![VLFatPalmer](https://upload.wikimedia.org/wikipedia/commons/2/2d/VLFatPalmer.JPG)
Very low frequency or VLF is the
ITU
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
designation
for
radio frequencies
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...
(RF) in the range of 3–30
kHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
, corresponding to
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten
myriameter
The following are examples of orders of magnitude for different lengths.
__TOC__
Overview
Detailed list
To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10^ ...
s (an obsolete metric unit equal to 10 kilometers). Due to its limited
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
,
audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
*Digital audio, representation of sound ...
(voice) transmission is highly impractical in this band, and therefore only low
data rate coded signals are used. The VLF band is used for a few
radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The basic principles a ...
services, government
time radio stations (broadcasting time signals to set
radio clock
A radio clock or radio-controlled clock (RCC), and often (incorrectly) referred to as an atomic clock is a type of quartz clock or watch that is automatically synchronized to a time code transmitted by a radio transmitter connected to a time st ...
s) and for secure military communication. Since VLF waves can penetrate at least 40 meters (131 ft) into saltwater, they are used for
military communication
Military communications or military signals involve all aspects of communications, or conveyance of information, by armed forces. Military communications span from pre-history to the present. The earliest military communications were delivered b ...
with
submarine
A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
s.
Propagation characteristics
Because of their long wavelengths, VLF radio waves can
diffract
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
around large obstacles and so are not blocked by mountain ranges, and can propagate as
ground wave
Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in rad ...
s following the curvature of the Earth and so are not limited by the horizon. Ground waves are less important beyond several hundred to a thousand miles, and the main mode of long-distance propagation is an
Earth-ionosphere waveguide mechanism.
[
] The Earth is surrounded by a conductive layer of
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s and
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s in the upper atmosphere at the bottom of the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
called the
D layer
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
at 60–90 km (37–56 miles) altitude,
[
] 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
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
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
A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and microwav ...
(TM) mode.
VLF waves have very low path attenuation, 2–3 dB per 1,000 km,
with little of the "
fading
In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...
" experienced at higher frequencies.
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 and
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 km have been realized.
However, atmospheric noise ("
sferics
A radio atmospheric signal or sferic (sometimes also spelled "spheric") is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major a ...
") is high in the band,
including such phenomena as "
whistlers", caused by
lightning
Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
.
* VLF waves can penetrate
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
to a depth of at least 10–40 meters (30–130 feet), 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 Electron precipitation (also called energetic electron precipitation or EEP) is an atmospheric phenomenon that occurs when previously trapped electrons enter the Earth's atmosphere, thus creating communications interferences and other disturbances. ...
.
* VLF waves used to communicate with submarines have created an artificial bubble around the Earth that can protect it from
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s and
coronal mass ejection
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
s; this occurred through interaction with high-energy radiation particles.
Antennas
A major practical drawback to the VLF band is that because of the length of the waves, full size resonant antennas (
half wave dipole
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole w ...
or
quarter wave monopole antennas) cannot be built because of their physical height. Vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30 kHz (10 km wavelength) would be high. So practical transmitting antennas are
electrically short An electrically small or electrically short antenna is an Antenna (radio), antenna much shorter than the wavelength of the signal it is intended to transmit or receive. Electrically short antennas are generally less efficient and more challenging t ...
, a small fraction of the length at which they would be self-resonant.
[
][
] Due to their low
radiation resistance
Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna.
Radiation resistance is an ''effecti ...
(often less than one ohm) they are inefficient, radiating only 10% to 50% of the transmitter power at most,
with the rest of the power dissipated in the antenna/ground system resistances. Very high power transmitters (~1 megawatt) are required for long-distance communication, so the efficiency of the antenna is an important factor.
VLF transmitting antennas
High power VLF transmitting stations use capacitively-loaded
monopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane.
The driving signal from the transmitter is applied, o ...
s. These are very large wire antennas, up to several kilometers long.
