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U : CW abbreviation for "you"
UFB : CW abbreviation for "Ultra Fine Business" or "excellent"
UHF : Ultra High Frequency. Frequencies in the range 300MHz to 3 000MHz
UHF connector : A threaded co-axial RF connector (PL-259, SO-239) originally designed in the 1930's for use at frequencies in the 30-300MHz range. (These frequencies are now considered to be VHF.) Most commonly used in amateur radio stations to make RG-8 or RG-213 cable connections to HF transceivers.
UR : CW abbreviation for "your"
USB : (1) Upper Sideband, used primarily for single sideband operation above 10MHz
USB : (2) Universal Serial Bus, an interface to desktop PC peripherals