Lowest usable high frequency

The Lowest Usable Frequency (LUF) is the lowest, barely usable frequency in the short wave range skywave propagation, in which the field strength at the receiver is strong enough for a certain time interval at 90 % of the ionospheric undisturbed days in a month a sufficient signal -to-noise to guarantee ratio. If this frequency is exceeded, the signal is attenuated too much in the lower ionospheric layers and can not be reflected on the ionosphere.

Ionospheric attenuation depends on the electron density. Thus, the LUF will change depending on the day and season. Furthermore, the solar activity has a dominant influence on the electron density.