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When this happens, MW radio waves can easily be received many hundreds or even thousands of miles away as the signal will be reflected by the higher F layer.This can allow very long-distance broadcasting, but can also interfere with distant local stations.Initially, broadcasting in the United States was restricted to two wavelengths: "entertainment" was broadcast at 360 meters (833 k Hz), with stations required to switch to 485 meters (619 k Hz) when broadcasting weather forecasts, crop price reports and other government reports.This arrangement had numerous practical difficulties. Early transmitters were technically crude and virtually impossible to set accurately on their intended frequency and if (as frequently happened) two (or more) stations in the same part of the country broadcast simultaneously the resultant interference meant that usually neither could be heard clearly.In North America, the North American Regional 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 they are required to broadcast at higher powers of 10 to 50 k W.In the US and Canada the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2 megawatts daytime.
Most broadcast stations use groundwave to cover their listening area.
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 skywave signals of distant station may interfere with the signals of local stations on the same frequency.
The Commerce Department rarely intervened in such cases but left it up to stations to enter into voluntary timesharing agreements amongst themselves.
The addition of a third "entertainment" wavelength, 400 meters, did little to solve this overcrowding.
A tower in Dubai that opens today has earned the title of world’s tallest building with a height of 2,717 feet (828 meters). Actually, it grabbed that title during construction back in July 2007 when it passed Taipei 101, which stands 500 meters tall. Until its official opening today, the building’s exact height was a closely held secret known by only a few people. The tower, which had been known as the Burj Dubai, was renamed the Burj Khalifa, in honor of Sheik Khalifa bin Zayed Al Nahyan, the president of the neighboring emirate Abu Dhabi. Speaking of the acrophobia inducing elevator, it travels at speeds roughly 40 miles per hour (65 kilometers per hour) and reaches the observation deck in about 2 minutes. Once at the top, visitors can enjoy temperatures that are nearly 15 degrees cooler than at the building’s base. Dubai is built in the middle of the desert, so to withstand the UAE’s 120-degree blistering summer heat the tower is covered with 24,348 cladding panels. Many skyscrapers are built to bend with the wind—the Burj, which will be exposed to strong desert winds, more than others.