MUF, LUF, and Choosing the Right Frequency
The Propagation Window
For any given path between two stations, there's a range of frequencies that will work. Go too high, and the signal punches through the ionosphere into space. Go too low, and the D layer absorbs it. The sweet spot is between these limits.
Maximum Usable Frequency (MUF)
The highest frequency the ionosphere will return to Earth for a specific path. Above this, signals pass straight through into space.
The MUF varies with:
- Time of day: Higher during the day, drops at night
- Season: Can be higher in winter for F2 paths at mid-latitudes
- Solar cycle: Much higher during solar maximum (the 11-year cycle)
- Path length: Longer paths can use higher frequencies (lower angle of incidence)
Lowest Usable Frequency (LUF)
The lowest frequency that provides a usable signal. Below this, D-layer absorption is too great. The LUF is higher during the day and lower at night.
Frequency of Optimum Traffic (FOT)
The "Goldilocks" frequency — not too high (might skip out), not too low (might be absorbed). Operating at 85% of the MUF gives the most reliable communication.
| Time | Best bands | Why |
|---|---|---|
| Daytime | 20m, 17m, 15m, 12m, 10m | High MUF, D-layer absorbs low bands |
| Evening | 40m, 30m, 20m | D layer fading, F layer still good |
| Night | 160m, 80m, 40m | No D-layer absorption, low MUF |
| Sunrise/sunset | 160m, 80m (grey line!) | Special propagation — see next lesson |
Skip Distance and Skip Zone
When a signal goes up to the ionosphere and comes back down, it doesn't return right next to the transmitter:
- Skip distance: The minimum distance where the skywave signal returns to Earth
- Skip zone (dead zone): The area between where the ground wave fades out and where the skywave returns — no signal here!
Higher frequency = longer skip distance. On 10m, the skip might be 1000+ km. On 40m, it might be 300 km. This is why you sometimes can't hear local stations on 20m — they're in your skip zone.