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Learn HF Propagation

Plain-English guides that explain what the numbers on this site mean and how to use them on the air. Start with the basics, then dive into each index.

What Is HF Propagation?HF propagation is the way shortwave signals (3 to 30 MHz) travel around the world by bouncing off the ionosphere instead of going straight into space. Solar Flux Index (SFI) ExplainedThe solar flux index is the single most useful number for judging HF conditions: the higher it is, the higher the frequencies the ionosphere can reflect. K-Index and A-Index ExplainedThe K and A indices measure how disturbed Earth’s magnetic field is. For radio amateurs they are the early warning that HF paths, especially polar ones, are about to degrade. Sunspot Number and the Solar CycleSunspots are the oldest record of solar activity, and the 11-year rise and fall of their number sets the rhythm of HF propagation. Gray Line Propagation for DXAround sunrise and sunset, a band of twilight sweeps around the Earth. Along this gray line, low-band signals can travel exceptionally well, sometimes halfway around the world. Solar Flares and Radio BlackoutsA strong solar flare can wipe out HF signals on the sunlit side of the Earth within minutes. Knowing the flare classes tells you how bad it is and how long it will last. Aurora and Sporadic-E on VHFTwo phenomena open the VHF bands well beyond line of sight: radio aurora during geomagnetic storms, and sporadic-E in the summer months. Neither depends on the solar flux. HF Bands by Day and NightEach amateur band has its own personality. This guide gives the typical behaviour of every HF band so you can pick the right one for the time of day.

Where to start

New to propagation? Read What Is HF Propagation?, then HF bands by day and night. To read the live numbers, continue with the solar flux index and the K-index and A-index. Quick answers are in the FAQ.

Data: NOAA Space Weather Prediction Center, NASA SDO. Band conditions are estimates computed by Map For HAM and are indicative only. Methodology and credits · Data feeds · Widgets