If you’ve ever opened DXLook and seen “Good” or “Poor” next to a band like 20 meters, you might’ve wondered — where do those ratings come from?
This post explains how we determine those band conditions, what values like SFI, A, K, and Sunspot Number actually mean, and how they impact your HF experience on the air.
The Data Behind the Bands
Every few minutes, DXLook fetches real-time space weather data from official sources like NOAA and SIDC. We use these numbers to understand what’s happening in the ionosphere — the layer of the atmosphere that makes long-distance HF communication possible.
The four key values we use are:
🌞 Solar Flux Index (SFI)
The Solar Flux Index measures how much radio energy the sun is emitting at a wavelength of 10.7 cm. Higher values usually mean better ionization in the upper atmosphere, which helps signals on higher bands (like 10m and 12m) travel farther.
- High SFI = better propagation, especially on high frequencies
- Low SFI = tougher conditions, especially above 20 meters
🧲 K-index
This number tracks how disturbed the Earth’s magnetic field is, based on 3-hour intervals. It’s a short-term warning signal for geomagnetic storms.
- Low K (0–2) = stable conditions, good for HF
- High K (5 or above) = stormy conditions, signals degrade
🧲 A-index
This is a daily average of the K-index, smoothed out over time. It gives a broader picture of how active (or quiet) the geomagnetic field has been.
- A below 15 means things are calm
- A above 40 means conditions are very disturbed
☀️ Sunspot Number (SN)
This is the number of visible sunspots on the sun today. More sunspots usually mean better HF propagation because they boost the sun’s radio output and help ionize the upper atmosphere.
We also account for the accuracy of this number: if the sunspot count has high uncertainty, we reduce its influence on the final result.
Other Factors We Monitor
Beyond those four values, DXLook also checks:
⚡ Solar Flares
Big solar flares can wipe out HF propagation temporarily. If there’s an X-class flare (the strongest kind), we downgrade all band conditions to “Poor.” Even medium flares (M-class) can hurt the higher bands. This data comes from real-time GOES satellite reports.
💨 Solar Wind
We monitor the speed and density of the solar wind. When it's fast and dense, it shakes up the magnetic field, raises the noise floor, and weakens signal strength. We use this to estimate the HF noise level across bands — shown on DXLook as something like “S2–S4.”
Band Groupings and Ratings
Instead of rating every band individually, we group them into four ranges:
- 80m–40m (lower bands)
- 30m–20m (mid-range)
- 17m–15m (upper-mid)
- 12m–10m (high bands)
Each group gets a day and night rating, because the ionosphere behaves very differently depending on solar illumination.
We look at all the inputs above — solar activity, geomagnetic conditions, flare presence, and solar wind — and decide if conditions for that band group are Good, Fair, or Poor. For example:
- 20m might be “Good” during the day if SFI is high and magnetic activity is quiet.
- 80m might be “Fair” at night unless there’s a flare or a big solar wind spike.
If there’s a geomagnetic storm or solar flare in progress, we may downgrade the ratings accordingly, even if the SFI looks strong.
Noise Levels
We also estimate how “noisy” the bands are — not in terms of local interference, but the natural background noise coming from solar and geomagnetic sources.
If the geomagnetic field is quiet and the solar wind is slow, we show very low noise levels (like “S1–S2”). But if the wind is fast or there are disturbances, we raise the estimated noise level, which helps explain why signals might not be as strong as expected — even on an open band.
Updates Every 10 Minutes
DXLook fetches and recalculates all this data every ten minutes. It’s not a forecast — it’s a live snapshot of what’s happening right now. We do this so that the band ratings and noise levels reflect real-world conditions as closely as possible.
So when you check the 15m rating and it says “Good,” you can be confident that it’s based on current data, not guesswork.
Final Thoughts
The sun controls a lot more than we like to admit. When it’s quiet and stable, HF comes alive. When it throws a tantrum, signals vanish or get buried in noise.
At DXLook, we make sense of all the moving parts — solar flux, sunspots, magnetic fields, flares, and more — and turn them into simple, clear ratings that any operator can use.
If you ever wondered why 10 meters worked like magic yesterday but is dead today, now you know who’s to blame: the sun.
73 de AK6FP
Rodrigo
https://dxlook.com