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low frequency
Low frequency (low freq) or LF is the ITU designation〔(【引用サイトリンク】url=http://www.itu.int/dms_pubrec/itu-r/rec/v/R-REC-V.431-7-200005-I!!PDF-E.pdf )〕 for radio frequencies (RF) in the range of 30 kHz–300 kHz. As its wavelengths range from one to ten kilometres, it is also known as the kilometre band or kilometre wave.
LF radio waves exhibit low signal attenuation, 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 as the "longwave" band. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN), information, and weather systems. A number of time signal broadcasts are also broadcast in this band.
==Propagation==
Because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and follow the curvature of the Earth. This mode of propagation, called ''ground wave'', is the main mode in the LF band. The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies. Low frequency ground waves can be received up to 2000 km from the transmitting antenna.
Low frequency waves can also 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 km from the transmitting antenna.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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