MUF - Maximum Usable Frequency

The MUF depends on:

• Ionospheric electron density: higher = higher MUF
• Angle of incidence: lower angles allow higher frequencies
• Link distance: longer links have higher MUF

The approximate formula is: MUF ≈ foF2 × sec(θ) where θ is the angle of incidence.

The foF2 (F2 layer critical frequency) is the maximum frequency reflected at vertical incidence. For oblique links:

• MUF at 3000 km ≈ foF2 × 3
• MUF at 1500 km ≈ foF2 × 2

Example: if foF2 = 8 MHz, the MUF for 3000 km is about 24 MHz (12m band open).

MUF maps show the maximum usable frequency toward different directions. How to interpret them:

• Warm colors (red/orange): High MUF, higher bands open
• Cool colors (blue/green): Low MUF, use lower bands
• Terminator: the day/night line often offers interesting conditions

Besides MUF, consider:

• FOT (Frequency of Optimum Traffic): about 85% of MUF, the most reliable frequency
• LUF (Lowest Usable Frequency): below this frequency, absorption is too high

The ideal band is between LUF and FOT, not necessarily at MUF!