Understanding MUF vs. Critical Frequency

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

Figure 1: Daytime HF refractions and reflections

The Critical Frequency

The critical frequency ↗ (f_oF₂ or CF) is the highest frequency below which a radio wave is refracted by the F2-region at vertical incidence, independent of transmitting power.

Figure 2: Vertical refraction from the F₂ region

Calculation of f_o (the plasma frequency): , where N_max is the density of free electrons.
N_max also determines the refractive index ↗ of the ionosphere: μ = √(1 - (81N_max / f_o²)) (known as the Appelton formula) ↗.

Measurement of f_o: Ionosondes determine the critical frequency, which varies significantly based on location and time.

Variations of f_o: The critical frequency varies with several factors: time of day, geographic latitude, season, solar activity, and geophysical conditions.

Maximum Usable Frequency (MUF) is the highest frequency that can propagate via the ionosphere under specific conditions. Frequencies exceeding MUF penetrate the ionosphere and escape to outer space, leading to a loss of communication.

Figure 3: MUF illustration

The MUF is calculated using the formula:

MUF = f_oF₂ × sec(θ) ↗

• f_oF₂: Critical frequency of the F2 region.
• θ: Angle of incidence relative to the vertical.
• As a rule of thumb, the MUF is approximately 3-4 times the f_oF₂;
  i.e., incident angle θ = 70°-75°; transmission angle α = 15°-20°.

For vertical incidence (θ = 0), MUF equals f_oF₂. For oblique paths, MUF increases with sec(θ) ↗.

See the recent MUF charts.

1. Higher density of free electrons in the ionosphere supports higher frequencies.
2. Increased solar activity raises the MUF, peaking at noon and in summer.

Total visits since 17 August 2022.
Repeat visits counted as new after 24 hours.