Low frequency
Low frequency waves enable long-distance ground wave communication.
Low frequency (LF) is the International Telecommunication Union's term for radio frequencies between 30 and 300 kHz. Because their wavelengths span 10 to 1 kilometers, they are also called the kilometre band or kilometre waves. The band just below LF is very low frequency (VLF), and the one just above is medium frequency (MF).
LF radio waves lose very little signal strength over distance, which makes them good for long-range communication. In Europe, parts of Asia, and Northern Africa, a portion of the LF spectrum is used for AM broadcasting under the name "longwave." In the Western Hemisphere, LF is mainly used for aircraft beacons, navigation (like the now mostly defunct LORAN system), weather systems, and information broadcasts. Several time signal stations also operate in this band.
The primary way LF waves travel is as ground waves: they move just above the Earth's surface, following the terrain. These ground waves can cross hills and reach hundreds of kilometers beyond the horizon. Atmospheric radio noise increases as frequency drops, and at LF and below, this noise is much stronger than the thermal noise from a receiver's amplifier circuits. This means weak signals can be amplified without noticeably raising the noise level, so even inefficient antennas much smaller than the wavelength can receive them.
Because of their long wavelengths, LF waves can diffract around obstacles like mountain ranges and follow the Earth's curve. As ground waves travel, the Earth absorbs them, so signal power drops exponentially with distance—though less sharply than at higher frequencies. LF ground waves can be received up to 2,000 kilometers from the transmitter. They must be vertically polarized (electric field vertical, magnetic field horizontal), so vertical monopole antennas are used for transmission.
LF waves can also travel long distances by reflecting off the ionosphere's E or F layers (actually a refraction process), though this skywave or "skip" propagation is less common than at higher frequencies. Skywave signals can be detected more than 3,000 kilometers from the transmitter.
**Radio broadcasting** AM broadcasting is allowed in the longwave band from 148.5 to 283.5 kHz in Europe and parts of Asia.
- Frequency range
- 30–300 kHz
- Wavelength range
- 10–1 km
- Main propagation mode
- Ground waves
- Maximum ground wave reception distance
- Up to 2000 km
- Skywave reception distance
- Exceeding 3000 km
- European longwave broadcast band
- 148.5–283.5 kHz
- Amateur radio allocation
- 135.7–137.9 kHz
Lore & Background
Low frequency radio waves can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This ground wave propagation is the main mode in the LF band, with radio waves traveling horizontally through the atmosphere just above the surface. Ground waves are absorbed by the Earth as they travel, so signal strength decreases exponentially with distance, but attenuation is lower than at higher frequencies. Ground waves must be vertically polarized, so vertical monopole antennas are used for transmitting. Low frequency waves can also occasionally travel long distances by reflecting from the ionospheric E layer or F layers, a method called skywave or 'skip' propagation, though this is not as common as at higher frequencies.
In Europe and Japan, many low-cost consumer devices since the late 1980s have contained radio clocks with an LF receiver for time signals. Since these frequencies propagate by ground wave only, the precision of time signals is not affected by varying propagation paths. In the United States, such devices became feasible for the mass market only after the output power of the WWVB time signal radio station was increased in 1997 and 1999. The JJY time signal radio station broadcasts on the exact same frequency and has a similar timecode.
Radio signals below 50 kHz are capable of penetrating the ocean to depths of approximately 200 metres; the longer the wavelength, the deeper the radio waves can go. The British, German, Indian, Russian, Swedish, United States, and possibly other navies communicate with submarines on these frequencies. 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, and it is rumoured that they are to construe a sudden halt in transmission as an indicator that the UK is under attack.
Reader's Guide
Low frequency remains significant for its unique propagation characteristics that enable reliable long-distance communication without reliance on satellite or ionospheric reflection. The band's ground wave propagation allows signals to travel over hills and far beyond the horizon, up to several hundred kilometers from the transmitter, with lower attenuation than higher frequencies. This makes LF valuable for time signal broadcasts, where precision is unaffected by varying propagation paths, and for military submarine communications, as signals below 50 kHz can penetrate ocean depths of approximately 200 metres.
The legacy of LF includes systems like the LORAN-C radio navigation system operating on 100 kHz and the Decca Navigator System between 70 and 129 kHz, though the last Decca chains closed in 2000. The Ground Wave Emergency Network (GWEN) operated between 150 and 175 kHz until replaced by satellite communications in 1999. Today, non-directional beacons for aeronavigation operate on 190–300 kHz in parts of the world without longwave broadcasting, and differential GPS telemetry transmitters operate between 283.5 and 325 kHz. The amateur radio allocation at 135.7–137.9 kHz has enabled contacts exceeding 10,000 km, demonstrating the band's continued utility for experimental communication.
Did You Know?
- LF ground waves can be received up to 2000 km from the transmitting antenna.
- Skywave signals from LF can be detected at distances exceeding 3000 km.
- Radio signals below 50 kHz can penetrate the ocean to depths of approximately 200 metres.
- The world record distance for a two-way amateur contact on LF is over 10,000 km from near Vladivostok to New Zealand.
Frequently Asked Questions
Who is Low frequency?
Low frequency, often abbreviated LF, is the ITU-designated radio band covering 30 to 300 kHz, corresponding to wavelengths of 10 down to 1 kilometer. Because of those metre-scale wavelengths, enthusiasts commonly nickname it the 'kilometre band' or 'kilometre waves.'
What are Low frequency's powers and abilities?
LF waves barely shed energy as they travel, letting a single ground-wave signal carry up to roughly 2,000 km without a repeater. When a skywave does form, reception can stretch past 3,000 km, making LF one of the longest-reach bands in the spectrum.
Where does Low frequency sit among its spectrum neighbours?
LF is sandwiched between very low frequency (VLF) on the lower side and medium frequency (MF) on the upper side. It is the bridge band where wavelengths shrink from the tens of kilometres of VLF down toward the single-kilometre range of MF.
What is Low frequency's role in broadcasting?
In Europe, parts of Asia, and Northern Africa, a slice of the LF band—specifically 148.5 to 283.5 kHz—is licensed for AM radio and marketed to listeners as 'longwave.' In the Western Hemisphere that same frequency range is largely reserved for other services rather than public broadcasting.
Why is Low frequency important to the radio community?
LF's minimal path loss and reliable ground-wave propagation make it a go-to choice whenever a signal must cross vast land or sea areas without satellite or microwave infrastructure. Its combination of long reach and relatively simple receiver technology keeps it a staple of long-distance communication.
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