General Guidelines for Correlation of Propagation Indices to Actual HF Propagation Conditions

Propagation tutorial by PA9X

N0NBH's propagation tutorial (PDF)

NM7M: The Little Pistol's Guide to HF Propagation (PDF)

Propagation Broadcasts and Forecasts Demystified (PDF)

RSGB Propagation Studies Committee (lots of info on different propagation modes)

Understanding 10 (11) Meter Sporadic-E (understanding ES or “short skip”)

Understanding Solar Indices by Ian Poole, G3YWX