How does ionospheric propagation work?
Ionospheric propagation (skywave) occurs when HF signals are refracted back to Earth by ionized layers 60–500 km above the surface. The critical frequency fo is the highest frequency reflected at vertical incidence; at oblique incidence the maximum usable frequency is approximately fo·secθ (the secant law). ITU-R P.533 provides the standard method for predicting HF circuit performance.[1][2]
What are the ionospheric layers?
- D layer (≈ 60–90 km): daytime only; absorbs lower HF.
- E layer (≈ 90–150 km): regular daytime reflection; sporadic-E patches can support VHF.
- F layer (≈ 150–500 km): splits into F1 and F2 by day; F2 supports most long-distance HF.[2][1]
How does the antenna affect skywave?
The elevation (takeoff) angle needed depends on path length and layer height: long paths need low angles (a few degrees to about 15°), while NVIS needs near-vertical radiation. Antenna height above ground and ground conductivity shape the elevation pattern; see HF antennas.[3]
Why does propagation vary?
Ionization follows solar radiation, so daily, seasonal and roughly 11-year solar-cycle variations change usable frequencies; geomagnetic storms can disrupt propagation. Faraday rotation randomizes polarization of skywave signals.[1]
Frequently asked questions
What is the MUF?
The highest frequency that will be refracted back to Earth over a given path.
Why does lower HF work better at night?
The absorbing D layer disappears after sunset.
Does VHF use the ionosphere?
Occasionally via sporadic-E and, at solar maximum, F2 at low VHF.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Recommendation ITU-R P.533: Method for the prediction of the performance of HF circuits”. ITU-R. current edition.
- “Antennas and Radiowave Propagation”. R. E. Collin, McGraw-Hill. 1985.
- “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.