オメガ局|it was dismantled in 1998]].
| width4            = 220
| footer            = 
}}

A major practical drawback to the VLF band is that because of the length of the waves, full size resonant antennas ([[half wave dipole]] or [[monopole antenna|quarter wave monopole]] antennas) cannot be built because of their physical height.<ref name="Watt" />{{rp|pages=14}}  Vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30&nbsp;kHz (10&nbsp;km wavelength) would be {{convert|2.5|km|ft|abbr=off}} high. So practical transmitting antennas are [[electrical length|electrically short]], a small fraction of the length at which they would be self-resonant.<ref name="Seybold">{{Cite book |last=Seybold |first=John S. |url=https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57 |title=Introduction to RF Propagation |publisher=John Wiley and Sons |year=2005 |isbn=978-0471743682 |pages=55–58}}</ref><ref name="Johnson-1993">{{Cite book |url=http://seklad69associates.com/seklad69associates.com/EEG_808_and_815_files/Antenna%20Engineering%20Handbook.pdf |title=Antenna Engineering Handbook |publisher=McGraw-Hill |year=1993 |isbn=007032381X |editor-last=Johnson |editor-first=Richard C. |edition=3rd}}</ref>{{rp|pages=&nbsp;24.5–24.6}} Due to their low [[radiation resistance]] (often less than one ohm) they are inefficient, radiating only 10% to 50% of the transmitter power at most,<ref name="Hunsucker" /><ref name="Watt" />{{rp|page=14}} with the rest of the power dissipated in the antenna/ground system resistances. Very high power transmitters (~1&nbsp;megawatt) are required for long-distance communication, so the efficiency of the antenna is an important factor.

[[File:Flattop antenna 1912.png|thumb|A "triatic" or "[[Inverted-L antenna|flattop]]" antenna, another common VLF transmitting antenna. It consists of vertical radiator wires each connected at top to parallel horizontal capacitive topload wires stretching up to a kilometer, supported on tall towers. The transverse support cables suspending the horizontal wires are called "triatics".]]

=== VLF transmitting antennas ===
High power VLF transmitting stations use capacitively-toploaded [[monopole antenna]]s. These are very large wire antennas, up to several kilometers long.<ref name="NAVELEX-0101-113">{{Cite book |url=http://www.navy-radio.com/manuals/0101-1xx/0101_113-03.pdf |title=Naval Shore Electronics Criteria - VLF, LF, and MF communications systems |date=August 1972 |publisher=U.S. Navy |location=Washington, DC |id=Manual NAVELEX 0101-113 |department=Naval Electronics Systems Command}}</ref>{{rp|pages=&nbsp;3.9–3.21}}<ref name=Johnson-1993/>{{rp|pages=&nbsp;24.8–24.12}} They consist of a series of steel [[radio mast]]s, linked at the top with a network of cables, often shaped like an umbrella or clotheslines.<ref name="Watt" />{{rp|page=p.14}} Either the towers themselves or vertical wires serve as [[monopole antenna|monopole]] radiators, and the horizontal cables form a ''capacitive top-load'' to increase the current in the vertical wires, increasing the radiated power and efficiency of the antenna. High-power stations use variations on the [[umbrella antenna]] such as the "delta" and "[[Umbrella antenna#Trideco antenna|trideco]]" antennas, or multiwire [[T-aerial|flattop]] (triatic) antennas.<ref name="Watt">{{Cite book |last=Watt |first=Arthur D. |url=https://archive.org/details/VLFRadioEngineering/page/n143/mode/2up |title=VLF Radio Engineering |publisher=Pergamon Press |year=1967}}</ref>{{rp|pages=p.129–162}}  For low-power transmitters, inverted-L and [[T-aerial|T antennas]] are used.

Due to the low radiation resistance, to minimize power dissipated in the ground these antennas require extremely low resistance [[ground (electricity)|ground]] (Earthing) systems, consisting of radial networks of buried copper wires under the antenna. To minimize [[dielectric loss]]es in the soil, the ground conductors are buried shallowly, only a few inches in the ground, and the ground surface near the antenna is sometimes protected by copper ground screens. [[Counterpoise (ground system)|Counterpoise]] systems have also been used, consisting of radial networks of copper cables supported several feet above the ground under the antenna.

