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In the UHF band, signals from earth-based transmitters are not returned by the ionosphere to the surface; they always pass into space. Conversely, signals from space always penetrate the ionosphere and reach the surface. The global "shortwave" propagation familiar to users of lower frequencies is unknown at UHF. The troposphere can cause bending, ducting, and scattering at UHF, extending the range of communication significantly beyond the visual horizon. Auroral, meteor-scatter, and EME (earth-moon-earth, also called moonbounce) propagation are sometimes observed, but these modes do not offer reliable communication and are of interest primarily to amateur radio operators. In the upper portion of the band, waves can be focused or collimated by dish antennas of modest size.
The UHF band is extensively used for satellite communication and broadcasting, in cellular telephone and paging systems, and by third-generation (3G) wireless services. Because the frequency is high and the band is vast (a span of 2.7 gigahertz from the low end to the high end), wideband modulation and spread spectrum modes are practical. Channels and subbands within the UHF portion of the radio spectrum are allocated by the International Telecommunication Union (ITU).
'UHF (ultrahigh frequency)' is part of the:
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