[
][ They consist of a series of steel ]radio mast
Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television. There are two main types: guyed and self-supporting structures. They are among the tallest human-made ...
s, linked at the top with a network of cables, often shaped like an umbrella or clotheslines. Either the towers themselves or vertical wires serve as 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 An umbrella antenna is a capacitively top-loaded wire monopole antenna, consisting in most cases of a mast fed at the ground end, to which a number of radial wires are connected at the top, sloping downwards.
They are used as transmitting antennas ...
such as the "delta" and " trideco" antennas, or multiwire flattop
A flattop is a type of haircut where the hair on the top of the head is cut and styled upright to form a flat profile when viewed from the front or side.
Styling
In the most classic and mainstream style of flattop for men and boys, the hair ...
(triatic) antennas.[
] For low-power transmitters, inverted-L and T antennas are used.
Due to the low radiation resistance, to minimize power dissipated in the ground these antennas require extremely low resistance ground (Earthing) systems, consisting of radial networks of buried copper wires under the antenna. To minimize dielectric loss
Dielectric loss quantifies a dielectric material's inherent dissipation of electromagnetic energy (e.g. heat). It can be parameterized in terms of either the loss angle ''δ'' or the corresponding loss tangent tan ''δ''. Both refer to the ...
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 protected by copper ground screens. 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
A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
is required at the antenna feed point to cancel the capacitive reactance
In electrical circuits, reactance is the opposition presented to alternating current by inductance or capacitance. Greater reactance gives smaller current for the same applied voltage. Reactance is similar to resistance in this respect, but does ...
of the antenna to make it resonant
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscilla ...
. At VLF the design of this coil is challenging; it must have low resistance at the operating RF frequency, high , and must withstand the extremely high voltage on the antenna. The RF resistance is usually reduced by using litz wire
Litz wire is a particular type of multistrand wire or cable used in electronics to carry alternating current (AC) at radio frequencies. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies ...
.
The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a high tuned circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
. VLF antennas have very narrow bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
and to change the transmitting frequency requires a variable inductor (variometer
In aviation, a variometer – also known as a rate of climb and descent indicator (RCDI), rate-of-climb indicator, vertical speed indicator (VSI), or vertical velocity indicator (VVI) – is one of the flight instruments in an aircraft used to in ...
) to tune the antenna. The large VLF antennas used for high-power transmitters usually have bandwidths of only 50–100 hertz, and when transmitting frequency-shift keying
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
(FSK), the usual mode, the resonant frequency of the antenna must sometimes be dynamically shifted with the modulation, between the two FSK frequencies. The high results in very high voltages (up to 200 kV) on the antenna and very good insulation is required. 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 air breakdown, corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
, and arcing from the antenna.
Dynamic antenna tuning
The bandwidth of large capacitively loaded VLF antennas is so narrow (50–100 Hz) that even the small frequency shifts of FSK and MSK modulation may exceed it, throwing the antenna out of resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
, 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 , 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
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
between the two output frequencies with the modulation.[ This is accomplished with a ]saturable reactor
A saturable reactor in electrical engineering is a special form of inductor where the magnetic core can be deliberately saturated by a direct electric current in a control winding. Once saturated, the inductance of the saturable reactor drops dr ...
in series with the antenna loading coil
A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
. This is a ferromagnetic core inductor
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
with a second control winding through which a DC current flows, which controls the inductance by magnetizing the core, changing its 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
Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, there are about 40 lightning flashes per second – ≈3.5 million lightning discharges ...
in the band. At VLF frequencies atmospheric radio noise
In radio reception, radio noise is unwanted random radio frequency electrical signals, fluctuating voltages, always present in a radio receiver in addition to the desired radio signal. Radio noise near in frequency to the radio signal being receive ...
is far above the receiver noise introduced by the receiver circuit and determines the receiver signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
. 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
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) d ...
s are usually used for reception.
Modulation
Because of the small bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
of the band, and the extremely narrow bandwidth of the antennas used, it is impractical to transmit audio signal
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...
s ( AM or FM radiotelephony
A radiotelephone (or radiophone), abbreviated RT, is a radio communication system for conducting a conversation; radiotelephony means telephony by radio. It is in contrast to ''radiotelegraphy'', which is radio transmission of telegrams (messa ...
