What is MUF (Maximum Usable Frequency)?
The Maximum Usable Frequency (MUF) is the highest frequency that can be used to communicate between two points via ionospheric reflection.
- If you transmit below the MUF, your signal can refract off the ionosphere and reach distant locations.
- If you transmit above the MUF, your signal passes through the ionosphere into space and is lost.
MUF is dynamic — it changes depending on:
- Time of day (higher during daytime)
- Solar activity (more sun = higher MUF)
- Season (higher in summer)
- Latitude and path distance
Typical MUF Ranges:
- Low solar activity: 10–18 MHz (lower bands open)
- High solar activity: 20–30+ MHz (higher bands like 15m, 12m, 10m open)
What is Absorption?
Absorption refers to the loss of radio signal strength as waves pass through the ionosphere, especially through the lower D-layer.
Key points about absorption:
- Strongest during daytime, when the D-layer is fully ionized by sunlight.
- Low frequencies (like 80m, 40m) are more heavily absorbed.
- Solar flares and geomagnetic storms greatly increase D-layer absorption, causing HF blackouts.
In general:
- Lower bands (80m, 40m) suffer more daytime absorption.
- Higher bands (20m, 17m, 15m, 10m) benefit from lower absorption and higher MUF.
How MUF and Absorption Affect Ham Radio
| Condition | Effect on HF Communication |
|---|---|
| MUF is high (20–30 MHz) | High bands (15m, 12m, 10m) open for DX. |
| MUF is low (10–18 MHz) | Lower bands (30m, 40m) best for long-distance. |
| High daytime D-layer absorption | 80m and 40m become noisy or lose range. |
| Solar flare increases absorption | Sudden HF blackouts on sunlit side of Earth. |
Ideal for DX:
- Pick a frequency just below the current MUF for maximum distance and best signal quality.
Warning signs of high absorption:
- Increased background noise (“hiss”) on low bands.
- Sudden loss of signals after a solar flare or during geomagnetic storms.
In Conclusion
Understanding MUF and absorption is critical for successful HF operations:
- Use high frequencies during the day when the MUF is higher.
- Use lower frequencies at night or when the MUF drops.
- Be ready for absorption events during solar flares and geomagnetic storms.
By adjusting your band choices and transmission strategies based on MUF and D-layer absorption, you can dramatically improve your operating success.