A large [[loading coil]] is required at the antenna feed point to cancel the [[capacitive reactance]] of the antenna to make it [[resonant]]. At VLF the design of this coil is challenging; it must have low resistance at the operating RF frequency, [[Q factor|high {{mvar|Q}}]], must handle very high currents, and must withstand the extremely high voltage on the antenna. These are usually huge air core coils 2–4 meters high wound on a nonconductive frame, with RF resistance reduced by using thick [[litz wire]] several centimeters in diameter, consisting of thousands of insulated strands of fine wire braided together.<ref name="Watt" />{{rp|page=p.95}}

The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a [[Q factor|high {{mvar|Q}}]] [[tuned circuit]]. VLF antennas have very narrow [[bandwidth (signal processing)|bandwidth]] and to change the transmitting frequency requires a variable inductor ([[Transformer types#Variometer and variocoupler|variometer]]) to tune the antenna. The large VLF antennas used for high-power transmitters usually have bandwidths of only 50–100&nbsp;hertz. The high {{mvar|Q}} results in very high voltages (up to 250&nbsp;kV)<ref name="Watt" />{{rp|page=p.58}} on the antenna and very good insulation is required.<ref name="Watt" />{{rp|page=p.14,19}} Large VLF antennas usually operate in 'voltage limited' mode: the maximum power of the transmitter is limited by the voltage the antenna can accept without [[electrical breakdown|air breakdown]], [[corona discharge|corona]], and arcing from the antenna.

=== Dynamic antenna tuning ===
The bandwidth of large capacitively loaded VLF antennas is so narrow (50–100&nbsp;Hz) that even the small frequency shifts of FSK and MSK modulation may exceed it, throwing the antenna out of [[resonance]], causing the antenna to reflect some power back down the feedline. The traditional solution is to use a "bandwidth resistor" in the antenna which reduces the {{mvar|Q}}, increasing the bandwidth; however this also reduces the power output. A recent alternative used in some military VLF transmitters is a circuit which dynamically shifts the antenna's [[resonant frequency]] between the two output frequencies with the modulation.<ref name=Johnson-1993/>{{rp|page=&nbsp;24.7}}<ref name=NAVELEX-0101-113/>{{rp|page=&nbsp;3.36}} This is accomplished with a [[saturable reactor]] in series with the antenna [[loading coil]]. This is a [[magnetic core|ferromagnetic core]] [[inductor]] with a second control winding through which a DC current flows, which controls the inductance by magnetizing the core, changing its [[magnetic permeability|permeability]]. The keying datastream is applied to the control winding. So when the frequency of the transmitter is shifted between the '1' and '0' frequencies, the saturable reactor changes the inductance in the antenna resonant circuit to shift the antenna resonant frequency to follow the transmitter's frequency.

=== VLF receiving antennas ===
The requirements for receiving antennas are less stringent, because of the high level of natural [[atmospheric noise]] in the band. At VLF frequencies atmospheric [[radio noise]] is far above the [[Noise figure|receiver noise]] introduced by the receiver circuit and determines the receiver [[signal-to-noise ratio]]. So small inefficient receiving antennas can be used, and the low voltage signal from the antenna can simply be amplified by the receiver without introducing significant noise. Ferrite [[loop antenna]]s are usually used for reception.

== Modulation ==
Because of the small [[bandwidth (signal processing)|bandwidth]] of the band, and the extremely narrow bandwidth of the antennas used, it is impractical to transmit [[audio signal]]s ([[amplitude modulation|AM]] or [[frequency modulation|FM]] [[radiotelephony]]).<ref name="Holtet">{{Cite conference |date=17–27 April 1974 |editor-last=Holtet |editor-first=J.A. |title=ELF-VLF Radio Wave Propagation |url=https://books.google.com/books?id=K3HyCAAAQBAJ&pg=PA373 |conference=NATO Advanced Study Institute |publisher=Springer Science and Business Media |pages=372–373 |isbn=9789401022651 |place=Spåtind, Norway}}</ref> A typical AM radio signal with a bandwidth of 10&nbsp;kHz would occupy one third of the VLF band. More significantly, it would be difficult to transmit any distance because it would require an antenna with 100 times the bandwidth of current VLF antennas, which due to the [[Chu-Harrington limit]] would be enormous in size. Therefore, only text data can be transmitted, at low [[bit rate]]s. In military networks   [[frequency-shift keying]] (FSK) [[modulation]] is used to transmit [[radioteletype]] data using 5&nbsp;bit [[ITA2]] or 8&nbsp;bit [[ASCII]] character codes. A small frequency shift of 30–50&nbsp;hertz is used due to the small bandwidth of the antenna.

