Lowest Usable Frequency
The Lowest Usable Frequency (LUF) is the lowest, just usable frequency in the shortwave range in the case of skywave propagation , at which the field strength at the receiving location is strong enough to ensure sufficient signal-to-noise level for a certain time interval on 90% of the ionospheric undisturbed days of a month. To ensure relationship. If this frequency is not reached, the signal in the lower ionospheric layers is attenuated too much and can no longer be reflected by the ionosphere .
The ionospheric attenuation depends on the electron density. Thus, the LUF changes depending on the time of day and time of year . Furthermore, solar activity has a dominant influence on electron density.