HFpropagation

HF propagation refers to radio wave propagation in the high-frequency band, roughly 3 to 30 MHz, which enables long-range communication by reflecting signals off the Earth's ionosphere. This mode is widely used by amateur radio, shortwave broadcasting, aviation, and maritime services.

The dominant mechanism is skywave propagation: a transmitted signal travels upward, is refracted by ionospheric layers and returns to Earth, allowing beyond-line-of-sight communication. Groundwave effects exist at the lower end of the HF band but are generally limited for long distances. The efficiency of skywave propagation depends on ionospheric conditions, time of day, season, and solar activity.

Ionospheric layers play a key role. The daytime D layer causes absorption that attenuates lower frequencies, reducing range for those frequencies. The E and F layers provide reflection, enabling long-range paths. The F2 layer typically governs the maximum usable frequency for a given path; factors such as solar flux and sunspot activity raise or lower the critical and maximum usable frequencies. The highest usable frequency (MUF) varies with angle, elevation, time of day, season, and solar conditions, while the lowest usable frequency (LUF) defines the lower edge of a viable path.

Propagation is also affected by geomagnetic activity, solar storms, and radio noise. Forecasts rely on ionospheric measurements, sunspot indices, and geomagnetic indices. Practically, HF users select bands and times with favorable MUF, often exploiting greyline periods around sunrise and sunset. Antennas such as dipoles, inverted-V, verticals, and directional beams are employed to optimize skywave paths.