In high power VLF transmitters, to increase the allowable data rate, a special form of FSK called [[minimum-shift keying]] (MSK) is used. This is required due to the high {{mvar|[[Q factor|Q]]}} of the antenna.<ref name=NAVELEX-0101-113/>{{rp|pages=&nbsp;3.2–3.4, §3.1.1}} The huge capacitively-loaded antenna and [[loading coil]] form a high {{mvar|Q}} [[tuned circuit]], which stores oscillating electrical energy. The {{mvar|Q}} of large VLF antennas is typically over 200; this means the antenna stores far more energy (200&nbsp;times as much) than is supplied or radiated in any single cycle of the transmitter current. The energy is stored alternately as [[electrostatic energy]] in the topload and ground system, and magnetic energy in the vertical wires and loading coil. VLF antennas typically operate "voltage-limited", with the voltage on the antenna close to the limit that the insulation will stand, so they will not tolerate any abrupt change in the voltage or current from the transmitter without arcing or other insulation problems. As described below, MSK is able to modulate the transmitted wave at higher data rates without causing voltage spikes on the antenna.

The three types of [[modulation]] that have been used in VLF transmitters are:
;[[Continuous Wave]] (CW), Interrupted Continuous Wave (ICW), or On-Off Keying: [[Morse code]] [[radiotelegraphy]] transmission with unmodulated carrier. The carrier is turned on and off, with carrier on representing the Morse code "dots" and "dashes" and carrier off representing spaces. The simplest and earliest form of radio data transmission, this was used from the beginning of the 20th&nbsp;century to the 1960s in commercial and military VLF stations. Because of the high [[Q factor|antenna {{mvar|Q}}]] the carrier cannot be switched abruptly on and off but requires a long time constant, many cycles, to build up the oscillating energy in the antenna when the carrier turns on, and many cycles to dissipate the stored energy when the carrier turns off. This limits the data rate that can be transmitted to 15–20&nbsp;words/minute. CW is now only used in small hand-keyed transmitters, and for testing large transmitters.
;[[Frequency-shift keying]] (FSK): FSK is the second oldest and second simplest form of digital radio data modulation, after CW. For FSK, the carrier shifted between two frequencies, one representing the binary digit '1' and the other representing binary '0'. For example, a frequency of 9070&nbsp;Hz might be used to indicate a '1' and the frequency 9020&nbsp;Hz, 50&nbsp;Hz lower, to indicate a '0'. The two frequencies are generated by a continuously running [[frequency synthesizer]]. The transmitter is periodically switched between these frequencies to represent 8&nbsp;bit [[ASCII]] codes for the characters of the message. A problem at VLF is that when the frequency is switched the two [[sine wave]]s usually have different [[phase (waves)|phase]]s, which creates a sudden phase-shift transient which can cause arcing on the antenna. To avoid arcing, FSK can only be used at slow rates of 50–75&nbsp;bit/s.
;[[Minimum-shift keying]] (MSK): A [[continuous phase modulation|continuous phase]] version of FSK designed specifically for small bandwidths, this was adopted by naval VLF stations in the 1970s to increase the data rate and is now the standard mode used in military VLF transmitters. If the two frequencies representing '1' and '0' are 50&nbsp;Hz apart, the standard frequency shift used in military VLF stations, their phases coincide every 20&nbsp;ms. In MSK the frequency of the transmitter is switched only when the two sine waves have the same phase, at the point both sine waves cross zero in the same direction. This creates a smooth continuous transition between the waves, avoiding transients which can cause stress and arcing on the antenna. MSK can be used at data rates up to 300&nbsp;bit/s, or about 35&nbsp;[[ASCII]] characters (8&nbsp;bits each) per second, approximately 450&nbsp;words per minute.

