The MUF in one sentence

The MUF is the highest frequency the ionosphere will bend back to earth for a particular path at a particular time. Above it, your signal passes straight through into space. Just below it, signals are usually at their strongest.

Why distance changes the MUF

A signal hitting the ionosphere at a shallow angle bends back more easily than one going straight up. So a long path has a much higher MUF than a short one:

• Straight up (NVIS), the usable frequency is the critical frequency, foF2, often only 3 to 8 MHz.
• For a single hop of about 3,000 km, the MUF can be three times higher. That’s the “MUF(3000)” number you see on propagation charts.

That’s why 20 m can be wide open to Europe while you can’t reach Calgary on it.

The LUF: the bottom end

There’s also a lowest usable frequency (LUF). Below it, daytime D-layer absorption soaks up the signal before it gets anywhere. The usable window is between the LUF and the MUF. At noon in summer it can be narrow; at night the LUF drops and the low bands open.

Using the MUF to pick a band

• Find the MUF for your path. PropMap estimates it live for a typical 3,000 km path.
• Choose the highest band comfortably below it, roughly 80 to 90 percent of the MUF. That band usually has the strongest signals and least absorption.
• As the MUF drops in the evening, step down band by band: 15 m to 20 m to 40 m.

The MUF is a forecast, not a law. Sporadic-E, grey line and scatter modes regularly beat it, so always listen.

Last updated October 2026.