↑ Understanding CF vs MUF

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

Critical Frequency (CF)

The Critical Frequency ↗ (foF2) is the highest frequency below which a radio wave is refracted by the F2-region at vertical incidence, independent of transmitting power.

CF
Figure 1: Vertical refraction from F2 region

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 2: Daytime HF refractions
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.
The project "Understanding HF Propagation" explores HF skywave propagation, including LUF and MUF concepts.
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