== Applications ==
[[Image:Grimetonmasterna.jpg|thumb|right|[[T antenna|Flattop antenna]] towers of the [[Grimeton Radio Station|Grimeton VLF transmitter]], Varberg, Sweden]]

=== Early wireless telegraphy ===
Historically, this band was used for long distance transoceanic radio communication during the [[wireless telegraphy]] era between about 1905 and 1925. Nations built networks of high-power LF and VLF [[radiotelegraphy]] stations that transmitted text information by [[Morse code]], to communicate with other countries, their colonies, and naval fleets. Early attempts were made to use radiotelephone using [[amplitude modulation]] and [[single-sideband modulation]] within the band starting from 20&nbsp;kHz, but the result was unsatisfactory because the available bandwidth was insufficient to contain the [[sideband]]s.

In the 1920s the discovery of the [[skywave]] (skip) radio propagation method allowed lower power transmitters operating at [[high frequency]] to communicate at similar distances by reflecting their radio waves off a layer of [[ionization|ionized]] atoms in the [[ionosphere]], and long-distance radio communication stations switched to the [[shortwave]] frequencies. The [[Grimeton VLF transmitter]] at Grimeton near Varberg in [[Sweden]], one of the few remaining transmitters from that era that has been preserved as a historical monument, can be visited by the public at certain times, such as on [[Alexanderson Day]].

=== Navigation beacons and time signals ===
Due to its long propagation distances and stable phase characteristics, during the 20th&nbsp;century the VLF band was used for long range [[hyperbolic navigation|hyperbolic]] [[radio navigation]] systems which allowed ships and aircraft to determine their geographical position by comparing the phase of radio waves received from fixed VLF [[navigation beacon]] transmitters.

The worldwide [[Omega (navigation system)|Omega]] system used frequencies from 10 to 14&nbsp;kHz, as did Russia's [[Alpha (navigation)|Alpha]].

VLF was also used for [[standard time and frequency signal|standard time and frequency]] broadcasts. In the US, the [[time signal]] station [[WWVL]] began transmitting a 500&nbsp;W signal on 20&nbsp;kHz in August&nbsp;1963. It used frequency-shift keying ([[Frequency-shift keying|FSK]]) to send data, shifting between 20&nbsp;kHz and 26&nbsp;kHz. The WWVL service was discontinued in July&nbsp;1972.

=== Geophysical and atmospheric measurement ===
Naturally occurring signals in the VLF band are used by [[geophysicist]]s for long range lightning location and for research into atmospheric phenomena such as the aurora. Measurements of [[Whistler (radio)|whistlers]] are employed to infer the physical properties of the [[magnetosphere]].<ref>{{Cite web |title=AWDANet |url=http://plasmon.elte.hu/awdanet.htm |website=plasmon.elte.hu}}</ref>

[[Geophysicists]] use VLF-[[electromagnetic]] receivers to measure conductivity in the near surface of the Earth.<ref>{{Cite web |title=Geonics Limited - VLF Receivers |url=http://www.geonics.com/html/vlfsystems.html |access-date=13 June 2014}}</ref>

VLF signals can be measured as a [[Geophysics|geophysical]] [[electromagnetic]] survey that relies on transmitted currents inducing secondary responses in conductive geologic units. A VLF anomaly represents a change in the attitude of the electromagnetic vector overlying conductive materials in the subsurface.

=== Mine communication systems ===
VLF can also penetrate soil and rock for some distance, so these frequencies are also used for [[through-the-earth mine communications]] systems.

=== Military communications ===
Powerful VLF transmitters are used by the military to communicate with their forces worldwide. The advantage of VLF frequencies is their long range, high reliability, and the prediction that in a [[nuclear war]] VLF communications will be less disrupted by nuclear explosions than higher frequencies. Since it can penetrate seawater VLF is used by the military to [[Communication with submarines|communicate with submarines]] near the surface, while [[Extremely low frequency|ELF]] frequencies are used for deeply submerged subs.

