↑ Understanding LUF and MUF

This page is part of the project "Understanding HF Propagation."
by Doron Tal, 4X4XM

Maximum Usable Frequency (MUF) is the highest frequency that can propagate via the ionosphere under specific conditions. Frequencies exceeding MUF penetrate the ionosphere and continue into space.

During the day, the ionospheric E and F regions refract skywaves below the MUF:

MUF
Figure 1: Daytime HF refractions above 10 MHz
Factors affecting MUF:
  1. Higher electron density in the ionosphere supports higher frequencies.
  2. Increased solar irradiation raises the MUF, peaking at noon and in summer.

Lowest Usable Frequency (LUF)

During daylight, the ionospheric D-region absorbs skywaves below approximately 10 MHz.

LUF
Figure 2: Daytime HF absorption below 10 MHz
Factors affecting LUF:
  1. The D-region absorbs frequencies below the LUF, preventing signals from reaching higher layers.
  2. From sunrise to noon, increased solar irradiation raises D-region absorption, resulting in a higher LUF.
  3. Intense solar flares can elevate the LUF beyond the MUF, causing radio blackouts.
  4. Frequencies below 10 MHz experience greater absorption compared to higher bands.

Window of Usable Frequency Range

The range between LUF and MUF defines the usable "HF skywave window."

Diurnal cycle MUF-LUF
Figure 3: Typical diurnal cycle based on NPS training materials
The project "Understanding HF Propagation" explores HF skywave propagation, including LUF and MUF concepts.
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