I first became interested in the Sun’s activity way back before I obtained my amateur radio licence, so that was over fifty years ago. Any errors here are of course mine.
Solar and sun-related activity plays a critical role in HF and to a lesser extent VHF propagation for amateur radio. Here’s a structured overview of how solar phenomena affect HF and VHF Key Solar Phenomena.
1. Solar Flux (SFI)
- Measures solar radiation at 10.7 cm (2800 MHz).
- Higher SFI = better ionisation of the F2 layer, improving HF propagation.
- Typical values:
- Below 70 = poor HF conditions
- 100–150 = good for 20–10m
- Above 150 = potential for 6m (50 MHz) F2 openings
2. Sunspots
- Magnetic storms on the Sun’s surface.
- More sunspots mean increased UV and X-ray output, which enhances the ionosphere.
- 11-year solar cycle: more sunspots at solar maximum, better HF propagation.
3. Solar Flares
- Sudden bursts of radiation (X-rays, UV).
- Can cause Shortwave Fadeouts (SWF), especially on the dayside of Earth.
- More likely to disrupt HF than VHF.
4. Coronal Mass Ejections (CMEs)
- Massive bursts of solar plasma and magnetic fields.
- Can disturb Earth’s magnetosphere and trigger geomagnetic storms (see K-index below).
Effects on HF Propagation (3–30 MHz)
Positive Effects:
- High SFI and sunspot numbers = better F2 layer ionisation
- Good for long-distance DX on 20, 17, 15, 12, and 10 metres
- At solar peak, even 6m (50 MHz) can open via F2
Negative Effects:
- Solar flares = sudden HF blackouts (especially below 10 MHz)
- CMEs and geomagnetic storms = disrupted F2 layer, poor signal paths
- Low solar activity = limited DX, higher bands go “dead”
Effects on VHF Propagation (30–300 MHz)
Direct effects are limited, but:
- 50 MHz (6m): most influenced, can experience F2 openings during high solar activity
- 70 MHz (4m): some F2 impact, mostly Sporadic-E related
- 144 MHz (2m) and up: rarely affected directly, but indirect effects occur (e.g., auroral propagation during geomagnetic storms)
Ionospheric Layers Summary
| Layer | Affected by | Role in Propagation | Frequencies |
|---|---|---|---|
| D | Solar flares, daytime sun | Absorbs lower HF (esp. 160–40m) | Negative impact |
| E | Solar UV, seasonal | Supports short-skip & Sporadic-E | HF & low VHF |
| F1/F2 | Solar flux, sunspots | Main long-haul DX layer | HF and 6m during high sola |
Useful Indices and Tools
| Index | Meaning | Ideal for DX |
|---|---|---|
| SFI (Solar Flux Index) | 70–300+ | >100 for good HF |
| K-index | Geomagnetic activity (0–9) | 0–3 |
| A-index | 24h average of K-index | <15 |
| X-ray Flux | Real-time flare impact | Low or stable is best |
| Auroral Index (AU, AL) | Indicates aurora potential | High = aurora on 6m/2m |
Propagation Enhancements from Solar Activity
| Band | Effect from Solar Max | Notes |
|---|---|---|
| 160m–80m | Poorer day conditions (D-layer) | Better at night |
| 40m–20m | Stable, reliable DX | Day and night paths improve |
| 15m–10m | Greatly improved | 10m often wide open |
| 6m (50 MHz) | F2 possible | True DX (e.g. EU to SA, JA) |
| 2m (144 MHz) | Aurora, rare F2 | Usually tropo or aurora paths |
Resources for Monitoring
- NOAA Space Weather
- SolarHam
- DXHeat Propagation
- VOACAP Online Propagation Predictor
At the time of writing in July 2025, one can say that in the last few months and year there has been more auroral activity but rather less in the way of Sporadic E, which is what one might expect at or near the peak of the solar cycle.
We had an interesting opening over the evening of 14th and the following morning. It started with a couple of very strong French stations in JN18. I then found the following all worked on SSB on 14th:
Amateur Radio News from Jon Stow, G4MCU 