Examples of naval VLF transmitters are
* Britain's [[Skelton Transmitting Station]] in Skelton, Cumbria
* Germany's [[VLF transmitter DHO38|DHO38]] in [[Rhauderfehn]], which transmits on 23.4&nbsp;kHz with a power of 800&nbsp;kW
* U.S. [[Jim Creek Naval Radio Station]] in [[Oso, Washington]] state, which transmits on 24.8&nbsp;kHz with a power of 1.2&nbsp;MW
* U.S. [[VLF Transmitter Cutler|Cutler Naval Radio Station]] at [[Cutler, Maine]] which transmits on 24&nbsp;kHz with 1.8&nbsp;MW.

Since 2004 the [[US Navy]] has stopped using ELF transmissions, with the statement that improvements in VLF communication has made them unnecessary, so it may have developed technology to allow submarines to receive VLF transmissions while at operating depth.

High power land-based and aircraft transmitters in countries that operate submarines send signals that can be received thousands of miles away. Transmitter sites typically cover great areas (many [[acres]] or square kilometers), with transmitted power anywhere from 20&nbsp;kW to 2,000&nbsp;kW. Submarines receive signals from land based and aircraft transmitters using some form of towed antenna that floats just under the surface of the water – for example a [[Buoyant Cable Array Antenna]] (BCAA).

Modern receivers use sophisticated [[digital signal processing]] techniques to remove the effects of atmospheric noise (largely caused by lightning strikes around the world) and adjacent channel signals, extending the useful reception range. Strategic nuclear bombers of the United States Air Force receive VLF signals as part of hardened nuclear resilient operations.

Two alternative character sets may be used: 5&nbsp;bit [[ITA2]] or 8&nbsp;bit [[ASCII]]. Because these are military transmissions they are almost always [[encrypted]] for security reasons. Although it is relatively easy to receive the transmissions and convert them into a string of characters, enemies cannot decode the encrypted messages; military communications usually use unbreakable [[one-time pad]] [[cipher]]s since the amount of text is so small.

==Amateur use==
The frequency range below 8.3&nbsp;kHz is not allocated by the [[International Telecommunication Union]] and in some nations may be used license-free.
Radio amateurs in some countries have been granted permission (or have assumed permission) to operate at frequencies below 8.3&nbsp;kHz.<ref>{{Cite web |title=Sub 9&nbsp;kHz spectrum in the Amateur Service |url=http://www.cept.org/Documents/wg-fm/24788/fm-15-119_sub-9-khz-spectrum-in-the-amateur-service |access-date=13 May 2017}}</ref>

Operations tend to congregate around the frequencies 8.27&nbsp;kHz, 6.47&nbsp;kHz, 5.17&nbsp;kHz, and 2.97&nbsp;kHz.<ref>{{Cite web |title=Some recent milestones with amateur radio experiments at VLF |url=http://abelian.org/vlf/amateur-radio |access-date=13 May 2017}}</ref> Transmissions typically last from one hour up to several days and both receiver and transmitter must have their frequency locked to a stable reference such as a [[GPS disciplined oscillator]] or a [[rubidium standard]] in order to support such long duration coherent detection and decoding.

===Amateur equipment===
Radiated power from amateur stations is very small, ranging from 1&nbsp;μW to 100&nbsp;μW for fixed base station antennas, and up to 10&nbsp;mW from kite or balloon antennas. Despite the low power, stable propagation with low attenuation in the [[Earth-ionosphere waveguide|earth-ionosphere cavity]] enable very narrow bandwidths to be used to reach distances up to several thousand kilometers. The modes used are [[QRP operation#QRSS|QRSS]], [[Multiple frequency-shift keying|MFSK]], and coherent [[Phase-shift keying|BPSK]].

