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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).
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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.
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 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.[1]
Medium wave radio stations are allocated an AM broadcast band from 526.5 kHz to 1606.5 kHz[2] in Europe; in North America this extends from 535 kHz to 1705 kHz.[3]
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.[4]
There is an amateur radio band known as 160 meters or 'top-band' 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.[5] RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.[6]
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.[5][7]
Propagation at MF is often via ground waves. Ground wave propagation at these frequencies follows the curvature of the Earth over conductive surfaces such as the sea and damp earth. At sea, MF communications can typically be heard over several hundred miles.[8]
Conversely, MF Skywave propagation depends on the various layers in the Ionosphere as the signal is refracted back down to earth by the E and F layers. When heavily ionised, such as during the day, in summer and especially at times of high solar activity, the D layer (At a lower altitude than the refractive E and F layers) can be electronically noisy and absorptive of MF waves.
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.
Even a quarter-wave antenna at MF can be physically large (25 to 250 metres (82 to 820 ft), 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.
On the other hand, ferrite is very efficient at MF and so a compact and efficient reception antenna can be made from a ferrite rod with a coil of fine wire wound around it. These are common in AM radios and 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.
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