Understanding MUF vs. Critical Frequency in Skywave Propagation This page is part of the project "Understanding HF Propagation." by Doron Tal, 4X4XM The Critical Frequency The critical frequency ↗ (foF2 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 1: Vertical refraction from the F2 region Calculation of fo (the plasma frequency): , where Nmax is the density of free electrons. Nmax also determines the refractive index ↗ of the ionosphere: μ = √(1 - (81Nmax / fo²)) (known as the Appelton formula) ↗. Measurement of fo: Ionosondes determine the critical frequency, which varies significantly based on location and time. Variations of fo: 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 continue into space. During the day, the ionospheric E and F regions refract skywaves below the MUF: Figure 2: Daytime HF refractions The MUF is calculated using the formula: MUF = foF2 × sec(θ) ↗ foF2: 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 foF2; i.e., incident angle θ = 70°-75°; transmission angle α = 15°-20°. For vertical incidence (θ = 0), MUF equals foF2. For oblique paths, MUF increases with sec(θ) ↗. See the recent MUF charts. Factors affecting MUF: Higher density of free electrons in the ionosphere supports higher frequencies. Increased solar activity raises the MUF, peaking at noon and in summer. The project "Understanding HF Propagation" explores HF skywave propagation, including LUF and MUF concepts. Total visits since 17 August 2022. Repeat visits counted as new after 24 hours.

The ionosphere plays a crucial role in the propagation of radio waves, which are used for communication between amateur radio operators. The incident angle, which is the angle at which a wave hits the ionosphere, can be acute or obtuse.

The maximum usable frequency (MUF) is the highest frequency that allows for reliable communications between two points via "reflection" from the ionosphere. The MUF depends on factors such as the critical frequency, angle of incidence, and distance between the transmitter and receiver. Signals above the MUF will penetrate the ionosphere (escape to outer space) and not be bounced back to Earth, leading to a loss of communication.

The critical frequency refers to the highest frequency at which a radio wave can be refracted or bent back to Earth by the ionosphere. This frequency is dependent on the electron density in the ionosphere and is used to determine the MUF and select signals for communication.

The MUF is determined through empirical observations and measurements and is influenced by factors such as signal-to-noise ratio and minimum acceptable signal strength. Real-time monitoring and prediction systems are often used to adjust the estimated MUF for current conditions.

The MUF map provides a date and time stamp, estimated solar flare index number, and sunspot number, which are useful information for determining the MUF and selecting suitable frequencies for communication.