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Medium frequency (MF) refers to radio frequencies (RF) in the range of 300 kHz to 3 MHz. Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten down to one hectometers (1,000 to 100 m). Frequencies immediately below MF are denoted low frequency (LF), and the next higher frequencies are known as high frequency (HF). It is used for AM radio broadcasting, navigational radio beacons, and maritime ship-to-shore communication, among other uses.

Uses and applications


Non-directional navigational radio beacons (NDBs) for maritime and aircraft navigation occupy a band from 190 to 435 kHz, which overlaps from the LF into the bottom part of the MF band.

Following WRC-2012, the amateur service received a new allocation between 472 and 479 kHz for narrow band modes and secondary service, after extensive propagation and compatibility studies made by the ARRL 600 meters Experiment Group and their partners around the world.

500 kHz was for many years the maritime distress and emergency frequency, and there are more NDBs between 510 and 530 kHz. Navtex, which is part of the current Global Maritime Distress Safety System (GMDSS) occupies 518 kHz and 490 kHz for important digital text broadcasts. In recent years, some limited amateur radio operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden.

A medium frequency radio when used for Global Maritime Distress Safety System (GMDSS) is mandatory for sea area A2, which is not found in the United States. The United States has rescue 21 in place sea areas A1 and A2, due to this fact vessels operating off the coast of the United States are required to carry equipment for sea area A3.

The minimum requirement for MF radios in marine vessels which operate in sea area A2, is that they must be capable of transmitting and receiving on all distress and safety frequencies in the (marine) bands between 1,605 kHz and 27,500 kHz using DSC, radiotelephony and narrow-band direct printing (NBDP). On top of that the radio must also have watchkeeping receiver capable of maintaining DSC watch on 2187.5 kHz, 8,414.5 kHz and on at least one of the distress and safety DSC frequencies 4,207.5 kHz, 6,312 kHz, 12,577 kHz or 16,804.5 kHz, and at any time it should be possible to select any of those frequencies.

Medium wave radio stations are allocated an AM broadcast band from 526.5 kHz to 1606.5 kHz in Europe; in North America this extends from 535 kHz to 1705 kHz.

Many home-portable or cordless telephones, especially those that were designed in the 1980s, transmit low power FM audio signals between the table-top base unit and the handset on frequencies in the range 1600 to 1800 kHz.

An amateur radio band known as 160 meters or 'top-band' is placed between 1800 and 2000 kHz (allocation depends on country and starts at 1810 kHz outside the Americas). Amateur operators transmit CW morse code, digital signals and SSB voice signals on this band.

There are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz. These include, as examples, the French MRCC on 1696 kHz and 2677 kHz, Stornoway Coastguard on 1743 kHz, the US Coastguard on 2670 kHz and Madeira on 2843 kHz. RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.

2182 kHz is the international calling and distress frequency for SSB maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band.

Lastly, there are aeronautical and other mobile SSB bands from 2850 kHz to 3500 kHz, crossing the boundary from the MF band into the HF radio band.

Propagation


Propagation at MF wavelengths is via ground waves and skywaves. Ground waves follow the curvature of the Earth. At MW wavelengths they can cover a radius of several hundred miles from the transmitter, with longer distances over water and damp earth. MW broadcasting stations use ground waves to cover their listening areas.

MF can also travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are reflected (actually refracted) back to Earth by the ionosphere 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 absorptive of MF 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.

Late 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 on a quiet frequency, but can have the opposite effect in many other cases. For example, due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasters provided they transmit several hundred miles apart. On nights of good MF propagation, distant stations may appear superimposed onto local ones causing interference. In North America, the North American Radio Broadcasting Agreement (NARBA) sets aside certain channels for nighttime use over extended service areas via skywave by a few specially licensed AM broadcasting stations. These channels are called clear channels, and the stations, called clear-channel stations, are required to broadcast at higher powers of 10 to 50 kW.

Antennas


Transmitting antennas commonly used on this band include monopole mast radiators, top-loaded wire monopole antennas such as the inverted-L and T antennas, and wire dipole antennas. Ground wave propagation, the most widely-used type at these frequencies, requires vertically polarized antennas like monopoles.

Even a quarter-wave antenna at MF can be physically large (25 to 250 m, depending for which part of the band), and a half-wave dipole will be twice that size. Given the requirements for gaining an adequate height and for a good earth, this can make demands on establishing an efficient antenna system for an MF transmitter.

Receiving antennas do not have to be as efficient as transmitting antennas since the signal to noise ratio is determined by atmospheric noise, so antennas small in comparison to the wavelength can be used. The most common receiving antenna is the ferrite loopstick antenna, made from a ferrite rod with a coil of fine wire wound around it. In addition to their use in AM radios they are also used in portable radio direction finder (RDF) receivers. The reception pattern of ferrite rod antennas has sharp nulls along the axis of the rod, so that reception is at its best when the rod is at right angles to the transmitter, but fades to nothing when the rod points exactly at the transmitter. Other types of loop antennas and random wire antennas are also used.