Low frequency (LF) is the
ITU designation
for
radio frequencies (RF) in the range of 30–300
kHz. Since its wavelengths range from 10–1
km, respectively, it is also known as the kilometre band or kilometre waves.
LF radio waves exhibit low signal
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...
, making them suitable for long-distance communications. In Europe and areas of
Northern Africa and Asia, part of the LF spectrum is used for
AM broadcasting
AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transm ...
as the "
longwave
In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dati ...
" band. In the western hemisphere, its main use is for aircraft beacons, navigation (
LORAN
LORAN (Long Range Navigation) was a hyperbolic navigation, hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee (navigation), Gee system but operated at lower frequencies in order ...
, mostly defunct), information, and weather systems. A number of
time signal broadcasts also use this band. The main mode of transmission used in this band is
ground waves, in which LF radio waves travel just above the Earth's surface, following the terrain. LF ground waves can travel over hills, and can travel far beyond the horizon, up to several hundred kilometers from the transmitter.
Propagation
Because of their long
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
, low frequency
radio wave
Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
s can
diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ''
ground wave'', with the radio waves traveling horizontally through the atmosphere just above the surface of the Earth, is the main mode in the LF band.
[
] Ground waves are absorbed by the Earth as they travel, so the signal strength (power density) decreases exponentially with distance from the transmitting antenna, limiting transmission distance. The attenuation of signal strength with distance is lower than at higher frequencies. Low frequency ground waves can be received up to from the transmitting antenna. Ground waves must be
vertically polarized (the
electric field
An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
is vertical while the
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
is horizontal), so vertical
monopole antennas are used for transmitting.
Low frequency waves can also occasionally travel long distances by reflecting from 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 ...
(the actual mechanism is one of
refraction
In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...
), although this method, called ''
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 curvatur ...
'' or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionospheric
E layer or
F layers. Skywave signals can be detected at distances exceeding from the transmitting antenna.
Uses
Radio broadcasting
AM broadcasting
AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transm ...
is authorized in the
longwave
In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dati ...
band on frequencies between 148.5 and 283.5 kHz in Europe and parts of Asia.
Standard time signals

In Europe and Japan, many low-cost consumer devices have since the late 1980s contained
radio clock
A radio clock or radio-controlled clock (RCC), and often colloquially (and incorrectly) referred to as an "atomic clock", is a type of Quartz clock, quartz clock or watch that is automatically Synchronization, synchronized to a time code transmi ...
s with an LF receiver for these signals. Since these frequencies propagate by
ground wave only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, and the receiver. In the United States, such devices became feasible for the mass market only after the output power of
WWVB
WWVB is a longwave time signal radio station near Fort Collins, Colorado, and is operated by the National Institute of Standards and Technology (NIST). Most radio clock, radio-controlled clocks in North America use WWVB's transmissions to set th ...
was increased in 1997 and 1999.
JJY transmitting broadcast on the exact same frequency, and has a similar
timecode.
Military
Radio signals below 50 kHz are capable of penetrating ocean depths to approximately ; the longer the wavelength, the deeper they go. The British, German, Indian, Russian, Swedish, United States,
and possibly other
navies communicate with
submarine
A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...
s on these frequencies.
In addition,
Royal Navy
The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...
nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the
BBC Radio 4
BBC Radio 4 is a British national radio station owned and operated by the BBC. The station replaced the BBC Home Service on 30 September 1967 and broadcasts a wide variety of spoken-word programmes from the BBC's headquarters at Broadcasti ...
transmission on 198 kHz in waters near the UK. It is rumoured that they are to construe a sudden halt in transmission, particularly of the morning news programme
''Today'', as an indicator that the UK is under attack, whereafter their
sealed orders take effect.
The United States has four LF stations maintaining contact with its submarine force:
Aguada, Puerto Rico,
Keflavik, Iceland,
Awase, Okinawa, and
Sigonella, Italy, using AN/FRT-95 solid state transmitters.
