Medium frequency

Medium frequency
Frequency range
 0.3 to 3 MHz
Wavelength range
 1000 to 100 m
MF's position in the electromagnetic spectrum.

Medium frequency (MF) is the ITU designation[1] for 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 to one hectometer (1,000 to 100 m). Frequencies immediately below MF are denoted low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons and maritime ship-to-shore communication.

Uses and applications

Mast radiator of a commercial MF AM broadcasting station, Chapel Hill, North Carolina, USA

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 World Radiocommunication Conference 2012 (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 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.[2]

Medium wave radio stations are allocated an AM broadcast band from 526.5 kHz to 1606.5 kHz[3] in Europe; in North America this extends from 535 kHz to 1705 kHz.[4]

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.[5]

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.[6] RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.[7]

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.

More broadcast stations are found in the 120 meter band from 2300 to 2495 kHz. These frequencies are mostly used in tropical areas. Although these are medium frequencies, 120 meters is generally treated as one of the shortwave bands.

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.[8]

Propagation

Propagation at MF wavelengths is via ground waves and skywaves. Ground waves follow the curvature of the Earth. At MW wavelengths they can bend (diffract) over hills, and cover a radius of several hundred miles from the transmitter, with longer distances over water and damp earth.[9] 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,

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. Due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasting stations several hundred miles apart. On nights of good skywave propagation, the signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency. 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

Ferrite loopstick receiving antenna used in AM radios
Cage T antenna used by amateur radio transmitter on 1.5 MHz.

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.

The most common transmitting antenna, the quarter wave monopole, is physically large at these frequencies (25 to 250 metres (82 to 820 ft) requiring a tall radio mast. Usually the metal mast itself is used as the antenna, and is mounted on a large porcelain insulator to isolate it from the ground; this is called a mast radiator. The monopole antenna, particularly if electrically short requires a good, low resistance Earth ground connection for efficiency, since the ground resistance is in series with the antenna and consumes transmitter power. Commercial radio stations use a ground system consisting of many heavy copper cables, buried a few feet in the earth, radiating from the base of the antenna to a distance of about a quarter wavelength. In areas of rocky or sandy soil where the ground conductivity is poor, above ground counterpoises are used.

Lower power transmitters often use electrically short quarter wave monopoles such as inverted-L or T antennas, which are brought into resonance with a loading coil at their base.

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 (also known as a ferrite rod aerial), 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.

See also

References

  1. ↑ "Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications" (PDF). ITU. Retrieved 20 February 2013.
  2. ↑ The 500 KC Amateur Radio Experimental Group
  3. ↑ "United Kingdom Frequency Allocation Table 2008" (PDF). Ofcom. p. 21. Retrieved 2010-01-26.
  4. ↑ "U.S. Frequency Allocation Chart" (PDF). National Telecommunications and Information Administration, U.S. Department of Commerce. October 2003. Retrieved 2009-08-11.
  5. ↑ How to listen to cordless telephone conversations
  6. ↑ MF/HF SSB Frequencies
  7. ↑ http://www.hffax.de/Northwood-95.txt
  8. ↑ http://www.ntia.doc.gov/osmhome/allochrt.pdf U.S. Government Frequency Allocation Chart
  9. ↑ "Ground wave MF and HF propagation" (PDF). Introduction to HF Propagation. IPS Radio and Space Services, Sydney Australia. Retrieved 27 September 2010.

Further reading

External links

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