Propagation physics

What is LUF? The Lowest Usable Frequency on HF

LUF (Lowest Usable Frequency) is the lowest frequency that can complete a given HF path before D-layer absorption attenuates the signal below detectability. Where the Maximum Usable Frequency (MUF) sets the upper boundary of the workable window, LUF sets the lower one. The two together describe the band of frequencies a path actually carries at a given moment.

Why it matters for HF operating

LUF is what makes 80 meters and 160 meters daytime bands feel dead even when the MUF is high. The signal is not failing to reflect; it is being absorbed by the D-layer on the way up and the way down. Knowing the LUF for your target path tells you the lowest band worth trying. If the LUF is sitting at 5 MHz on a winter afternoon, 80 meters is going to be a long shot regardless of MUF or activity.

Key values to know

  • Drivers. D-layer absorption (driven by solar elevation and X-ray flux), distance, time of day, and season.
  • Daily pattern. LUF rises through the morning, peaks near local noon, and drops sharply at sunset.
  • Typical ranges. Quiet daytime LUF for short paths sits around 3 to 5 MHz. During an X-class flare on the sunlit side, LUF can spike above 20 MHz, producing a short-wave fadeout where most of HF goes unusable for an hour.
  • Frequency dependence. Absorption scales roughly as 1 / frequency squared, which is why higher bands recover much faster from solar events.

Common misconceptions

LUF is sometimes described as “the band that is open lowest.” A more precise framing is “the floor the path imposes through absorption.” Two paths with similar geometry can have very different LUFs depending on how much of the path lies in daylight. A path that crosses a sunlit zone at noon has a much higher LUF than the same path at midnight.

  • MUF: the upper end of the workable window
  • D layer: the layer that produces LUF
  • Solar flares: the events that drive LUF spikes
  • Ionosphere: the broader system LUF sits inside

SkyWave shows D-layer absorption alongside MUF on the Go screen so you can see both the floor and the ceiling of the workable HF window. See it in the app →