Medium frequency
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Uses and applications
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 USA, 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
Propagation at MF is usually 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] MF ground-wave propagation depends on the ionosphere's D-layer. When heavily ionised, such as during the day, especially in summer, and more especially at times of high solar activity, this atmospheric layer can be electronically noisy and absorptive of MF waves. For this reason, many MF transmitters, whether for broadcast or communication purposes, need to be high-powered to punch the power through the noise and losses.[8]Late at night, especially in winter months, and particularly 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. 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.
Transmission and reception
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.
See also
- Electromagnetic spectrum
- Maritime broadcast communications net
- Types of radio emissions
- Global Maritime Distress Safety System
- Navtex
References
- ^ The 500 KC Amateur Radio Experimental Group
- ^ "United Kingdom Frequency Allocation Table 2008". Ofcom. p. 21. http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/UK-FAT-Table-2008/ukfat08.pdf. Retrieved 2010-01-26.
- ^ "U.S. Frequency Allocation Chart". National Telecommunications and Information Administration, U.S. Department of Commerce. October 2003. http://www.ntia.doc.gov/osmhome/allochrt.pdf. Retrieved 2009-08-11.
- ^ totse.com | How to listen to cordless telephone conversations
- ^ a b MF/HF SSB Frequencies
- ^ http://www.hffax.de/Northwood-95.txt
- ^ http://www.ntia.doc.gov/osmhome/allochrt.pdf U.S. Government Frequency Allocation Chart
- ^ a b "Ground wave MF and HF propagation". Introduction to HF Propagation. IPS Radio and Space Services, Sydney Australia. http://www.ips.gov.au/Category/Educational/Other%20Topics/Radio%20Communication/Intro%20to%20HF%20Radio.pdf. Retrieved 27 September 2010.
Further reading
- Charles Allen Wright and Albert Frederick Puchstein, "Telephone communication, with particular application to medium-frequency alternating currents and electro-motive forces". New York [etc.] McGraw-Hill Book Company, inc., 1st ed., 1925. LCCN 25008275
External articles
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
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