Longwave
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.
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