Reading Propagation Forecasts
Propagation forecasts tell you what band conditions to expect before you sit down at the radio — letting you choose the right band for the contact you want to make, time your operating session for best conditions, and avoid frustration on bands that are closed or disturbed. Modern propagation forecasting combines real-time solar data, computer modelling, and crowdsourced spotting networks to give operators better tools than ever before for predicting when and where a band will be open.
Solar Flux Index (SFI)
The Solar Flux Index is measured at 10.7 cm wavelength by a radio telescope in Penticton, Canada and is published daily. It is the best single-number indicator of ionospheric conditions for HF propagation. An SFI above 150 generally supports good propagation on 15m, 12m, and 10m. An SFI of 100–150 typically gives good 20m conditions and marginal higher-band conditions. Below 80, the higher bands (15m, 12m, 10m) are frequently closed. The SFI is a lagging indicator — it reflects conditions from the previous 27-day solar rotation as well as current activity, so rapid changes in SFI (caused by solar flares or new active regions rotating into view) can be more significant than the absolute number.
A-index and K-index
The A-index is a daily summary of geomagnetic disturbance on a scale of 0 to 400 (though values above 100 are extreme). It is derived from averaging eight 3-hour K-index values through the day. The K-index is the 3-hour geomagnetic activity index on a quasi-logarithmic scale of 0–9. For HF propagation: K-index 0–2 is quiet and good for most paths; K-index 3 is unsettled and may degrade polar paths; K-index 4 is active and degrades HF at high latitudes; K-index 5+ (geomagnetic storm) can significantly disrupt HF propagation globally, particularly on paths through polar and auroral regions. Low K-index combined with high SFI gives the best HF conditions.
VOACAP
VOACAP (Voice of America Coverage Analysis Program) is a sophisticated HF propagation prediction program originally developed for shortwave broadcasters. The online version at voacap.com allows you to enter your transmitter location, receiver location, antenna type, and power, then generates predicted signal levels and reliability for all HF bands throughout the 24-hour day. VOACAP uses statistical models based on decades of ionospheric data to predict median propagation conditions for any path. It is excellent for planning — telling you which band is most likely to support a specific path at a specific time. It is a prediction of median conditions, not a real-time forecast.
DX clusters and spotting networks
Real-time crowdsourced spotting gives you actual propagation data right now, not a model prediction. When an operator works or hears a DX station, they post a spot to a DX cluster network. Aggregator sites like DXWatch.com, DX Summit, and the Cluster column in logging software show these spots in real time. If you see spots of stations in Japan appearing on 15m, the band is open to Japan right now — more reliable than any model for the current moment. The Reverse Beacon Network (reversebeacon.net) adds automated CW and digital spotting from software receivers worldwide, providing even more comprehensive real-time propagation data without human intervention.
PSKReporter
PSKReporter (pskreporter.info) collects automated propagation reports from WSJT-X, Fldigi, and other software and displays them on a world map showing which stations are hearing which stations right now. It is invaluable for understanding band conditions because it shows the actual spots being decoded at this moment across all HF bands. Click on any band on PSKReporter and you see a real-time picture of propagation — red lines showing who is hearing whom and in what directions. If your signal is being spotted 10,000 km away on 10m, the band is open to that part of the world even if you cannot hear anyone there.
WSPR as a propagation beacon
WSPR (Weak Signal Propagation Reporter) is a mode where stations transmit automated low-power beacons that are received and reported by other WSPR stations worldwide. WSPRnet.org shows a real-time map of WSPR reception reports across all bands. Running WSPR on a band for even 30 minutes tells you exactly where your signal is reaching — a circle of red dots at 5,000 km on 20m tells you the band is open to those regions. Many operators run WSPR continuously to build a database of their station's propagation performance. It is the most systematic way to understand what your antenna actually radiates and where.
| Indicator | Good Sign | Bad Sign | Where to Check |
|---|---|---|---|
| SFI | Above 150 for high bands | Below 80 for 10–15m | swpc.noaa.gov, hamqsl.com |
| K-index | 0–2 quiet | 5+ storm | swpc.noaa.gov, WWV at :18 past hour |
| DX cluster spots | Spots from target region on target band | No spots from region on band | DXWatch.com, DX Summit |
| RBN | CW stations spotted from target region | No spots from target region | reversebeacon.net |
| PSKReporter | FT8 decodes from target region | No decodes from target region | pskreporter.info |
| WSPR map | Reception reports from target region | No reports from target region | wspr.net |
Is there a simple way to check if a band is open right now?
Yes — open PSKReporter.info, select the band you are interested in, and look at the map. If you see lines connecting stations across your target path, the band is open to that region right now. This takes about 10 seconds and is more accurate than any model prediction. Alternatively, tune to the FT8 frequency for the band and watch WSJT-X for 30 seconds — if you are decoding stations from your target region, propagation is there.
What does it mean when the band sounds like white noise?
A band that sounds like flat white noise with no signals is either closed (no propagation supporting that frequency) or below the MUF. The higher HF bands (10m, 12m, 15m) close when the MUF drops below their frequency — typically at night or during solar minimum. A closed band sounds like receiver noise floor — flat, hissy, and devoid of the characteristic heterodynes and signal remnants of an active band. Check PSKReporter or the cluster for the band — if nobody worldwide is spotting anything on it, the band is closed.
How accurate are propagation predictions a week in advance?
Weekly propagation forecasts are essentially educated guesses for most operators. Solar flux levels can change rapidly due to new active regions rotating into view or flare activity. The NOAA Space Weather Prediction Center publishes 27-day outlooks (based on the previous solar rotation) that provide rough guidance on expected SFI trends. Geomagnetic activity is very difficult to predict more than 1–3 days in advance. For operating planning beyond 3 days, use the VOACAP median predictions for your planned path rather than specific event forecasts.