In radio,longwave,long wave orlong-wave,[1] and commonly abbreviatedLW,[2] refers to parts of theradio spectrum withwavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the early 20th century, when the radio spectrum was considered to consist of longwave (LW),medium-wave (MW), andshort-wave (SW) radio bands. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short' (i.e.VHF,UHF, andmicrowave).
In contemporary usage, the termlongwave is not defined precisely, and its intended meaning varies. It may be used for radio wavelengths longer than 1,000 m[2] i.e. frequencies[note 1] up to 300 kilohertz (kHz),[3][4] including theInternational Telecommunication Union's (ITU's)low frequency (LF, 30–300 kHz) andvery low frequency (VLF, 3–30 kHz) bands. Sometimes the upper limit is taken to be higher than 300 kHz, but not above the start of themedium wave broadcast band at 520 kHz.[5]
In Europe, Africa, and large parts of Asia (International Telecommunication Union Region 1), where a range of frequencies between 148.5 and 283.5 kHz is used forAM broadcasting[6] in addition to themedium-wave band, the termlongwave usually refers specifically to this broadcasting band, which falls wholly within thelow frequency band of the radio spectrum (30–300 kHz). The "Longwave Club of America" (United States) is interested in "frequencies below the AM broadcast band"[5] (i.e., all frequencies below 520 kHz).
Because of their longwavelength,radio waves in this frequency range candiffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, calledground wave, is the main mode in the longwave band.[7] The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies, and falls with frequency.Low frequency ground waves can be received up to 2,000 kilometres (1,200 mi) from the transmitting antenna.Very low frequency waves below 30 kHz can be used to communicate at transcontinental distances, can penetrate saltwater to depths of hundreds of feet, and are used by the military tocommunicate with submerged submarines.
Low frequency waves can also occasionally travel long distances by reflecting from theionosphere (the actual mechanism is one ofrefraction), although this method, calledskywave or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionosphericE layer orF layers. Skywave signals can be detected at distances exceeding 300 kilometres (190 mi) from the transmitting antenna.[8]
Non-directional beacons transmit continuously for the benefit ofradio direction finders in marine and aeronautical navigation. They identify themselves by acallsign inMorse code. They can occupy any frequency in the range 190–1750 kHz. In North America, they occupy 190–535 kHz. In ITU Region 1 the lower limit is 280 kHz.
There are institutional broadcast stations in the range that transmit coded time signals to radio clocks. For example:
Radio-controlled clocks receive their time calibration signals with built-in long-wave receivers. They use long-wave, rather thanshort-wave ormedium-wave, because long-wave signals from the transmitter to the receiver always travel along the same direct path across the surface of theEarth, so the time delay correction for the signal travel time from the transmitting station to the receiver is always the same for any one receiving location.
Longwaves travel bygroundwaves that hug the surface of the Earth, unlikemediumwaves andshortwaves. Those higher-frequency signals do not follow the surface of theEarth beyond a few kilometers, but can travel asskywaves, 'bouncing' off different layers of theionosphere at different times of day. These differentpropagation paths can make the time lag different for every signal received. The delay between when the long-wave signal was sent from thetransmitter (when the coded time was correct) and when the signal is received by the clock (when the coded time is slightly late) depends on the overland distance between the clock and the transmitter and thespeed of light through the air, which is also very nearly constant. Since the time lag is essentially the same, a single constant shift forward from the time coded in the signal can compensate for all long-wave signals received at any one location from the same time signal station.
The militaries of the United Kingdom, Russian Federation, United States, Germany, India and Sweden use frequencies below 50 kHz to communicate with submergedsubmarines.
In the ITU Radio Regulations the band 135.7–137.8 kHz is allocated (on a secondary basis) toAmateur radio worldwide, subject to a power limit of 1 watt EIRP. Many countries' regulators license amateurs to use it.
In North America during the 1970s, the frequencies 167, 179, and 191 kHz were assigned to the short-livedPublic Emergency Radio of the United States.
Nowadays, in the United States,Part 15 of FCC regulations allow unlicensed use of the 160–190 kHz band a transmitter / amplifier output power to the antenna of at most 1 watt, with an antenna at most 15 meters (49 feet) high; this is calledLow Frequency Experimental Radio (LowFER). The 190–435 kHz band is used fornavigational beacons.
Frequencies from 472–479 kHz are available to licensed amateurs as the new630 m band, part of the now-defunctmaritime band, but this is often considered amedium wave sub-band.
Swedish station SAQ, located at theVarberg Radio Station facility in Grimeton, is the last remaining operationalAlexanderson alternator long-wave transmitter. Although the station ended regular service in 1996, it has been maintained as aWorld Heritage Site, and makes at least two demonstration transmissions yearly, on 17.2 kHz.[9]
Longwave is used for broadcasting only within ITU Region 1. The long-wave broadcasters are located in Europe, North Africa andMongolia.
Typically, a larger geographic area can be covered by a long-wave broadcast transmitter compared to amedium-wave one. This is because ground-wave propagation suffers lessattenuation due toground conductivity at lower frequencies.[10]
Many countries have stopped using LW for broadcasting because of low audience figures, a lack of LW on new consumer receivers, increasing interference levels, the energy inefficiency of AM and high electricity costs at transmitters.
In 2014 and 2015 Russia closed all of its LW broadcast transmitters.[11]
As of 2024 more than half of LW frequencies are unoccupied and some of the remaining services are scheduled for closure.BBC Radio 4 (UK) announced that it will stop distinct programming for LW broadcasts in 2024 in an effort to transition listeners to other means of listening. A closure date for LW broadcasts has not yet been announced.[12] Theradio teleswitch service for electricity meters is broadcast with the long wave signal. In October 2024 the Radio Teleswitch Taskforce said that it will end on 30 June 2025.[13]
With the adoption of theGeneva Frequency Plan of 1975, long-wave carrier frequencies are exact multiples of 9 kHz; ranging from 153 to 279 kHz. One exception was a French-language station,Europe 1 in Germany, which retained its prior channel spacing until the long-wave service was terminated in 2019. Other exceptions are all Mongolian transmitters, which are 2 kHz above the internationally recognized channels.[clarification needed]
Until the 1970s, some long-wave stations in northern and eastern Europe and the Soviet Union operated on frequencies as high as 433 kHz.[14]
Some radio broadcasters, for instanceDroitwich transmitting station in the UK, derive their carrier frequencies from anatomic clock, allowing their use asfrequency standards. Droitwich also broadcasts a low bit-rate data channel, using narrow-shift phase-shift keying of the carrier, forRadio Teleswitch Services.
Because long-wave signals can travel very long distances, someradio amateurs andshortwave listeners engage in an activity calledDXing. DXers attempt to listen in to far away transmissions, and they will often send a reception report to the sending station to let them know where they were heard. After receiving a report, the sending station may mail the listener aQSL card to acknowledge this reception.
Reception of long-wave signals at distances in excess of 17,000 kilometres (11,000 mi) have been verified.[15]
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