).[
] A typical AM radio signal with a bandwidth of 10 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
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
s. In military networks frequency-shift keying
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
(FSK) modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
is used to transmit radioteletype
Radioteletype (RTTY) is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations connected by radio rather than a wired link. Radioteletype evolved from earlier landline teleprinter o ...
data using 5 bit ITA2
The Baudot code is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII ...
or 8 bit ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
character codes. A small frequency shift of 30–50 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
In digital modulation, minimum-shift keying (MSK) is a type of continuous-phase frequency-shift keying that was developed in the late 1950s by Collins Radio employees Melvin L. Doelz and Earl T. Heald. Similar to OQPSK, MSK is encoded with b ...
(MSK) is used. This is required due to the high of the antenna.[ The huge capacitively-loaded antenna and ]loading coil
A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
form a high tuned circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
, which stores oscillating electrical energy. The of large VLF antennas is typically over 200; this means the antenna stores far more energy (200 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
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
that have been used in VLF transmitters are:
;Continuous Wave
A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...
(CW), Interrupted Continuous Wave (ICW), or On-Off Keying: Morse code
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
radiotelegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
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 century to the 1960s in commercial and military VLF stations. Because of the high antenna 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 words/minute. CW is now only used in small hand-keyed transmitters, and for testing large transmitters.
;Frequency-shift keying
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
(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 Hz might be used to indicate a '1' and the frequency 9020 Hz, 50 Hz lower, to indicate a '0'. The two frequencies are generated by a continuously-running frequency synthesizer
A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radioteleph ...
. The transmitter is periodically switched between these frequencies to represent 8 bit ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
codes for the characters of the message. A problem at VLF is that when the frequency is switched the two sine wave
A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...
s usually have different phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
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 bit/s.
;Minimum-shift keying
In digital modulation, minimum-shift keying (MSK) is a type of continuous-phase frequency-shift keying that was developed in the late 1950s by Collins Radio employees Melvin L. Doelz and Earl T. Heald. Similar to OQPSK, MSK is encoded with b ...
(MSK): A 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 Hz apart, the standard frequency shift used in military VLF stations, their phases coincide every 20 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 bit/s, or about 35 ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
characters (8 bits each) per second, approximately 450 words per minute.
Applications
Early wireless telegraphy
Historically, this band was used for long distance transoceanic radio communication during the wireless telegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
era between about 1905 and 1925. Nations built networks of high-power LF and VLF radiotelegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
stations that transmitted text information by Morse code
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
, to communicate with other countries, their colonies, and naval fleets. Early attempts were made to use radiotelephone using amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
and single-sideband modulation
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modul ...
within the band starting from 20 kHz, but the result was unsatisfactory because the available bandwidth was insufficient to contain the sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands co ...
s.
In the 1920s the discovery of the skywave
In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature of ...
(skip) radio propagation method allowed lower power transmitters operating at high frequency
High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten ...
to communicate at similar distances by reflecting their radio waves off a layer of ionized
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
atoms in the ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
, and long-distance radio communication stations switched to the shortwave
Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
frequencies. The Grimeton VLF transmitter at Grimeton near Varberg in Sweden
Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
, 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
Alexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2 July, is the day of the open house at the Swedish go ...
.
Navigation beacons and time signals
Due to its long propagation distances and stable phase characteristics, during the 20th century the VLF band was used for long range hyperbolic
Hyperbolic is an adjective describing something that resembles or pertains to a hyperbola (a curve), to hyperbole (an overstatement or exaggeration), or to hyperbolic geometry.
The following phenomena are described as ''hyperbolic'' because they ...
radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The basic principles a ...
systems which allowed ships and aircraft to determine their geographical position by comparing the phase of radio waves received from fixed VLF navigation beacon
A beacon is an intentionally conspicuous device designed to attract attention to a specific location. A common example is the lighthouse, which draws attention to a fixed point that can be used to navigate around obstacles or into port. More mod ...
transmitters.
