IonoFlux field guide
MUF (maximum usable frequency)
MUF explained with an easy HF propagation example.
In simple terms
MUF is the approximate upper frequency limit for a radio wave to complete a path by ionospheric reflection. It changes with location, time, distance and solar conditions.
How to read it
A frequency below MUF is more likely to return, but too low can suffer more noise and absorption. The best operating frequency is usually below the MUF, not exactly on it.
A practical example
If a path MUF is 18 MHz, 17 m may be a good candidate while 15 m is less reliable. Terrain, antennas and actual ionosphere conditions still matter.