Low frequency
In Europe and areas of Northern Africa and Asia, part of the LF spectrum is used for AM broadcasting as the "longwave" band.
Because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth.
In Europe and Japan, many low-cost consumer devices have since the late 1980s contained radio clocks with an LF receiver for these signals.
Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 metres (660 ft); the longer the wavelength, the deeper they go.
The British, German, Indian, Russian, Swedish, United States,[5] and possibly other navies communicate with submarines 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.
GWEN was a land based military radio communications system which could survive and continue to operate even in the case of a nuclear attack.
[10] Very slow Morse Code from G3AQC in the UK was received 3,275 miles (5,271 km) away, across the Atlantic Ocean, by W1TAG in the US on 21-22 November 2001 on 72.401 kHz.
[a] In the United States, there is an exemption within FCC Part 15 regulations permitting unlicensed transmissions in the frequency range of 160–190 kHz.
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.
[14] In parts of the world where there is no longwave broadcasting service, Non-directional beacons used for aeronavigation operate on 190–300 kHz (and beyond into the MW band).
[15] The commercial "Datatrak" radio navigation system operates on a number of frequencies, varying by country, between 120–148 kHz.
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 human exposure to radio waves.
Amateur radio operators have achieved good LF reception using active antennas: A short whip with a built-in pre-amplifier.
Due to the long wavelengths in the band, nearly all LF antennas are electrically short, shorter than one quarter of the radiated wavelength, so their low radiation resistance makes them inefficient, requiring very low resistance grounds and conductors to avoid dissipating transmitter power.