The worldwide Omega
Omega (; capital: Ω, lowercase: ω; Ancient Greek ὦ, later ὦ μέγα, Modern Greek ωμέγα) is the twenty-fourth and final letter in the Greek alphabet. In the Greek numeric system/isopsephy (gematria), it has a value of 800. The wo ...
system used frequencies from 10 to 14 kHz, as did Russia's Alpha
Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...
.
VLF was also used for standard time and frequency broadcasts. In the US, the time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.
Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, audi ...
station WWVL began transmitting a 500 W signal on 20 kHz in August 1963. It used frequency-shift keying (FSK FSK may refer to:
* FSK (band), a German band
* Federal Counterintelligence Service, (Russian ') of Russia
* Fiskerton railway station, in England
* Forskolin, a diterpene
* Forsvarets Spesialkommando, a Norwegian special forces unit
* Fort Scott M ...
) to send data, shifting between 20 kHz and 26 kHz. The WWVL service was discontinued in July 1972.
Geophysical and atmospheric measurement
Naturally occurring signals in the VLF band are used by geophysicist
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...
s for long range lightning location and for research into atmospheric phenomena such as the aurora. Measurements of whistlers
Whistler may refer to:
* Someone who whistles
Places
Canada
* Whistler, British Columbia, a resort town
** Whistler railway station
** Whistler Secondary School
* Whistler Blackcomb, a ski resort in British Columbia
* Whistler Mountain, Briti ...
are employed to infer the physical properties of the magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
.
Geophysicists
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...
use VLF-electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
receivers to measure conductivity in the near surface of the Earth.
VLF signals can be measured as a geophysical
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...
electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
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.
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
Nuclear warfare, also known as atomic warfare, is a theoretical military conflict or prepared political strategy that deploys nuclear weaponry. Nuclear weapons are weapons of mass destruction; in contrast to conventional warfare, nuclear w ...
VLF communications will be less disrupted by nuclear explosions than higher frequencies. Since it can penetrate seawater VLF is used by the military to communicate with submarines near the surface, while ELF
An elf () is a type of humanoid supernatural being in Germanic mythology and folklore. Elves appear especially in North Germanic mythology. They are subsequently mentioned in Snorri Sturluson's Icelandic Prose Edda. He distinguishes "ligh ...
frequencies are used for deeply submerged subs.
Examples of naval VLF transmitters are
* Britain's Skelton Transmitting Station
The Skelton Transmitting Station is a radio transmitter site at near Skelton, Cumbria, England, about north-west of Penrith, run by Babcock International and owned by the Ministry of Defence. Since the Belmont Mast in Lincolnshire was shor ...
in Skelton, Cumbria
* Germany's DHO38 in Rhauderfehn
Rhauderfehn is a municipality in the Leer district, in Lower Saxony, Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and t ...
, which transmits on 23.4 kHz with a power of 800 kW
* U.S. Jim Creek Naval Radio Station
Jim Creek Naval Radio Station is a United States Navy very low frequency (VLF) radio transmitter facility at Jim Creek near Oso, Washington. The primary mission of this site is to communicate orders one-way to submarines of the Pacific fleet. ...
in Oso, Washington
Oso is a census-designated place (CDP) in Snohomish County, Washington, United States. It is located to the west of Darrington, south of the North Fork of the Stillaguamish River and approximately 50 air miles (80 km) from Seattle. The popu ...
state, which transmits on 24.8 kHz with a power of 1.2 MW
* U.S. Cutler Naval Radio Station at Cutler, Maine
Cutler is a town in Washington County, Maine, United States. The town was named after Joseph Cutler, an early settler, who hailed from Newburyport, Massachusetts. The population was 524 at the 2020 census.
Demographics
2010 census
As of th ...
which transmits on 24 kHz with 1.8 MW.
Since 2004 the US Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage of ...
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.
Submarine and subterranean communications
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
The acre is a unit of land area used in the imperial and US customary systems. It is traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, of a square mile, 4,840 square ya ...
or square kilometers), with transmitted power anywhere from 20 kW to 2,000 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
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
(BCAA).
