The ionosphere — your HF mirror
What the ionosphere is
The ionosphere is a region of the upper atmosphere, roughly 60–400 km altitude, where solar ultraviolet and X-ray radiation ionises gas molecules to create free electrons. These electrons refract (bend) radio waves back toward Earth, enabling long-distance communication far beyond line-of-sight. The degree of ionisation — and therefore the highest frequency that can be reflected (the MUF) — is directly controlled by solar activity.
Ionospheric layers
F2 layer — 200–400 km — day and night (weakened)
The primary reflector for long-distance HF DX. Highest electron density of all layers. SFI directly controls how high the MUF reaches here — when SFI exceeds 150 and has been sustained for several days, 10m and 12m can open for intercontinental paths. F2 persists at night (unlike lower layers), which is why 20m often stays open for DX after dark.
F1 layer — 150–200 km — daytime only
Merges with F2 at night and disappears. Contributes to MUF calculations for 14–21 MHz during daylight hours. Less significant than F2 for DX but relevant for mid-distance paths. No practical impact after sunset.
E layer — 90–150 km — mainly daytime
Reflects 3–10 MHz for regional paths (100–2000 km). Also produces Sporadic-E (Es) — intense, unpredictable ionised patches that can suddenly open 6m, 10m, even 2m to distant stations. Sporadic-E is completely unrelated to solar flux; it is a summer phenomenon (Nov–Jan in the southern hemisphere) driven by upper-atmospheric wind shear.
D layer — 60–90 km — daytime only
The enemy during solar flares. Normally absorbs signals below ~5 MHz in daytime — hence why 40m and 80m are poor during the day. During an X-ray flare, D-layer ionisation surges and absorbs the entire HF spectrum. Lower frequencies are absorbed first and recover last. Disappears completely at night — why 40m and 80m open dramatically after dark.
Why bands behave differently by time of day
Daytime: D-layer absorbs lower HF (160m/80m/40m poor). F2 supports 14–28 MHz DX if SFI is adequate.
Grey-line: D-layer rapidly disappearing/appearing. Low-angle paths along the terminator see reduced absorption — a prime DX window on 40m and 80m especially.
Night: D and F1 layers gone. 40m, 80m, 160m open dramatically. F2 weakens, lowering the MUF. 10m and 15m close.
Three distinct event types — different timing, different impact
Solar flare — radio blackout Immediate
A burst of X-ray and EUV radiation from the Sun, travelling at the speed of light — arrives 8 minutes after the eruption. Massively ionises the D-layer on the sunlit side of Earth. Lower HF frequencies are absorbed first; higher frequencies last and recover first. A strong X-class flare can cause complete HF blackout (NOAA R4–R5). Recovery takes minutes to several hours. Night side of Earth unaffected. After the flare ends and D-layer de-ionises, conditions sometimes briefly improve above pre-flare levels.
Solar radiation storm — polar cap absorption 15–60 min after flare
Energetic protons accelerated by the flare arrive within 15–60 minutes. Funnelled by Earth's magnetic field into polar regions, they ionise the upper atmosphere and create a dense absorbing layer — Polar Cap Absorption (PCA). PCA can last several days and renders all transpolar HF paths useless: Europe–Japan, Europe–North America via Arctic, and similar high-latitude routes go dark. Mid-latitude paths (such as ZS1 to Europe running northeast) are less severely affected but still see elevated absorption on paths clipping the sub-auroral zone.
Coronal mass ejection (CME) — geomagnetic storm 1–3 days after flare
A massive plasma cloud ejected from the solar corona, taking 1–3 days to reach Earth. When it strikes the magnetosphere it compresses and distorts it, driving the K-index to G3–G5 levels. Global ionospheric disruption follows: the auroral oval expands to mid-latitudes, the ionosphere becomes irregular, and signal paths relying on a smooth F2 become faded and distorted. Recovery takes 1–2 days. Following a CME storm, a brief window of unusually enhanced conditions often follows — watch for it. The 27-day solar rotation means active regions and coronal holes recur predictably.
NOAA space weather scales
Radio blackout (R)
R1–R2: Minor HF degradation
R3: Wide HF blackout
R4: HF propagation mostly lost
R5: Complete HF blackout
Geomag storm (G)
G1: K=5, minor
G2: K=6, moderate
G3: K=7, strong
G4–G5: K=8–9, severe–extreme
Radiation storm (S)
S1: Minor PCA
S2: Moderate polar absorption
S3: High-latitude HF blackout
S4–S5: Full polar cap blackout
The 27-day solar rotation rule
The sun rotates once every ~27 days as seen from Earth. Active sunspot regions and coronal holes that produce bad conditions tend to reappear on the next rotation. Mark your logbook on days of exceptional 10m openings or severe geomagnetic storms — check the conditions again 27 days later. Coronal holes are particularly reliable repeaters: a high-speed solar wind stream will arrive roughly every 27 days for several rotations. This single observation makes 27-day-ahead predictions viable without any specialised software.
Tropospheric propagation — VHF, UHF and microwave
How the troposphere differs from the ionosphere
The ionosphere (60–400 km) is driven by solar radiation and dominates HF propagation (3–30 MHz). The troposphere is the lowest 10–15 km of the atmosphere — the weather layer. Tropospheric propagation is governed by temperature, humidity and pressure gradients, not solar activity. It primarily affects VHF (50–300 MHz), UHF (300 MHz–3 GHz) and microwave bands. A severe geomagnetic storm destroying HF has zero effect on tropospheric propagation — you can work DX on 2m or 70cm while HF is completely blacked out.
