Radio Propagation Theory — Page 30, Lesson 28

Let’s pick up with the ionosphere itself — the region of the atmosphere that makes sky-wave propagation possible. I want to walk you through its structure, because everything about long-range HF radio depends on these layers. The ionosphere is divided into layers, and the key idea is that ionization — the process where solar radiation strips electrons from atoms, creating charged particles — is most intense at the centre of each layer, and it decreases towards the lower and upper edges. So each layer is densest in its middle, not uniform. Now, the lowest layer sits at an average altitude of 75 kilometres. It’s called the D-region, or D-layer. This one is fairly diffuse — spread out and weak — and for practical purposes it forms at sunrise and disappears at sunset. That’s a critical point: the D-layer only exists during daylight. Above that, at an average altitude of 125 kilometres, we have the E-layer. Unlike the D-layer, the E-layer is present throughout the full 24 hours — it doesn’t vanish at night. But it does move: it reduces in altitude at sunrise and increases in altitude after sunset. The final layer of significance is the F-layer, at an average altitude of 225 kilometres. This one has a fascinating behaviour — it splits into two at sunrise and rejoins at sunset. The lower part, called the F1-layer, reduces in altitude at sunrise and increases in altitude after sunset, just like the E-layer does. The upper part, the F2-layer, behaves differently — its altitude depends on the time of year. In summer it increases in altitude and may reach altitudes in excess of 400 kilometres, and in winter it reduces in altitude. Now, let’s talk about the overall behaviour of ionization. Overall, the levels of ionization increase from sunrise to midday local time, and then decrease until sunrise the following morning. But — and this is important — those levels are continually fluctuating, because the intensity of high-energy radiation from the sun fluctuates. So it’s entirely possible for ionization levels to decrease temporarily during the morning, or increase temporarily during the afternoon. The trend is upward to midday, but it’s not a smooth curve — it wobbles. Finally, let’s consider what this means for operations. The structure of the ionosphere gives stable conditions by day and by night. But around dawn and dusk — the transitional periods — the ionosphere is in a state of change, which leads to what can best be described as electrical turbulence. The practical consequence is that around dawn and dusk, radio navigation and communication systems that use the ionosphere are subject to excessive interference and disruption. So if you’re planning HF communications, those twilight periods are when you expect trouble. Let me just tie the layers together for you: D-layer at 75 km, daylight only; E-layer at 125 km, always present, dropping at sunrise and rising after sunset; F-layer at 225 km, splitting into F1 and F2 at sunrise, with F2 reaching over 400 km in summer. And remember the ionization peaks at layer centres, fluctuates with solar radiation, and the dawn/dusk transitions are when the ionosphere becomes electrically turbulent. That’s the foundation for understanding how sky waves get refracted back to Earth.