Low frequency
Encyclopedia
Low frequency or low freq or LF refers to radio frequencies
(RF) in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa
and of Asia, part of the LF spectrum is used for AM broadcasting
as the longwave
band. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN
), information, and weather systems. Time signal
stations MSF, HBG, DCF77
, JJY
and WWVB
are found in this band. Also known as the kilometre band or kilometre wave as the wavelengths range from one to ten kilometres.
Propagation by reflection (the actual mechanism is one of refraction) from the ionosphere
is also possible. The refraction can take place at the E layer or F layers
. These waves, called skywave
s, can be detected at distances exceeding 300 km from the transmitting antenna.
In the frequency range 40 kHz–80 kHz, there are several standard time and frequency stations, such as
In Europe and Japan, many low-cost consumer devices have since the late 1980s contained radio clock
s with an LF receiver for these signals. Since these frequencies propagate by ground wave only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, and the receiver. In the United States, such devices became feasible for the mass market only after the output power of WWVB
was increased in 1997 and 1999.
communicate with submarine
s on these frequencies.
In addition, Royal Navy
nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the BBC Radio 4
transmission on 198 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.
In the US, 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.
operators in several countries in Europe, New Zealand, Canada and French overseas dependencies. The world record distance for a two-way contact is over 10,000 km from near Vladivostok
to New Zealand
. As well as conventional Morse code
many operators use very slow computer controlled Morse code (QRSS) or specialized digital communications modes. The 2007 World Radiocommunication Conference
(WRC-07) made this band a worldwide amateur radio allocation.
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 W ERP (effective radiated power
). This was withdrawn on 30 June 2003 after a number of extensions in favor of the European-harmonized 136 kHz band. A 1-watt transmission of very slow Morse Code between G3AQC (in the UK) and W1TAG (in the US) spanned the Atlantic Ocean
for 3275 miles (5,270.6 km) on November 21–22, 2001.
In the United States there is a special license free allocation in the longwave range called LowFER
. This experimental allocation between 160 kHz and 190 kHz is sometimes called the "Lost Band". Unlicensed operation by the public is permitted south of 60 degrees north latitude, except where interference would occur to ten licensed location service stations located along the coasts. Regulations for use include a power output of no more than 1 watt, a combined antenna/ground-lead length of no more than 15 meters, and a field strength of no more than 4.9 microvolts/meter. Also, emissions outside of the 160 kHz–190 kHz band must be attenuated by at least 20 dB below the level of the unmodulated carrier. Many experimenters in this band are amateur radio operators.
marine meteorological information on LF is the German Meteorological Service (Deutscher Wetterdienst or 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.
s or NDB's 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
-C radio navigation system operates on 100 kHz.
In the past, the Decca Navigator System
operated between 70 kHz and 129 kHz. The last Decca chains were closed down in 2000.
Differential GPS
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 and 148 kHz.
radio broadcasting service operates on frequencies between 148.5 and 283.5 kHz in Europe and parts of Asia.
Antennas (aerials) used at these low frequencies are usually mast radiator
s, which are fed at the bottom and which are insulated from ground, or mast antennas fed by the guy ropes (such masts are usually grounded). T-antennas and L-antennas are used when antenna height is an issue. Long wire antennas are also used in rare cases. Nearly all LF antennas are shorter than one quarter of the radiated wavelength. The only longwave transmission antenna realized with a height corresponding to a half radiated wavelength was Warsaw Radio Mast
. Low height antennas need loading coils of high inductance. These coils have high power losses due to ohmic heating of the coil wire. The addition of a horizontal section ("top hat") improves the efficiency of electrically short LF antennas without increasing the height of the antenna or its supporting structures.
The height of antennas differ by usage.
For some non-directional beacon
s (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 and 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 1000 kilowatts. 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.
For broadcasting stations often directional antennas are 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.
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 exposure to high-power radio waves.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa
North Africa
North Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Morocco, South Sudan, Sudan, Tunisia, and...
and of Asia, part of the LF spectrum is used for AM broadcasting
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
as the longwave
Longwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
band. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN
LORAN
LORAN is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment to determine the location and speed of the receiver....
), information, and weather systems. Time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...
stations MSF, HBG, DCF77
DCF77
DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH , on behalf of the Physikalisch-Technische Bundesanstalt, Germany's...
, JJY
JJY
JJY is the call sign of a low frequency time signal radio station.The station is located in Japan and broadcasts from two sites, one on Mount Otakadoya, near Fukushima, and the other on Mount Hagane, located on Kyushu Island...
and WWVB
WWVB
WWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks in most of North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic...
are found in this band. Also known as the kilometre band or kilometre wave as the wavelengths range from one to ten kilometres.
