Propagation on HF is driven almost entirely by the Sun. These are the numbers every cluster page and club net quotes, and what they actually tell you.
The numbers
- Solar flux index (SFI) — radio noise from the Sun at 2800 MHz, a proxy for ionisation. Below
- Sunspot number (SSN) — the visible count, closely tracking SFI. The 11-year solar cycle takes
- A index — a daily average of geomagnetic disturbance, 0 to 400. Under 10 is quiet; over 20
- K index — the same thing every three hours, 0 to 9, logarithmic. K of 0–2 is good, 4 is
about 70 the high bands are dead; 100–150 opens 15 m and 12 m; above 200, 10 m and 6 m come alive worldwide. Higher is better, with no downside.
it from near zero at minimum to well over 150 at maximum.
degrades the high latitudes and the polar paths.
unsettled, 5 and up is a geomagnetic storm and the low bands go long-path or nowhere.
The short version: high flux and low A/K is what you want.
Events
- Solar flares — X-ray bursts that ionise the D layer on the daylit side and cause a sudden
- Coronal mass ejections — plasma arriving one to three days after a flare. They trigger
- Coronal holes — recurring high-speed solar wind streams, roughly every 27 days with the Sun's
- Aurora — bad for normal HF, excellent if you want to bounce VHF signals off the auroral
ionospheric disturbance: HF simply disappears for minutes to an hour. Classed A/B/C/M/X.
geomagnetic storms, aurora, and absorption on polar paths; 160 m and 80 m suffer most.
rotation. Mildly disruptive and quite predictable.
curtain (rough, buzzy CW and SSB on 6 m and 2 m).
Where to look
- NOAA Space Weather Prediction Center — the authoritative source for alerts and forecasts.
- solarham.com and hamqsl.com — the familiar at-a-glance amateur summaries and widgets.
- VOACAP Online — a predicted path between two locations for a given mode and power.
- PSK Reporter and the Reverse Beacon Network — where your signal is actually being heard
right now. This beats any prediction.
Using it
Predictions tell you where to look; the beacon networks tell you the truth. If the flux is high and K is low, try the high bands in daylight and expect long grey-line paths at dawn and dusk. During a storm, drop to 40 m and 30 m, work regionally, and remember that the weak-signal modes still get through when voice cannot.