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The microwave band, usually taken to be from 1GHz to 30GHz has been subdivided into a number of sub-bands which have been given letter designations such as X band (8GHz to 12GHz), but differing definitions are used and can cause confusion. 30GHz to 100GHz is commonly referred to as the millimetric band.

https://www.sciencedirect.com/topics/engineering/radio-frequency-spectrum#:~:text=The microwave band, usually taken,to as the millimetric band.
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