Types of tropospheric propagation
Evaporation duct
Coastal & maritimeForms just above the sea surface (typically 5–40 m) where evaporating water creates a sharp humidity gradient. The gradient causes radio waves to refract downward into a near-surface waveguide. More or less permanently present over warm open ocean. Very common along coastlines. Enables VHF/UHF paths of several hundred kilometres over water. Effective mainly up to around 1 GHz. Cape Town's False Bay and Atlantic coast can regularly produce these conditions.
Surface duct — temperature inversion
Primary VHF/UHF DX modeThe classic tropospheric DX mechanism. Normally temperature decreases with altitude. When a warm air mass overrides cooler surface air — a temperature inversion — the refractive index profile bends radio waves back toward Earth, creating a surface waveguide. On 2m this can extend contacts to 500–1500 km; on 70cm and 23cm even further. Triggered by: stable anticyclonic conditions, clear still evenings, coastal sea breezes, and large blocking high pressure systems. The South Atlantic high is a key driver for ZS operators.
Elevated duct
Less predictable — very long pathsA ducting layer elevated above the surface, typically 500–3000 m altitude, associated with subsidence inversions in anticyclones. Signals must couple into the duct at a shallow elevation angle. Can produce extremely long paths (1000–3000 km) when two stations couple in at each end. More common over sea and subtropical regions. In southern Africa, these form along the subsidence inversion associated with the South Atlantic anticyclone. Harder to predict and couple into, but spectacular when it works.
Troposcatter
Always present — inherently weakNot a duct — a forward scatter mechanism. Radio waves scatter off turbulent refractive index variations in the troposphere (roughly 1–10 km altitude). Always present regardless of weather — no "opening" needed. Signal levels are inherently low (typically −100 to −120 dBm on a 2m path of a few hundred km), requiring good antennas or digital weak-signal modes. FT8 on 144 MHz has made troposcatter genuinely workable at modest power levels. Paths of 200–600 km on 2m are achievable with a Yagi and 50–100 W even under flat conditions.
Predicting tropospheric openings
Hepburn tropo forecast — dxinfocentre.com/tropo.html — the standard amateur tool. Plots predicted ducting index across regions, updated daily from NWP models. If your region shows index 80+ it's worth monitoring the band.
850 hPa temperature maps — a positive temperature anomaly at ~1500 m altitude indicates a subsidence inversion building. ECMWF or GFS model data from Windy.com gives a 5-day outlook.
DXMaps 144 MHz beacon page — if beacons beyond 300 km are suddenly audible, a duct is open. Check before calling CQ on 2m.
PSKReporter 144 MHz view — FT8 spots at distances well beyond normal tell you the duct is working. Real-time confirmation is far more reliable than forecasts alone.
Time of day — late afternoon and early evening see the strongest surface inversions as the ground radiates heat. Pre-dawn also excellent. Midday convective mixing destroys inversions.
Anticyclone persistence — the best ducting events last 2–5 days under a stalled high pressure system. Single-night events are possible but multi-day setups are where the big DX happens.
Understanding the numbers
Solar Flux Index (SFI / F10.7)
Measured daily at Penticton, Canada at 2800 MHz (10.7 cm). A proxy for ionising radiation reaching Earth's ionosphere. Range: ~50 (solar min) to 300+ (solar max). SFI directly controls maximum electron density in the F2 layer, setting the MUF. Key thresholds: <80 = poor (upper bands closed); 80–120 = below average (20m marginal); 120–150 = good (17m/15m opening); 150–200 = very good (12m/10m opening); 200+ = exceptional. SFI must be sustained for several days for the F2 to fully build up — a single spike doesn't instantly open 10m.
A-index
A daily average of geomagnetic activity derived from eight 3-hourly K readings. Gives a smoother view of the day's overall conditions. A = 0–7: quiet; 8–15: unsettled; 16–29: active; 30–49: minor storm; 50–99: major storm; 100+: severe. Use K for real-time band decisions; use A to assess whether yesterday's conditions were settled and whether the ionosphere is recovering from a storm. Two consecutive days with A ≤ 5 means the ionosphere is in good shape.
Sunspot number (SSN)
A longer-term indicator of solar cycle phase. Correlates with SFI over weeks and months but less directly actionable for day-to-day operating. More useful for understanding where we are in the 11-year solar cycle. Solar Cycle 25 peaked higher than forecast in 2025 — we are on the descending slope but still well above minimum, meaning conditions remain generally favourable for HF DX compared to the 2019–2020 minimum years.
MUF and LUF
The Maximum Usable Frequency (MUF) is the highest frequency refracted back to Earth for a given path — signals above the MUF pass straight through into space. The Lowest Usable Frequency (LUF) is the lowest frequency where signal strength exceeds noise after D-layer absorption. The usable HF window lies between LUF and MUF. As SFI rises, the MUF rises (opening higher bands). During a flare, the LUF rises dramatically (closing lower bands). Working near but below the MUF usually gives the strongest signals.
K-index scale
0–1Quiet — ideal conditionsAll HF bands. Push to highest frequency.
2Quiet — very good20m/17m solid, 15m+ possible
3UnsettledSlight high-latitude degradation
4Active — noticeable degradationStick to 20m and below
5Minor storm (G1)Poor. Lower bands only.
6Moderate storm (G2)Severe degradation, 40m marginal
7–9Strong–extreme storm (G3–G5)HF blackout likely