Propagation of LF signals
Low frequency radio signals can follow the curvature of the Earth. Radio waves reaching the receiver by this route are called ground waves. Their strength is not reduced by absorption as much as in higher frequencies. Ground wave can cover an area with a radius of 2000 km about the transmitting antenna.Propagation by reflection (the actual mechanism is one of refraction) from the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
is also possible. The refraction can take place at the E layer or F layers
F region
The F region of the ionosphere is home to the F layer of ionization, also called the Appleton layer, after the English physicist Edward Appleton. As with other ionospheric sectors, 'layer' implies a concentration of plasma, while 'region' is the area that contains the said layer...
. These waves, called skywave
Skywave
Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...
s, can be detected at distances exceeding 300 km from the transmitting antenna.
Standard time signals
- JJYJJYJJY is the call sign of a low frequency time signal radio station.The station is located in Japan and broadcasts from two sites, one on Mount Otakadoya, near Fukushima, and the other on Mount Hagane, located on Kyushu Island...
in Japan (40 kHz and 60 kHz) - MSF in AnthornAnthornAnthorn is a village in Cumbria, England. It is situated on the south side of the Solway Firth, on the Wampool estuary, about thirteen miles west of Carlisle...
, England (60 kHz) - WWVBWWVBWWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks in most of North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic...
in Fort Collins, ColoradoFort Collins, ColoradoFort Collins is a Home Rule Municipality situated on the Cache La Poudre River along the Colorado Front Range, and is the county seat and most populous city of Larimer County, Colorado, United States. Fort Collins is located north of the Colorado State Capitol in Denver. With a 2010 census...
, US (60 kHz) - HBGHBG PranginsHBG is a low frequency time signal transmitter for the Swiss time reference system. It currently operates on 75 kHz with 20 kW and is located in Prangins, Switzerland...
in PranginsPranginsPrangins is a municipality in the district of Nyon in the canton of Vaud in Switzerland. It is located on Lake Geneva.-Geography:Prangins has an area, , of . Of this area, or 54.7% is used for agricultural purposes, while or 13.1% is forested...
, Switzerland (75 kHz) (to be closed down 31 December 2011) - DCF77DCF77DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH , on behalf of the Physikalisch-Technische Bundesanstalt, Germany's...
in MainflingenMainflingenMainflingen is a small settlement of about 3800 inhabitants, a constituent community of the municipality Mainhausen in Hesse, Germany....
near Frankfurt am Main, Germany (77.5 kHz)
In Europe and Japan, many low-cost consumer devices have since the late 1980s contained radio clock
Radio clock
A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...
s with an LF receiver for these signals. Since these frequencies propagate by ground wave only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, and the receiver. In the United States, such devices became feasible for the mass market only after the output power of WWVB
WWVB
WWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks in most of North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic...
was increased in 1997 and 1999.
Military
Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 metres, the longer the wavelength, the deeper. The British, German, Indian, Russian, Swedish, United States and possibly other naviesNavy
A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...
communicate with submarine
Submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...
s on these frequencies.
In addition, Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the BBC Radio 4
BBC Radio 4
BBC Radio 4 is a British domestic radio station, operated and owned by the BBC, that broadcasts a wide variety of spoken-word programmes, including news, drama, comedy, science and history. It replaced the BBC Home Service in 1967. The station controller is currently Gwyneth Williams, and the...
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
Today programme
Today is BBC Radio 4's long-running early morning news and current affairs programme, now broadcast from 6.00 am to 9.00 am Monday to Friday, and 7.00 am to 9.00 am on Saturdays. It is also the most popular programme on Radio 4 and one of the BBC's most popular programmes across its radio networks...
, as an indicator that the UK is under attack, whereafter their sealed orders take effect.
In the US, the Ground Wave Emergency Network
Ground Wave Emergency Network
The Ground Wave Emergency Network was a command and control communications system intended for use by the United States government to facilitate military communications before, during and after a nuclear war...
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
An international 2.1 kHz allocation, the 2200-meter band (135.7 kHz to 137.8 kHz), is available to amateur radioAmateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operators in several countries in Europe, New Zealand, Canada and French overseas dependencies. The world record distance for a two-way contact is over 10,000 km from near Vladivostok
Vladivostok
The city is located in the southern extremity of Muravyov-Amursky Peninsula, which is about 30 km long and approximately 12 km wide.The highest point is Mount Kholodilnik, the height of which is 257 m...
to New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. As well as conventional Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
many operators use very slow computer controlled Morse code (QRSS) or specialized digital communications modes. The 2007 World Radiocommunication Conference
World Radiocommunication Conference
World Radiocommunication Conference is organized by ITU to review, and, as necessary, revise the Radio Regulations, the international treaty governing the use of the radio-frequency spectrum and the geostationary-satellite and non-geostationary-satellite orbits. It is held every three to four years...