Modern receivers use sophisticated digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
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 bit ITA2
The Baudot code is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII ...
or 8 bit ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
. Because these are military transmissions they are almost always encrypted
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
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
In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. In this technique, a plaintext is paired with a ran ...
cipher
In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...
s since the amount of text is so small.
VLF can also penetrate soil and rock for some distance, so these frequencies are also used for through-the-earth mine communications systems.
Amateur use
The frequency range below 8.3 kHz is not allocated by the International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
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 kHz.
Operations tend to congregate around the frequencies 8.27 kHz, 6.47 kHz, 5.17 kHz, and 2.97 kHz. 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 A GPS clock, or GPS disciplined oscillator (GPSDO), is a combination of a GPS receiver and a high-quality, stable oscillator such as a quartz or rubidium oscillator whose output is controlled to agree with the signals broadcast by GPS or other GNS ...
or a rubidium standard
A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency.
Synopsis
The Rb standard is the most inexpensive, compact, ...
in order to support such long duration coherent detection and decoding.
Amateur equipment
Radiated power from amateur stations is very small, ranging from 1 μW to 100 μW for fixed base station antennas, and up to 10 mW from kite or balloon antennas. Despite the low power, stable propagation with low attenuation in the earth-ionosphere cavity enable very narrow bandwidths to be used to reach distances up to several thousand kilometers. The modes used are QRSS, MFSK
Multiple frequency-shift keying (MFSK) is a variation of frequency-shift keying (FSK) that uses more than two frequencies. MFSK is a form of M-ary orthogonal modulation, where each symbol consists of one element from an alphabet of orthogonal wave ...
, and coherent BPSK
Phase-shift keying (PSK) is a digital modulation process which conveys Data (computing), data by changing (modulating) the Phase (waves), phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying ...
.
The transmitter generally consists of an audio amplifier of a few hundred watts, an impedance matching transformer, a loading coil
A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
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
A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio i ...
to digitise the signal. Extensive digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
is required to retrieve the weak signals from beneath interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extr ...
from power line harmonics and VLF radio atmospherics. Useful received signal strengths are as low as volts/meter (electric field) and tesla (magnetic field), with signaling rate
In telecommunication, data signaling rate (DSR), also known as gross bit rate, is the aggregate rate at which data passes a point in the transmission path of a data transmission system.
# The DSR is usually expressed in bits per second.
# The d ...
s typically between 1 and 100 bits per hour.
PC based reception
VLF signals are often monitored by radio amateur
An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators h ...
s using simple homemade VLF radio receiver
In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...
s based on personal computers (PCs).[
] 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
A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in th ...
(FFT) software in combination with a sound card allows reception of all frequencies below the Nyquist frequency
In signal processing, the Nyquist frequency (or folding frequency), named after Harry Nyquist, is a characteristic of a sampler, which converts a continuous function or signal into a discrete sequence. In units of cycles per second ( Hz), it ...
simultaneously in the form of spectrogram
A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time.
When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. When the data are represen ...
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. 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
A list of VLF-transmitters and LF-transmitters, which work or worked on frequencies below 100 kHz.
List of VLF transmissions
Demolished
References
{{Reflist
Time signal radio stations
Radio navigation
Submarines
Military radio s ...
See also
*Communication with submarines
Communication with submarines is a field within military communications that presents technical challenges and requires specialized technology. Because radio waves do not travel well through good electrical conductors like salt water, submerged ...
*OMEGA Navigation System
OMEGA was the first global-range radio navigation system, operated by the United States in cooperation with six partner nations. It was a hyperbolic navigation system, enabling ships and aircraft to determine their position by receiving very low ...
, 1971–1997
*Radio atmospheric
A radio atmospheric signal or sferic (sometimes also spelled "spheric") is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major at ...
References
Further reading
*
*
*
External links
Longwave club of America
Radio waves below 22 kHz
VLF Discussion Group
* Tomislav Stimac, "
'".
* PC-based VLF-reception
* Gallery of VLF-signals
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.
World Wide Lightning Location Network
Stanford University VLF group
Larry's Very Low Frequency site
List of VLF-transmitters
{{Authority control
Radio spectrum
Radio electronics