The transmitter generally consists of an audio amplifier of a few hundred watts, an impedance matching transformer, a [[loading coil]] and a large wire antenna. Receivers employ an electric field probe or magnetic loop antenna, a sensitive audio preamplifier, isolating transformers, and a PC [[sound card]] to digitise the signal. Extensive [[digital signal processing]] is required to retrieve the weak signals from beneath [[Electromagnetic interference|interference]] from [[Harmonics (electrical power)|power line harmonics]] and [[Radio atmospheric|VLF radio atmospherics]]. Useful received signal strengths are as low as {{val|3|e=-8}}&nbsp;volts/meter (electric field) and {{val|1|e=-16}}&nbsp;tesla (magnetic field), with [[signaling rate]]s typically between 1 and 100&nbsp;bits per hour.

===PC based reception===
[[File:VLF 18.1 kHz spectrogram.svg|thumb|Timing diagram of a frequency-shift keyed 18.1&nbsp;kHz VLF signal, picked up using a small [[loop antenna]] and a sound card. The Morse code says "..33376.."; the vertical stripes are distant lightning strikes.]]
VLF signals are often monitored by [[radio amateur]]s using simple homemade VLF [[radio receiver]]s based on personal computers (PCs).<ref>{{Cite book |last=Renato Romero, IK1QFK |title=Radio Nature |publisher=[[Radio Society of Great Britain]] |year=2008 |isbn=9781-9050-8637-5 |pages=77}}</ref><ref>{{Cite conference |last=Mardina Abdullah |display-authors=etal |year=2013 |title=Development of UKM-SID teaching module for space science education |conference=6th International Forum on Engineering Education (IFEE 2012) |volume=102 |pages=80–85 |doi=10.1016/j.sbspro.2013.10.716 |doi-access=free |orig-year=2012 |journal=Procedia - Social and Behavioral Sciences}}</ref> An aerial in the form of a coil of insulated wire is connected to the input of the soundcard of the PC (via a jack plug) and placed a few meters away from it. [[Fast Fourier transform]] (FFT) software in combination with a sound card allows reception of all frequencies below the [[Nyquist frequency]] simultaneously in the form of [[spectrogram]]mes.

Because CRT monitors are strong sources of noise in the VLF range, it is recommended to record the spectrograms with any PC CRT monitors turned off. These spectrograms show many signals, which may include VLF transmitters and the horizontal electron beam deflection of TV sets. The strength of the signal received can vary with a [[Sudden Ionospheric Disturbance|sudden ionospheric disturbance]]. These cause the ionization level to increase in the ionosphere producing a rapid change to the amplitude and phase of the received VLF signal.

== List of VLF transmissions ==
For a more detailed list, see [[List of VLF-transmitters]]

