Mediumwave
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Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. In most of the world, mediumwave serves as the most common band for broadcasting. The standard AM broadcast band is 525 kHz to 1715 kHz in North America, but remains only up to 1615 kHz elsewhere.
Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km away in New York City, due to groundwave propagation. The effectiveness of groundwave signals largely depends on ground conductivity—higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably. Many stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range.
In the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of ±5 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of ±4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands. In the US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts.
Stereo transmission is possible and offered by some stations in the US, Australia, South Africa, and France. However, there are multiple standards for AM stereo with C-QUAM being the legal one in the United States, and receivers that actually implement the technologies are relatively rare but not uncommon. Failed systems include Kahn Powerside and others.
In September 2002, the United States Federal Communications Commission approved the iBiquity in-band on-channel (IBOC) system of digital audio broadcasting, which is meant to improve the audio quality of signals. The Digital Radio Mondiale (DRM) IBOC system has been approved by the ITU for use outside the Americas.
Antennas
As aerials mostly mast radiators are used. Stations broadcasting with low power commonly use masts with heights of a quarter wavelength, while high power stations mostly use half wavelength. The usage of masts longer than 5/8 of radiated wavelength gives a bad radiation pattern. Usually mast antennas are insulated against ground and show a high voltage against ground during transmission, which complicates maintenance, installation of air safety warning lights or using the mast as a tower for UHF/VHF-radio, but there are several ways to use grounded masts or towers.
If grounded masts or towers are required, than cage aerials or longwire aerials are used. Another possibility consists of feeding the mast or the tower by cables running from the tuning unit to the guys or crossbars in a certain height. Directional aerials consist of multiple masts, which need not to be from the same height. It is also possible to realize directional aerials for mediumwave with cage aerials where some parts of the cage are fed with a certain phase difference.
Other type of aerials sometimes used for mediumwave are T- and L-aerials. The kind used depends on the need for grounded or insulated towers.
In some cases dipole aerials are used, which are spun between two masts or towers. Such aerials radiate toward the sky. The mediumwave transmitter at Berlin-Britz for transmitting RIAS used a cross dipole mounted on five 30.5 metre high guyed masts to transmit the skywave up to the ionosphere at nighttime.
Non-broadcast use
For most of the 20th century, the radio frequency 500 kHz was reserved world wide as the Morse code international calling and distress frequency for ships on the high seas. The frequency 2182 kHz is still used for this purpose, but employing voice transmission.
Other services that operate in medium wave include Navtex and the Amateur Radio 160-meter band. The obsolete LORAN-A system used medium wave.
See also
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Radio spectrum |