(WRC-07) made this band a worldwide amateur radio allocation.
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 W ERP (effective radiated power
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
). This was withdrawn on 30 June 2003 after a number of extensions in favor of the European-harmonized 136 kHz band. A 1-watt transmission of very slow Morse Code between G3AQC (in the UK) and W1TAG (in the US) spanned the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
for 3275 miles (5,270.6 km) on November 21–22, 2001.
In the United States there is a special license free allocation in the longwave range called LowFER
LowFER
LowFER is a form of two-way radio communications practiced on frequencies below 300 kHz.-Practices:...
. This experimental allocation between 160 kHz and 190 kHz is sometimes called the "Lost Band". Unlicensed operation by the public is permitted south of 60 degrees north latitude, except where interference would occur to ten licensed location service stations located along the coasts. Regulations for use include a power output of no more than 1 watt, a combined antenna/ground-lead length of no more than 15 meters, and a field strength of no more than 4.9 microvolts/meter. Also, emissions outside of the 160 kHz–190 kHz band must be attenuated by at least 20 dB below the level of the unmodulated carrier. Many experimenters in this band are amateur radio operators.
Meteorological information broadcasts
A regular service transmitting RTTYRadioteletype
Radioteletype is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations, later superseded by personal computers running software to emulate teleprinters, connected by radio rather than a wired link.The term radioteletype is used to...
marine meteorological information on LF is the German Meteorological Service (Deutscher Wetterdienst or 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 beaconNon-directional beacon
A non-directional beacon is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF...
s or NDB's 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
LORAN is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment to determine the location and speed of the receiver....
-C radio navigation system operates on 100 kHz.
In the past, the Decca Navigator System
Decca Navigator System
The Decca Navigator System was a low frequency hyperbolic navigation system that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings...
operated between 70 kHz and 129 kHz. The last Decca chains were closed down in 2000.
Differential GPS
Differential GPS
Differential Global Positioning System is an enhancement to Global Positioning System that provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations....
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 and 148 kHz.
Radio broadcasting
The longwaveLongwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
radio broadcasting service operates on frequencies between 148.5 and 283.5 kHz in Europe and parts of Asia.
Other applications
Some radio frequency identification (RFID) tags utilize LF. These tags are commonly known as LFID's or LowFID's (Low Frequency Identification). The LF RFID tags are near field devices.Antennas
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...
s, which are fed at the bottom and which are insulated from ground, or mast antennas fed by the guy ropes (such masts are usually grounded). T-antennas and L-antennas are used when antenna height is an issue. Long wire antennas are also used in rare cases. Nearly all LF antennas are shorter than one quarter of the radiated wavelength. The only longwave transmission antenna realized with a height corresponding to a half radiated wavelength was Warsaw Radio Mast
Warsaw radio mast
The Warsaw radio mast was the world's tallest structure until its collapse on 8 August 1991. It is the second tallest land-based structure ever built, being surpassed as tallest by the Burj Khalifa, completed in 2010....
. Low height antennas need loading coils of high inductance. These coils have high power losses due to ohmic heating of the coil wire. The addition of a horizontal section ("top hat") improves the efficiency of electrically short LF antennas without increasing the height of the antenna or its supporting structures.
The height of antennas differ by usage.
For some non-directional beacon
Non-directional beacon
A non-directional beacon is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF...
s (NDBs) the height can be as low as 10 meters, while for more powerful navigation transmitters such as DECCA
Decca Navigator System
The Decca Navigator System was a low frequency hyperbolic navigation system that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings...
, masts with a height around 100 meters are used. T-antennas have a height between 50 and 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 1000 kilowatts. The main type of LORAN-C antenna is insulated from ground.
LF (longwave) broadcasting stations
Longwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
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.
For broadcasting stations often directional antennas are 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.
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 exposure to high-power radio waves.
See also
- High frequencyHigh frequencyHigh frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
- Long wave
- LowFERLowFERLowFER is a form of two-way radio communications practiced on frequencies below 300 kHz.-Practices:...
- 2200-meter band
- Time signal
- WGU-20Wgu-20WGU-20, also known as "the last radio station," was operated by the United States Defense Civil Preparedness Agency in the mid-to-late 1970s.-Public Emergency Radio:...
- LowFID passive RFID tags
Further reading
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)".
- IK1QFK Home Page.