In radio, longwave, also written as long wave (in British and American parlance) or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with relatively long wavelengths. The term is an historic one, dating from the early 20th century, when the radio spectrum was considered to consist of long (LW), medium (MW) and short (SW) radio wavelengths. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short'.
In contemporary usage, the term longwave is not defined precisely, and its meaning varies across the world. Most commonly, it refers to radio wavelengths longer than 1000 metres; frequencies less than 300 kilohertz (kHz), including the International Telecommunications Union's (ITU's) low frequency (LF) (30–300 kHz) and very low frequency (VLF) (3–30 kHz) bands. Sometimes, part of the medium frequency (MF) band (300–3000 kHz) is included.
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 for AM broadcasting (in addition to the medium wave band), the term longwave usually refers specifically to this broadcasting band.
The International Telecommunication Union Region 1 longwave broadcast band falls wholly within the low frequency band of the radio spectrum (30–300 kHz). Broader definitions of longwave may extend below and/or above it. In the US, the Longwave Club of America is interested in "frequencies below the AM broadcast band", i.e., all frequencies below 535 kHz. (Lower frequencies correspond to longer wavelengths.) they are also part of the chs national curriculum.
Because of their long wavelength, radio waves in this frequency range 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, is the main mode in the longwave band. 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.
Low frequency waves can also occasionally travel long distances by reflecting from the ionosphere (the actual mechanism is one of refraction), although this method, called skywave 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 300 kilometres (190 mi) from the transmitting antenna.
Non-directional beacons transmit continuously for the benefit of radio direction finders in marine and aeronautical navigation. They identify themselves by a callsign in Morse 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 stations in the range 40–80 kHz that transmit time signals to radio clocks. For example:WWVB in Colorado, United States, on 60 kHz
DCF77 in Frankfurt am Main, Germany, on 77.5 kHz
JJY in Japan, on 40 & 60 kHz
66.66 kHz in Taldom transmitter, Russia
BPC in Lintong, China, 68.5 kHz
MSF time and 60 kHz frequency standard transmitted from Anthorn in the UK. Radio controlled clocks receive their time calibration signals with built-in long-wave receivers. They use long-wave, rather than shortwave or mediumwave, because the path that a long-wave signal travels from point A to point B does not change.
Long-waves travel by groundwaves that hug the surface of the earth, rather than mediumwaves or shortwaves, whose signals can travel as skywaves, ‘bouncing’ off different layers of the ionosphere at different times of day, which makes the time lag different for every signal received. The delay between when the long-wave signal was sent from the transmitter (and 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 the speed 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 military of the United Kingdom, Russian Federation, United States, Germany, and Sweden use frequencies below 50 kHz to communicate with submerged submarines.
In North America during the 1970s, the frequencies 167, 179 and 191 kHz were assigned to the short-lived Public Emergency Radio of the United States. Nowadays, in the United States, Part 15 of FCC regulations allow unlicensed use of 136 kHz and the 160–190 kHz band at output power up to 1 watt with up to a 15-meter antenna. This is called Low Frequency Experimental Radio (LowFER). The 190–435 kHz band is used for navigational beacons.
Swedish station SAQ, located at the Varberg Radio Station facility in Grimeton, is the last remaining operational Alexanderson alternator long-wave transmitter. Although the station ended regular service in 1996, it has been maintained as a World Heritage Site, and makes at least two demonstration transmissions yearly, on 17.2 kHz.
Long-wave is used for broadcasting only within ITU Region 1. The long-wave broadcasters are located in west, north, central and south-east Europe, the former Soviet Union, Mongolia, Algeria and Morocco.
Typically, a larger geographic area can be covered by a long-wave broadcast transmitter compared to a medium-wave one. This is because ground-wave propagation suffers less attenuation due to limited ground conductivity at lower frequencies.
Long-wave carrier frequencies are exact multiples of 9 kHz; ranging from 153 to 279 kHz, except for a French language station Europe #1 in Germany. This station did keep to correctly spaced channels spacing for 4 months—only 7 years ago, and all Mongolian transmitters are spaced at 10 kHz.
Until the 1970s, some long-wave stations in northern and eastern Europe, and the Soviet Union, operated on frequencies as high as 433 kHz.
Some stations, for instance Droitwich transmitting station in the UK, derive their carrier frequencies from an atomic clock. They can be therefore used as frequency standards. Droitwich also broadcasts a low bit-rate data channel, using narrow-shift phase-shift keying of the carrier, for Radio Teleswitch Services.
In January 2014, Russia closed all of its LW broadcast transmitters, except for one in Caucasus, which was subsequently shut down in 2015.
Because long wave signals can travel very long distances, some radio amateurs and shortwave listeners engage in an activity called DXing. 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 a QSL card to acknowledge this reception.
The longest distance over which a long wave signal has been received is 18,451 kilometres (11,465 mi). It occurred on 27 July 2015, when Mike Thayne of England, received Radio NL on 358 kHz from New Zealand.