In the U.S., the
Ground Wave Emergency Network or GWEN operated between 150 and 175 kHz, until replaced by satellite communications systems in 1999. GWEN was a land based military radio communications system which could survive and continue to operate even in the case of a nuclear attack.
Experimental and amateur
The 2007
World Radiocommunication Conference (WRC-07) made a worldwide amateur radio allocation in this band. An international 2.1 kHz allocation, the
(135.7–137.9 kHz) is available to
amateur radio
Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...
operators in several countries in Europe,
New Zealand, Canada, US,
[
]
and French overseas dependencies.
The world record distance for a two-way contact is over 10,000 km from near
Vladivostok
Vladivostok ( ; , ) is the largest city and the administrative center of Primorsky Krai and the capital of the Far Eastern Federal District of Russia. It is located around the Zolotoy Rog, Golden Horn Bay on the Sea of Japan, covering an area o ...
to
New Zealand
New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
.
[
]
As well as conventional
Morse code
Morse code is a telecommunications method which Character encoding, encodes Written language, text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code i ...
many operators use very slow computer-controlled Morse code (so-called
"QRSS") or specialized digital communications modes.
The UK allocated a 2.8 kHz sliver of spectrum from 71.6 kHz to 74.4 kHz beginning in April 1996 to UK amateurs who applied for a Notice of Variation to use the band on a noninterference basis with a maximum output power of 1 Watt
ERP. This was withdrawn on 30 June 2003 after a number of extensions in favor of the cross-European standard 136 kHz band.
[
]
Very slow Morse Code from G3AQC in the UK was received away, across the
Atlantic Ocean
The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...
, by W1TAG in the US on 21-22 November 2001 on 72.401 kHz.
In the United States, there is an exemption within FCC Part 15 regulations permitting unlicensed transmissions in the frequency range of 160–190 kHz. Longwave radio hobbyists refer to this as the '
LowFER' band, and experimenters, and their transmitters are called '
LowFERs'. This frequency range between 160 kHz and 190 kHz is also referred to as the band. Requirements include:
* The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt.
* The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.
* All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier.
* As an alternative to these requirements, a field strength of 2400/F(kHz) microvolts/meter (measured at a distance of 300 meters) may be used (as described in 47CFR15.209).
* In all cases, operation may not cause harmful interference to licensed services.
Many experimenters in this band are amateur radio operators.
Meteorological information broadcasts
A regular service transmitting
RTTY marine meteorological information in
SYNOP code on LF is the German Meteorological Service (
Deutscher Wetterdienst
The () or DWD for short, is the German Meteorological Service, based in Offenbach am Main, Germany, which monitors weather and meteorological conditions over Germany and provides weather services for the general public and for nautical, avi ...
o
DWD. The DWD operates station DDH47 on 147.3 kHz using standard ITA-2 alphabet with a transmission speed of 50
baud and FSK modulation with 85 Hz shift.
Radio navigation signals
In parts of the world where there is no longwave broadcasting service,
Non-directional beacons used for aeronavigation operate on 190–300 kHz (and beyond into the MW band). In Europe, Asia and Africa, the NDB allocation starts on 283.5 kHz.
The
LORAN
LORAN (Long Range Navigation) was a hyperbolic navigation, hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee (navigation), Gee system but operated at lower frequencies in order ...
-C radio navigation system operated on 100 kHz.
In the past, the
Decca Navigator System
The Decca Navigator System was a hyperbolic navigation, hyperbolic radio navigation system that allowed ships and aircraft to determine their position by using radio signals from a dedicated system of static radio transmitters. The system use ...
operated between 70 kHz and 129 kHz. The last Decca chains were closed down in 2000.
Differential GPS
Differential Global Positioning Systems (DGPSs) supplement and enhance the positional data available from global navigation satellite systems (GNSSs). A DGPS can increase accuracy of positional data by about a thousandfold, from approximately to ...
telemetry transmitters operate between 283.5 and 325 kHz.
