skypwavepropagation

Skywave propagation, sometimes spelled sky-wave propagation (often encountered in nonstandard spellings such as skypwavepropagation), refers to radio wave propagation in which signals are reflected or refracted by the Earth's ionosphere back toward the surface, enabling long-distance communication beyond line of sight. The mechanism relies on ionospheric layers—primarily the F2 layer, and to a lesser extent the E and D layers—that create a refractive medium for high-frequency (HF) radio waves in roughly the 3 to 30 MHz range.

The usable frequency range for a given path is described by the Maximum Usable Frequency (MUF) and the Lowest Usable Frequency (LUF). These values depend on ionospheric conditions, which vary with solar activity, time of day, season, and geographic location. Daytime D-layer absorption can attenuate lower HF signals, while nighttime reduction in D-layer effects and, typically, a higher F-layer peak enhance long-range propagation.

Propagation modes include single-hop paths, where a signal is reflected from the ionosphere once and returns to the Earth, and multi-hop paths, where successive ionospheric reflections extend reach to intercontinental distances. Path distance, frequency choice, and the angle of incidence to the ionosphere, along with the Earth's magnetic field, influence path quality and reliability.

Applications and limitations: skywave propagation is fundamental for international shortwave broadcasting, amateur radio, and some maritime and aviation communications. It is highly variable and sensitive to solar activity, geomagnetic storms, and atmospheric conditions, which can cause fading, distortion, or complete loss of a path.