MUF stands for Maximum Usable Frequency — it’s the highest frequency that can be used for skywave communication between two points, at a given time and ionospheric condition.
That sounds a bit abstract, but here’s what it means in practice: If you’re transmitting on a frequency above the MUF for your path, your signal goes right through the ionosphere and into space. If you’re below it, the ionosphere reflects it back to Earth — and someone might hear you.
So knowing the MUF between you and the region you’re trying to reach tells you whether 10 meters is even worth trying, or if you’re better off calling on 20 or 40.
MUF isn’t a fixed number. It changes based on solar radiation, time of day, and the distance between stations. In some cases, MUF may be 28 MHz for long-haul DX but only 14 MHz for local paths.
There are calculators and prediction models, but the most helpful MUF insights usually come from real-time data — actual signal paths and SNR trends. That tells you what’s working now, not just what might work.