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Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 MHz and 3 GHz, also known as the decimetre band as the wavelengths range from one meter to one decimetre. Radio waves with frequencies above the UHF band fall into the SHF (super-high frequency) or microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is high enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications. The IEEE defines the UHF radar band as frequencies between 300 MHz and 1 GHz.〔(IEEE Std 521-2002 ''Standard Letter Designations for Radar-Frequency Bands'' ).〕 Two other IEEE radar bands overlap the ITU UHF band: the L band between 1 and 2 GHz and the S band between 2 and 4 GHz. == Characteristics == (詳細はsolar wind, physical obstructions (such as mountains and buildings), and time of day all affect the signal transmission and the degradation of signal reception. All radio waves are partly absorbed by atmospheric moisture. Atmospheric absorption reduces, or attenuates, the strength of radio signals over long distances. The effects of attenuation degradation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, such as VHF TV signals. The ionosphere, a layer of the Earth's atmosphere, is filled with charged particles that can reflect some radio waves. Amateur radio enthusiasts primarily use this quality of the ionosphere to help propagate lower frequency HF signals around the world: the waves are trapped, bouncing around in the upper layers of the ionosphere until they are refracted down at another point on the Earth. This is called ''skywave transmission''. UHF TV signals are not carried along the ionosphere but can be reflected off of the charged particles down at another point on Earth in order to reach farther than the typical line-of-sight transmission distances; this is the skip distance. UHF transmission and reception are enhanced or degraded by tropospheric ducting as the atmosphere warms and cools throughout the day. Since the wavelengths of UHF signals are comparable to the size of buildings, trees, vehicles and other common objects, reflection and diffraction affects the propagation of UHF signals, especially in built-up urban areas. The main advantage of UHF transmission is the short wavelength that is produced by the high frequency. The size of transmission and reception antennas is related to the size of the radio wave. The UHF antenna is stubby and short. Smaller and less conspicuous antennas can be used with higher frequency bands. The major disadvantage of UHF is its limited broadcast range, often called ''line-of-sight'' between the TV station's transmission antenna and customer's reception antenna, as opposed to VHF's longer broadcast range. UHF is widely used in two-way radio systems and cordless telephones, whose transmission and reception antennas are closely spaced. Transmissions generated by two-way radios and cordless telephones do not travel far enough to interfere with local transmissions. Public safety, business communications and personal radio services such as GMRS, PMR446, and UHF CB are often found on UHF frequencies as well as IEEE 802.11 wireless LANs ("WiFi"). The widely adapted GSM and UMTS cellular networks use UHF cellular frequencies. A repeater propagates UHF signals when a distance greater than the line of sight is required. 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「Ultra high frequency」の詳細全文を読む スポンサード リンク
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