Medium wave
Mediumwave (MW) radio transmissions 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. In most of 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. 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 North American 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 Europe, each country is allocated a number of frequencies on which high power (up to 2.5 MW) can be used; the maximum power is also subject to international agreement. Other countries may only operate low-powered transmitters on the same frequency, again subject to agreement. For example, Russia operates a high-powered transmitter, located in its Kaliningrad exclave and used for external broadcasting, on 1323 kHz. The same frequency is also used by low-powered local radio stations in England; other parts of England can still receive the Russian broadcast. International mediumwave broadcasting in Europe has decreased markedly with the end of the Cold War and the increased availability of satellite and Internet TV and radio, although the cross-border reception of neighbouring countries’ broadcasts by expatriates and other interested listeners still takes place. Due to the high demand for frequencies in Europe, many countries operate single frequency networks; in Britain, BBC Radio 5 broadcasts from various transmitters on either 693 or 909 kHz. These transmitters are carefully synchronised to minimise interference from more distant transmitters on the same frequency.
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