Skywave

Skywave, or ionospheric propagation, is a mode of radio transmission in which signals are directed toward the upper atmosphere and are reflected or refracted back to Earth. This effect enables long-distance communication beyond the line of sight.

Most effective in the high-frequency (HF) band, roughly 3 to 30 MHz, skywave depends on the ionosphere, a layer of charged particles in the upper atmosphere. Waves strike the ionosphere at an angle, and, depending on frequency and incidence, are refracted and sometimes reflected back to the surface. If the frequency is below the layer's critical frequency, the wave is reflected; if above, it passes through. The apparent reflection point creates a hop, with the overall distance increasing with hops and path angle.

Diurnal and solar conditions strongly influence propagation. During daytime the D layer forms and absorbs lower HF, reducing skywave, while the F layer controls long-range reflection, particularly at night when higher frequencies can be used and absorption is lower. The maximum usable frequency (MUF) is the highest frequency usable for a given path, and virtual height describes the apparent reflection altitude. Multiple hops can extend distance beyond the horizon.

Variability arises from solar activity, seasonal changes, geomagnetic disturbances, and sporadic E events. Propagation is often subject to fading and multipath effects, making reception uneven.

Applications include amateur radio, international shortwave broadcasting, and emergency communications. While skywave provides global reach, it is inherently unpredictable and depends on ionospheric conditions, which can shift rapidly.