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2025-02-24 09:14 本頁面
【文章內(nèi)容簡介】 dio. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification. In North American broadcasting practice, transmitter power input to the antenna for mercial AM stations ranges from about 250 watts to 50,000 watts. Experimental licenses were issued for up to 500,000 watts radiated power, for stations intended for widearea munication during disasters including Cincinnati station WLW, which used 2 such power on occasion before World War II. WLW39。s superpower transmitter still exists at the station39。s suburban transmitter site, but it was demissioned in the early 1940s and no current mercial broadcaster in the US or Canada is authorized for such power levels. Some other countries do authorize higher power operation (for example the Mexican station XERF formerly operated at 250,000 watts). Antenna design must consider the coverage desired and must direct the transmitted signal so as not to interfere with other stations operating on the same or adjacent and shortwave radio signals act differently during daytime and nighttime. During the day, AM signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred miles (or kilometers) from the signal transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling AM radio stations to be heard much farther from their point of ori gin than is normal during the day. This phenomenon can be easily observed by scanning an AM radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. Such stations are monly referred to as the United States and Canada, some AM radio stations are granted clear channel status, meaning that they broadcast on frequencies with few other stations allocated, allowing an extended coverage area. Relatively few stations enjoy clearchannel status. Commercial broadcasters generally rely on the groundwave coverage only as their target market for hobby of listening to long distance signals is known as DX or DX39。ing, from an old telegraph abbreviation for distance. Several nonprofit hobbyist clubs are devoted exclusively to DXing the AM broadcast band, including the National Radio Club and International Radio Club of America. Similarly, people listening to short wave transmissions are SWLing. Frequency bandsAM radio
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