Medium frequency
Medium frequency spans 300 kHz to 3 MHz, key for AM broadcasting.
Medium frequency (MF) refers to radio frequencies from 300 kilohertz (kHz) to 3 megahertz (MHz), as defined by the International Telecommunication Union. This band includes the medium wave (MW) AM broadcast band and is sometimes called the hectometer band because its wavelengths range from 1000 to 100 meters (ten to one hectometers). The band just below MF is low frequency (LF), and the one above is high frequency (HF). MF is primarily used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.
Radio waves in the MF range travel via ground waves and skywaves. Ground waves hug the Earth's surface, bending over hills and traveling beyond the visual horizon, though mountains can block them. The ground absorbs these waves, so signal strength drops exponentially with distance. Typical MF stations cover a radius of several hundred kilometers or miles, with longer ranges over water or damp ground. Broadcasting stations rely on ground waves for their listening areas.
Skywave propagation allows MF waves to travel much farther. When radiated upward, they can be refracted back to Earth by charged particle layers in the ionosphere—specifically the E and F layers. However, the D layer, which sits lower, can absorb MF waves when heavily ionized, such as during the day, in summer, or during high solar activity. This absorption interferes with skywave propagation. At night, especially in winter or during low solar activity, the D layer nearly disappears. Then, MF waves can be refracted by the F layer and received hundreds or even thousands of miles away. This aids long-distance communication but can also cause interference with local stations. Because the medium wave broadcast band has limited channels, the same frequencies are reused by stations hundreds of miles apart. On nights with good skywave propagation, distant signals may reflect off the ionosphere and disrupt local stations on the same frequency. The North American Regional Broadcasting Agreement (NARBA) reserves certain channels—called clear channels—for nighttime skywave use by specially licensed AM stations, which must broadcast at higher powers of 10 to 50 kW.
A major use of MF is AM broadcasting.
- Frequency range
- 300 kHz to 3 MHz
- Wavelength range
- 1000 to 100 m
- Am broadcast band europe
- 526.5 kHz to 1606.5 kHz
- Am broadcast band north america
- 525 kHz to 1705 kHz
- Maritime distress frequency
- 2182 kHz
- Amateur band 160 meters
- 1800 to 2000 kHz
- Clear channel power range
- 10 to 50 kW
Lore & Background
Radio waves at MF wavelengths propagate via ground waves and reflection from the ionosphere (called skywaves). Ground waves travel just above the earth's surface, following the terrain. At these wavelengths, they can bend (diffract) over hills, and travel beyond the visual horizon, although they may be blocked by mountain ranges. Ground waves are progressively absorbed by the Earth, so the signal strength decreases exponentially with distance from the transmitting antenna. Typical MF radio stations can cover a radius of several hundred kilometres/miles from the transmitter, with longer distances over water and damp earth. MF broadcasting stations use ground waves to cover their listening areas.
MF waves can also travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are refracted back to Earth by layers of charged particles (ions) in the ionosphere, the E and F layers. However, at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is heavily ionised, such as during the day, in summer and especially at times of high solar activity. At night, especially in winter months and at times of low solar activity, the ionospheric D layer can virtually disappear. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer. This can be very useful for long-distance communication, but can also interfere with local stations.
Reader's Guide
Medium frequency remains significant for its enduring role in AM radio broadcasting, which occupies the medium wave band from 526.5 kHz to 1606.5 kHz in Europe and 525 kHz to 1705 kHz in North America. The band also supports maritime safety through the international calling and distress frequency of 2182 kHz for SSB voice communication, and includes Navtex broadcasts on 518 kHz and 490 kHz as part of the Global Maritime Distress Safety System. Non-directional navigational beacons for maritime and aircraft navigation occupy 190 to 435 kHz, overlapping from LF into MF. Amateur radio operators use the 160-meter band (1800–2000 kHz) for CW, digital, and voice modes, and have a secondary allocation at 472–479 kHz following WRC-2012. The band's propagation characteristics—ground waves for reliable local coverage and skywaves for long-distance night-time communication—have shaped frequency allocation agreements such as the North American Regional Broadcasting Agreement (NARBA), which designates clear channels for high-power stations. Transmitting antennas for MF are physically large, often using mast radiators up to 250 m tall, while receiving antennas like ferrite loopsticks are compact and efficient due to the high atmospheric noise floor. The legacy of MF is its foundational support for broadcast, maritime, and aeronautical communication, with ongoing amateur and emergency use.
Did You Know?
- The MF band is also called the hectometer band because wavelengths range from 1000 to 100 m.
- 2182 kHz is the international calling and distress frequency for SSB maritime voice communication.
- At night, the ionospheric D layer can virtually disappear, allowing MF signals to travel thousands of miles via skywave.
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