The term Lowest Usable Frequency (LUF) is used in antenna technology and communications engineering
. The Lowest Usable Frequency (LUF) is the lowest frequency that is not absorbed by the ionosphere or obscured by atmospheric noise when radiated. At the LUF frequency, a space wave can be used to establish a radio link between two locations on the Earth's surface. The Lowest Usable Frequency is related to the Maximum Usable Frequency (MUF) and the Frequency of Optimum Transmission (FOT).