The Kᵤ band is the portion of the electromagnetic spectrum in the microwave range of frequencies from 12 to 18 gigahertz. The symbol is short for "K-under", because it is the lower part of the original NATO K band, which was split into three bands because of the presence of the atmospheric water vapor resonance peak at 22.24 GHz, which made the center unusable for long range transmission. From Wikipedia
