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The 80 meter or 3.5 MHz band is a band of radio frequencies allocated for amateur radio use, from 3.5 to 4.0 MHz in IARU Region 2, and generally 3.5 to 3.8 or 3.9 MHz in Regions 1 and 3 respectively. The portion of the band used for phone (voice) communications is sometimes referred to as 75 meters. 80 meters is the most popular band for regional communications networks through the late afternoon and night time hours. It is usually reliable for short to medium distance contacts, with average distances ranging from local contacts within 200 miles/300 km out to a distance of 1,000 miles/1,600 km or more, depending on atmospheric and ionospheric conditions. ==Summary== The 80 meter band is favored for ragchews between amateurs within a range of 500 miles/800 km. During contests the band is filled with activity beginning before sunset and continuing all through the night. The 80-meter band begins at 3.5 MHz and goes up to 4.0 MHz. The upper part of the band, mostly used for voice, is often referred to as 75 meters, since the wavelength there is between 80 and 75 meters. In practice, the large physical size of antennas at this frequency (a quarter-wave vertical, for example, is approximately 65 feet/20 meters high) and the difficulty of ensuring significant low angle radiation from antennas to maximize the distance of "hops" are two of the challenges facing amateurs wishing to communicate over longer distances. Most amateurs interested in regional communication use low, wire antennas, such as dipoles, inverted vee dipole antennas or loop antennas on this band. Horizontally polarized antennas, when close to earth, produce predominantly high-angle radiation. High angles are useful for Near Vertical Incidence Skywave and other short distance propagation modes, but substantial distance can still be covered even with modest height antennas. 80 meters is plagued with noise, with the rural noise floor set by propagated noise from distant thunderstorms and man-made noise sources. Urban and suburban noise floor is often established by local noise, and is generally 10-20 dB stronger than the typical rural noise floor. On 80 meters, nearly all areas of the world are subject to local weather induced noises from thunderstorms. The Ionosphere's D-layer also affects 80 meters significantly by absorbing signals. During the daylight hours, a station in middle or high latitudes using 100 watts and a simple dipole antenna can expect a maximum communication range of 200 miles/300 km, extending to a few thousand miles or more at night. Global coverage can be routinely achieved during the late fall and winter by a station using modest power and common antennas. The higher background noise on 80 meters, especially when combined with higher ionospheric absorption, causes stations with higher effective radiated power to have a decided advantage in long distance communications. With very high antennas or large vertically polarized arrays and full legal power, reliable world-wide communications occurs over darkness paths. Mobile operation is possible, although the relative shortness of a practical mobile antenna (usually less than 10 feet/3 meters) compared to a quarter wavelength results in the need for significant inductive loading to achieve resonance. Since short antennas have very low radiation resistance, overall antenna efficiency is often limited to less than 10%. Additionally, the large inductance of the loading coil creates a very high Q antenna system, with an extremely narrow bandwidth. In spite of these difficulties (or perhaps ''because'' of them), dedicated mobile operators enjoy the challenge and routinely contact hundreds of stations, including DX stations at distances of up to 10,000 miles/16,000 km! 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「80-meter band」の詳細全文を読む スポンサード リンク
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