[
]
The commercial "
Datatrak" radio navigation system operates on a number of frequencies, varying by country, between 120–148 kHz.
Other applications
Some radio frequency identification (
RFID
Radio-frequency identification (RFID) uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID system consists of a tiny radio transponder called a tag, a radio receiver, and a transmitter. When tri ...
) tags utilize LF. These tags are commonly known as LFIDs or LowFIDs (low frequency identification). The LF RFID tags are
near-field devices, interacting with the
inductive near field, rather than with radiated waves (radio waves) that are the only part of the electromagnetic field that persists into the far field. As such, they are technically ''not'' radio devices nor radio antennas, even though they do operate at radio frequencies, and are ''called'' "antennas" in the RFID trade, but not in
radio engineering. It is more proper, and technically more informative to think of them as secondary coils of very loosely coupled
transformer
In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
s.
Antennas
Since the ground waves used in this band require
vertical polarization, vertical antennas are used for transmission.
Mast radiator
A mast radiator (or radiating tower) is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at l ...
s are most common, either insulated from the ground and fed at the bottom, or occasionally fed through guy-wires.
T-antennas and
inverted L-antennas are used when antenna height is an issue.
LF transmitting antennas for high power transmitters require large amounts of space, and have been the cause of controversy in Europe and the United States, due to concerns about possible health hazards associated with
human exposure to radio waves.
Longwave receiving antennas
Antenna requirements for LF reception are much more modest than for transmission. Although non-resonant long wire antennas are sometimes used, ferrite
loop antenna
A loop antenna is a antenna (radio), radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this p ...
s are far more popular because of their small size.
Amateur radio operators have achieved good LF reception using
active antennas: A short whip with a built-in
pre-amplifier.
Antenna heights
Due to the long wavelengths in the band, nearly all LF antennas are
electrically short, shorter than one quarter of the radiated wavelength, so their low radiation resistance makes them inefficient, requiring very low resistance grounds and conductors to avoid dissipating transmitter power.
These electrically short antennas need
loading coils at the base of the antenna to bring them into resonance. Many antenna types, such as the
umbrella antenna and L- and T-antenna, use capacitive top-loading (a "top hat"), in the form of a network of horizontal wires attached to the top of the vertical radiator. The
capacitance
Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
improves the efficiency of the antenna by increasing the current, without increasing its height.
The height of antennas differ by usage. For some
non-directional beacons (NDBs) the height can be as low as 10 meters, while for more powerful navigation transmitters such as
DECCA, masts with a height around 100 meters are used.
T-antennas have a height between 50–200 meters, while mast aerials are usually taller than 150 meters.
The height of mast antennas for
LORAN-C is around 190 meters for transmitters with radiated power below 500 kW, and around 400 meters for transmitters greater than The main type of LORAN-C antenna is insulated from ground.
LF
(longwave) broadcasting stations use mast antennas with heights of more than 150 meters or
T-aerials. The mast antennas can be ground-fed insulated masts or upper-fed grounded masts. It is also possible to use cage antennas on grounded masts.
Directional array antennas
For broadcasting stations, directional antennas are often required. They consist of multiple masts, which often have the same height. Some longwave antennas consist of multiple mast antennas arranged in a circle with or without a mast antenna in the center. Such antennas focus the transmitted power toward ground and give a large zone of fade-free reception. This type of antenna is rarely used, because they are very expensive and require much space and because fading occurs on longwave much more rarely than in the medium wave range. One antenna of this kind was used by
transmitter Orlunda in Sweden.
Footnotes
See also
*
2200-meter band
*
Ground Wave Emergency Network (GWEN)
*
Longwave
In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dati ...
*
LowFER
*
Passive RFID
*
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, au ...
*
WGU-20
References
Further reading
*
*
*
External links
*
*
{{DEFAULTSORT:Low Frequency
Radio spectrum