Propagation conditions
HF signals travel by the ionosphere, and the ionosphere follows the Sun. See current space weather from NOAA, where it is day, twilight and night, and why a band can skip over nearby stations.
Space weather now
The Sun drives HF propagation. These are NOAA's current readings and what they mean for radio, with times in UTC.
Loading current space weather from NOAA...
NOAA space weather scales now
NOAA scales: loading...
Planetary K index
K index: loading...
Solar X-rays (flares)
X-ray data: loading...
Solar flux (F10.7)
Solar flux: loading...
The Sun now
NOAA's GOES satellites photograph the Sun in extreme ultraviolet several times a minute. The newest image is about 1 MB.
Day, night and the gray line
The ionosphere follows the Sun. Watch where it is day, twilight and night, and see where NOAA expects the daytime D region to absorb HF signals.
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- Sunrise and sunset line
- End of civil twilight (Sun 6° down)
- Twilight bands darken to full night (Sun 18° down)
Try this
What is being heard now
WSPR stations send low-power signals around the clock, and receivers report what they decode to WSPRnet. The reports show which paths were open in the last hour with one end near your area, a reporting network of the kind the General exam describes (G3B04, G8C02).
This asks wspr.live, a volunteer-run copy of WSPRnet, for reports near the center of your four-character grid square. Nothing is sent until you press the button.
WSPR was designed for probing potential propagation paths using low-power transmissions; a message normally carries the sender's call sign, grid square and power, and can be decoded down to a signal-to-noise ratio of -31 dB in a 2500 Hz bandwidth (WSJT-X User Guide). Reports come from WSPRnet through wspr.live, run by volunteers with no guarantees on correctness, availability or stability; the raw reports can include duplicates and false decodes.
Skip zone and MUF explorer
Why a band can be open to stations 2,000 km away but silent for one 300 km away. Change the frequency and the state of the layer and watch which signals come back.
The highest frequency the layer returns when you transmit straight up.
At 14.2 MHz, rays steeper than 24.4° pass through the layer, leaving a skip zone of 1,152 km (716 mi). Signals land from 1,152 km to 3,836 km.
- Critical angle
- 24.4°
- Skip zone
- 1,152 km 716 mi
- MUF for 2,000 km
- 19.7 MHz your frequency returns
- Longest single hop
- 3,836 km 2,383 mi
- MUF for one hop
- Your operating frequency
- Skip distance
Try this
Apply it
- Check the NOAA scales before you operate. An R level means flare absorption on the sunlit side of Earth, S levels affect paths through the polar regions, and from G2 up HF can fade at higher latitudes (NOAA Space Weather Scales).
- If the D-region absorption on the map covers your path, try a higher band. Absorption hits lower frequencies harder (General pool, G3A02), and NOAA's map gives the highest frequency losing at least 1 dB (D-RAP).
- When darkness lowers the MUF on a path, move to a lower band to keep a contact going (Extra pool, E3A06). Frequencies just below the MUF have the least attenuation (General pool, G3B03).
- For long-distance contacts on the lower bands, operate around your sunrise and sunset. The 2012 to 2016 Extra pool described gray-line propagation as long-distance communication at twilight below 15 MHz, because D-layer absorption drops while the E and F layers stay strong (E3B09 to E3B11).
- Then listen. Reporting networks show where your signal is actually heard (General pool, G3B04); see checking conditions right now.
How it works
- Space weather comes from the NOAA Space Weather Prediction Center and is checked every five minutes while the page is open. Times are in UTC.
- Kp is a 3-hourly measure of disturbance in the Earth’s magnetic field from 0 to 9, and 5 or more is a geomagnetic storm. NOAA rates storms from G1 (Kp 5) to G5 (Kp 9).
- Flares are classed by peak X-ray flux from 0.1 to 0.8 nm, each letter from A to X ten times the last. NOAA radio blackouts run from R1 at an M1 flare to R5 at X20.
- NOAA’s D-Region Absorption Predictions (D-RAP) give the highest frequency that loses at least 1 dB to absorption from solar X-rays and energetic protons.
- Sun positions follow the NOAA solar calculator equations. NOAA says sunrise and sunset from them are theoretically accurate to within a minute between 72° north and south and within 10 minutes beyond, and that observed times may differ; NOAA no longer maintains the calculator itself. Sunrise, sunset and the three twilights use the US Naval Observatory definitions: the Sun’s center 0.833°, 6°, 12° and 18° below the horizon.
- The Sun’s elevation shown is geometric, without atmospheric refraction, which lifts the Sun by about half a degree at the horizon in NOAA’s refraction model.
- The E layer preset uses 3.8 MHz, about what the ITU-R P.1239 formula for the normal E layer gives at midday in a mid-latitude summer with a solar flux of 150.
- The skip explorer uses the secant law: a layer whose critical frequency is fc returns an oblique wave up to fc times the secant of its angle of incidence. The layer is a thin mirror at a fixed virtual height over an Earth of radius 6,370 km (NBS Monograph 80); real curved layers return up to about 20 percent more.
- Land outlines on the map are Natural Earth 1:110m data, which is in the public domain.
- The aurora overlay is NOAA's OVATION Prime short-term forecast, made from solar wind measured upstream of Earth, or from the Kp index with no lead time when that data is unavailable. Its data file gives values without units; NOAA's own maps label the scale as the probability of seeing aurora, in percent, and NOAA says a viewing probability can be estimated by assuming a linear relationship to the intensity. It loads only when you turn it on.
- The image of the Sun is the newest frame from NOAA's GOES Solar Ultraviolet Imager at 304 Å, loaded only when you ask (about 1 MB).
- WSPR reports come from WSPRnet through wspr.live, a volunteer-run database (free for non-commercial use, no guarantees, 20 requests a minute). The page asks only when you press the button, sends only the center of your four-character grid square, and shows the time of the newest report.
Learn more: How radio signals travel
Sources: NOAA Space Weather Scales (NOAA SWPC); Planetary K-index (NOAA SWPC); Solar Flares (Radio Blackouts) (NOAA SWPC); GOES X-ray Flux (NOAA SWPC); F10.7 cm Radio Emissions (NOAA SWPC); D Region Absorption Predictions (D-RAP) (NOAA SWPC); HF Radio Communications (NOAA SWPC); SWPC data service (NOAA SWPC); Solar Calculation Details (NOAA Global Monitoring Laboratory); Rise, Set, and Twilight Definitions (US Naval Observatory); Ionospheric Radio Propagation (NBS Monograph 80, K. Davies, 1965), sections 4.2 and 4.3 (National Bureau of Standards (NIST)); Introduction to HF Radio Propagation (Australian Bureau of Meteorology, Space Weather Services); Recommendation ITU-R P.1239, ITU-R reference ionospheric characteristics (foE, equations 12 to 17) (ITU-R); Aurora: 30 Minute Forecast (NOAA SWPC); GOES Solar Ultraviolet Imager (SUVI) (NOAA SWPC); WSPR Live (wspr.live); WSJT-X User Guide 2.7.0 (WSPR) (WSJT Development Group); 2012-2016 Extra Class question pool (E3B09 to E3B11, gray-line propagation) (NCVEC Question Pool Committee); 2023-2027 General question pool (NCVEC Question Pool Committee); 2024-2028 Extra Class question pool (NCVEC Question Pool Committee); Terms of Use (public domain map data) (Natural Earth)
Practice the exam questions: T3 Radio Wave Propagation (Technician), G3 Radio Wave Propagation (General), E3 Radio Wave Propagation (Amateur Extra)
Found something wrong or out of date? Report a problem with this tool on GitHub (a public issue; nothing is sent until you submit it there).