{| class="wikitable sortable"
|-
! Callsign
! Frequency
! Location of transmitter
! Remarks
|- align="center" valign="top"
|     — || 11.905&nbsp;kHz || various locations (Russia) || [[Alpha (radio navigation)|Alpha-Navigation]]
|- align="center" valign="top"
|     — || 12.649&nbsp;kHz || various locations (Russia) || [[Alpha (radio navigation)|Alpha-Navigation]]
|- align="center" valign="top"
|     — || 14.881&nbsp;kHz || various locations (Russia) || [[Alpha (radio navigation)|Alpha-Navigation]]
|- align="center" valign="top"
| [[HWU transmitter|HWU]] || 15.1&nbsp;kHz || Rosnay (France) || 400&nbsp;kW<ref name=Fidani-2011/>
|- align="center" valign="top"
|- align="center" valign="top"
|     — || 15.625&nbsp;kHz || — || Frequency for horizontal deflection of electron beam in [[CRT television]]s ([[576i]])
|- align="center" valign="top"
|     — || 15.734&nbsp;kHz || — || Frequency for horizontal deflection of electron beam in [[CRT television]]s ([[480i]])
|- align="center" valign="top"
| [[Noviken VLF Transmitter|JXN]] || 16.4&nbsp;kHz || [[Gildeskål Municipality]] (Norway) ||
|- align="center" valign="top"
| [[VLF transmitter Grimeton|SAQ]] || 17.2&nbsp;kHz || [[VLF transmitter Grimeton|Grimeton]] (Sweden) || Only active at special occasions {{nowrap|([[Alexanderson Day]])}}
|- align="center" valign="top"
| [[VLF Transmitter Cutler|NAA]] || 17.8&nbsp;kHz || VLF station (NAA) at [[Cutler, Maine|Cutler]], Maine (US)<ref>{{Cite web |title=Shortwave HF VHF UHF utilities monitoring |url=http://www.random-abstract.com/radio/ |website=Random Abstract}}</ref> || 
|- align="center" valign="top"
| RDL UPD UFQE UPP UPD8 || 18.1&nbsp;kHz || various locations, including [[Matochkin Shar]] (Russia)<ref name=Fidani-2011/> ||
|- align="center" valign="top"
| [[HWU transmitter|HWU]] || 18.3&nbsp;kHz || [[Le Blanc]] (France) || Frequently inactive for long periods
|- align="center" valign="top"
| RKS || 18.9&nbsp;kHz || various locations (Russia) || Rarely active
|- align="center" valign="top"
| [[Anthorn Radio Station|GQD]] || 19.6&nbsp;kHz || [[Anthorn radio station|Anthorn]] (UK) || Many operation modes.
|- align="center" valign="top"
| [[Naval Communication Station Harold E. Holt|NWC]] || 19.8&nbsp;kHz || [[Exmouth, Western Australia|Exmouth]], Western Australia (AUS) || Used for submarine communication, 1&nbsp;megawatt.<ref>{{Cite news |date=14 November 2008 |title=Naval base link to jet plunge |url=http://www.smh.com.au/news/travel/naval-base-link-to-qantas-plunge/2008/11/14/1226318890475.html |access-date=14 November 2008 |work=[[The Sydney Morning Herald]] |location=Sydney, AU}}</ref>
|- align="center" valign="top"
| ICV || 20.27&nbsp;kHz || [[Tavolara Island|Tavolara]] (Italy) ||
|-
|- align="center" valign="top"
| RJH63 RJH66 [[Vileyka VLF transmitter|RJH69]]  RJH77 RJH99 || 20.5&nbsp;kHz || various locations (Russia) || [[Beta (time signal)|Time signal transmitter Beta]]
|- align="center" valign="top"
| ICV || 20.76&nbsp;kHz || Tavolara (Italy) ||
|- align="center" valign="top"
| [[HWU transmitter|HWU]] || 20.9&nbsp;kHz || Saint-Assise (France)<!-- Saint-Assise seems not to exist as a location in France, according to Google Maps --><ref name="Fidani-2011">{{Cite journal |last=Fidani |first=Cristiano |date=8 December 2011 |title=The Central Italy Electromagnetic Network and the 2009 L'Aquila earthquake: Observed electric activity |url=http://www.mdpi.com/2076-3263/1/1/3/pdf |journal=Geosciences |publication-date=2012 |volume=1 |issue=1 |pages=3–25 |bibcode=2011Geosc...1....3F |doi=10.3390/geosciences1010003 |issn=2076-3263 |access-date=14 January 2021 |doi-access=free}}</ref> ||
|- align="center" valign="top"
| RDL || 21.1&nbsp;kHz || various locations (Russia) || rarely active
|- align="center" valign="top"
| [[Lualualei VLF transmitter|NPM]] || 21.4&nbsp;kHz || Hawaii (USA) ||
|- align="center" valign="top"
| [[HWU transmitter|HWU]] || 21.75&nbsp;kHz || Rosnay (France)<ref name=Fidani-2011/> ||
|- align="center" valign="top"
| [[Skelton Transmitting Station|GZQ]] || 22.1&nbsp;kHz || [[Skelton transmitting station|Skelton]] (UK) ||
|- align="center" valign="top"
| [[Ebino VLF transmitter|JJI]] || 22.2&nbsp;kHz || [[Ebino VLF transmitter|Ebino]] (Japan) ||
|- align="center" valign="top"
| RJH63 RJH66 [[Vileyka VLF transmitter|RJH69]]  RJH77 RJH99 || 23&nbsp;kHz || various locations (Russia) || Time signal transmitter Beta
|- align="center" valign="top"
| [[VLF transmitter DHO38|DHO38]] || 23.4&nbsp;kHz || near [[Rhauderfehn]] (Germany) || submarine communication
|- align="center" valign="top"
| [[VLF Transmitter Cutler|NAA]] || 24&nbsp;kHz || [[Cutler, Maine]] (USA) || Used for submarine communication, at {{nowrap|2&nbsp;megawatts}}<ref>{{Cite web |title=Cutler |url=http://www.globalsecurity.org/military/facility/cutler.htm |website=GlobalSecurity |series=Military Facilities}}</ref>
|- align="center" valign="top"
| [[Jim Creek Naval Radio Station|NLK]] || 24.6&nbsp;kHz || [[Oso, Washington]] (USA) || 192&nbsp;kW<ref name=Fidani-2011/>
|- align="center" valign="top"
| NLF || 24.8&nbsp;kHz || [[Arlington, Washington]] (USA) || Used for submarine communication.<ref>{{Cite web |title=20–25kHz |url=http://www.vlf.it/trond2/20-25khz.html |website=vlf.it}}{{full citation needed|date=January 2021}}<br/>{{Cite web |title=WA3248 |url=http://ludb.clui.org/ex/i/WA3248/ |url-status=dead |archive-url=https://web.archive.org/web/20110927004529/http://ludb.clui.org/ex/i/WA3248/ |archive-date=2011-09-27 |access-date=2009-07-13 |website=ludb.clui.org}}{{full citation needed|date=January 2021}}</ref>
|- align="center" valign="top"
| NML || 25.2&nbsp;kHz || [[LaMoure, North Dakota]] (USA) ||
|- align="center" valign="top"
| [[PNS Hameed|PNSH]] || 14–25.2?&nbsp;kHz || [[Karachi coast]], [[Sindh]] (Pakistan) ||
|- 
|}

