In amateur radio, what does a high frequency radio signal depend on for effective propagation?

For effective propagation of high-frequency (HF) radio signals, solar activity plays a vital role. HF radio waves typically rely on the ionosphere, a layer of the Earth's atmosphere that is ionized by solar radiation. During periods of high solar activity, such as solar flares or increased sunspot numbers, the ionization levels in the ionosphere are enhanced. This increased ionization allows HF signals to reflect off the ionosphere more effectively, enabling longer-distance communication.

This phenomenon is especially important because HF signals can travel beyond the horizon by bouncing off the ionosphere, which significantly extends their range. Conversely, during periods of low solar activity, the ionization in the ionosphere decreases, leading to poorer propagation conditions for HF signals, potentially limiting communication range.

In contrast, while atmospheric pressure, satellite positions, and ground conductivity can influence radio wave propagation in different ways, they do not have the direct and significant impact on HF signal propagation in the same manner as solar activity.