New operators tune across a dead band, hear nothing, and conclude the radio is broken or the hobby is dying. Experienced operators look at three numbers and a clock, pick a band, and make contacts.
The difference is not equipment. It is knowing that HF propagation is largely predictable, driven by the sun, and follows a daily rhythm you can learn in an afternoon.
The mechanism, briefly
HF signals travel beyond the horizon by refracting off the ionosphere — layers of the upper atmosphere ionised by solar radiation. Signals bend back toward Earth, sometimes bouncing repeatedly between ionosphere and ground to reach the other side of the planet.
Three layers matter:
- D layer (lowest, ~60–90 km). Present only in daylight. It does not refract — it absorbs, and it absorbs low frequencies hardest. This single fact explains why 160, 80 and 40 m are local by day and open up worldwide after sunset. The D layer dissolves within an hour or two of sundown.
- E layer (~100–125 km). Weak refraction most of the time. Its interesting behaviour is sporadic-E: intense, patchy, short-lived clouds of ionisation that reflect much higher frequencies, mainly 10 m and 6 m, occasionally 2 m. Peaks in late spring and early summer, with a smaller winter peak, and appears with no warning.
- F layer (~200–400 km, splitting into F1 and F2 by day). This is what does the work for long-distance HF. The F2 layer is the one carrying your signal to Japan.
Two derived numbers describe the usable window at any moment:
- MUF — Maximum Usable Frequency. Above this, signals punch through the ionosphere into space instead of bending back. Rises through the morning, peaks in the afternoon, falls after dark.
- LUF — Lowest Usable Frequency. Below this, D-layer absorption eats the signal. Highest at midday, drops away at night.
Your usable bands sit between the two. As the sun rises, the whole window slides upward; after sunset it slides down. That is the entire daily rhythm.
The daily cycle in practice
High bands by day, low bands by night. 20 m and above during daylight; 40 m and below after dark. Around sunrise and sunset, everything is briefly interesting at once.
| Time | Best bands | What you’ll find |
|---|---|---|
| Sunrise ±1 hr | 40, 30, 20 | Gray-line enhancement. Often the best DX window of the day. |
| Mid-morning | 20, 17, 15 | High bands opening east; 20 m reliable. |
| Midday | 15, 12, 10 | Highest bands peak — when the flux supports them. |
| Late afternoon | 20, 17, 15 | High bands close westward; 20 m holds. |
| Sunset ±1 hr | 20, 30, 40 | The second gray-line window. |
| Evening | 40, 80 | Regional nets, ragchews, steady traffic. |
| Late night | 80, 160 | Quiet, long-haul low-band work. Best in winter. |
Gray line deserves special mention. Along the moving terminator between day and night, the D layer has collapsed but the F layer is still ionised — briefly, absorption is gone while refraction remains. Signal paths that run along that line can be dramatically enhanced. Twice a day, for perhaps forty minutes, low-power stations work distances they cannot touch at any other hour. It is worth setting an alarm for.
The three numbers
Every propagation widget shows the same handful of figures. Only three matter day to day.
SFI — Solar Flux Index
Solar radio emission at 10.7 cm, used as a proxy for ionising radiation. Higher is better, and it drives the upper bands directly.
| SFI | Conditions |
|---|---|
| Under 70 | Poor. 10 and 12 m mostly dead; 20 m is your ceiling. |
| 70–100 | Fair. 20 m reliable, 15 m occasionally. |
| 100–150 | Good. 15 and 12 m open regularly, 10 m on good days. |
| 150–200 | Excellent. 10 m open daily, worldwide, on modest power. |
| Over 200 | Exceptional. Work the world on 10 m with a wire and 10 W. |
SFI follows the roughly eleven-year solar cycle. Cycle 25 peaked in 2024–2025 and is now in its declining phase — which still leaves several genuinely good years on the high bands before the next minimum.
A and K — geomagnetic disturbance
These measure how disturbed Earth’s magnetic field is. Lower is better, and they can undo a high SFI entirely.
K index is measured every three hours on a 0–9 scale:
- K 0–1 — quiet. Ideal.
- K 2–3 — unsettled. Normal operating; polar paths soften.
- K 4 — active. Noticeable degradation, especially north-south and over the pole.
- K 5+ — geomagnetic storm. High bands can shut down completely. Auroral absorption wrecks paths through high latitudes.
A index is a daily summary of the same thing, on a 0–400 scale. Under 10 is quiet; over 20 means a disturbed day; over 50 is a serious storm.
SFI 180 with K=6 is a bad day on the air. The geomagnetic numbers can override a strong flux completely, particularly for anything crossing high latitudes. Check both, and when K spikes, drop to the low bands and work regionally — or go do something else and come back tomorrow.
There is one consolation prize: during a storm, aurora can itself become a propagation mode on 6 m and 2 m, with a characteristic raspy, distorted note.
Tools worth using
Real-time MUF maps. prop.kc2g.com renders current MUF and foF2 as a world map from live ionosonde data. Look at it, see whether the MUF along your intended path clears the band you want, and pick accordingly. This is the single most useful propagation tool available, and it is free.
PSKReporter. pskreporter.info shows where your signal is actually being received, right now, by thousands of automated receivers. Transmit one FT8 cycle and you have an empirical map of your station’s reach in the next fifteen seconds. Nothing beats measurement.
WSPR. Transmit a beacon at 200 mW and watch wsprnet.org collect reports from around the world. The best passive way to characterise a new antenna.
DX cluster and DXMaps. What other people are working right now, which tells you what is open right now.
VOACAP for prediction rather than observation — statistical forecasts for a given path, power and antenna. Useful for planning a schedule days ahead, less so for deciding what to do in the next ten minutes.
A ten-second decision
- Glance at SFI and K. High flux, low K — go high. Low flux or high K — go low.
- Check the clock against the table above.
- Look at the MUF map for your intended direction.
- Tune the band and listen. The band is never dead just because your corner of it is quiet; move 40 kHz before concluding anything.
- If you hear nothing at all, transmit one FT8 cycle and check PSKReporter. Silence on receive and a map full of spots means the band is open and you are simply not hearing anyone — which is an antenna or noise problem, not a propagation problem.
That last distinction is worth internalising. “Band is dead” and “my receive is poor” produce identical experiences and have completely different fixes.
Next: FT8, End to End is the fastest way to start collecting real propagation data from your own station.