==See also==
*[[Communication with submarines]]
*[[OMEGA Navigation System]], 1971–1997
*[[Radio atmospheric]]

==References==
{{Reflist}}

==Further reading==
* {{Cite book |last=Romero, R. |title=Radio Natura |publisher=SANDIT S.r.l. |year=2006 |location=Albino, Italy |language=it}}
*{{Cite book |last=Klawitter, G. |title=Langwelle und Längstwelle |last2=Oexner, M. |last3=Herold, K. |publisher=Siebel Verlag GmbH |year=2000 |isbn=978-3-89632-043-8 |location=Meckenheim, DE |language=de}}
* {{Cite web |last=Friese |date=January 2006 |title=Very low wave reception with ferrite antennas 5-50 kHz |url=http://www.magnet-ferritantennen.de/assets/plugindata/poola/funkamateur012006.pdf}}

==External links==
* [http://www.lwca.org Longwave club of America]
* [http://www.vlf.it Radio waves below 22 kHz]
* [https://groups.io/g/VLF VLF Discussion Group]
* Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''".
* [[b:Electronics/VLF-reception with the PC|PC-based VLF-reception]]
* [[b:Electronics/Gallery of VLF-signals|Gallery of VLF-signals]]
* [http://spaceweather.com/glossary/inspire.html NASA live streaming ELF -> VLF Receiver] NOTE: As of 05/03/2014, the "Listen live" links are down, but the site has some previously recorded examples to listen to.
* [http://webflash.ess.washington.edu/ World Wide Lightning Location Network]
* [http://vlf.stanford.edu/research/introduction-vlf Stanford University VLF group]
* [http://moondog.astro.louisville.edu/index.html University of Louisville VLF Monitor]
* [http://www.vlfradio.com/ Larry's Very Low Frequency site]
* [http://www.markyd26uk.110mb.com/vlf.html Mark's Live Online VLF Receiver, UK] {{Webarchive|url=https://web.archive.org/web/20090228212920/http://markyd26uk.110mb.com/vlf.html |date=2009-02-28 }}
* [http://www.qsl.net/iw0bzd/VLF_TUBE_RX.htm IW0BZD VLF TUBE receiver]
* [http://www.ab9il.net/vlf/vlf1.html Internet based VLF listening guide with server list]
* [http://sidstation.loudet.org/stations-list-en.xhtml List of VLF-transmitters]

{{TimeSig}}
{{radio spectrum}}
{{EMSpectrum}}
{{Authority control}}

[[Category:Radio spectrum]]
[[Category:Radio electronics]]