Radio Propagation Theory — Page 30, Lesson 28

We're starting a fresh topic now: the ionosphere and how it affects radio propagation. This is the heart of why HF radio works the way it does, and it's all about layers of ionized gas high above the Earth. Let me set the scene. The ionosphere is a region of the atmosphere where solar radiation strips electrons from atoms, creating ions. This ionization isn't uniform — it's concentrated in distinct layers. And the key point I want you to take away first is that the ionization is most intense at the centre of each layer, decreasing towards the lower and upper edges. So each layer is like a band of maximum charge density, thinning out above and below. Now, the characteristics of these layers — their height, their density — vary with the levels of ionization. And the sun drives all of this. Let's walk through the three significant layers from bottom to top. The lowest is the D-region, also called the D-layer. It sits at an average altitude of 75 km. It's a fairly diffuse area — not sharply defined. For practical purposes, it forms at sunrise and disappears at sunset. That's a crucial operational fact: the D-layer only exists during daylight. Above that is the E-layer, at an average altitude of 125 km. Unlike the D-layer, it's present throughout the 24 hours — it never disappears. But it does move: the E-layer 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 km. This one has a fascinating behaviour. The F-layer splits into two at sunrise and rejoins at sunset. When it splits, you get the F1-layer and the F2-layer. The F1-layer reduces in altitude at sunrise and increases in altitude after sunset — similar to the E-layer. But the F2-layer's behaviour is dependent on time of year. In summer, it increases in altitude and may reach altitudes in excess of 400 km. In winter, it reduces in altitude. So let me give you a mental picture of the whole stack: D-layer at 75 km, present only by day. E-layer at 125 km, always there, dipping at sunrise and rising after sunset. F-layer at 225 km, splitting into F1 and F2 at sunrise, rejoining at sunset, with F2 climbing high in summer. Now, here's the dynamic part. Overall, the levels of ionization increase from sunrise to midday local time, then decrease until sunrise the following morning. That's the daily cycle. But — and this is important — the 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's noisy. Finally, let's talk about what this means for you as a pilot. The structure of the ionosphere gives stable conditions by day and by night. But around dawn and dusk, the ionosphere is in a transitional state. That transition leads to what can best be described as electrical turbulence. The practical result: 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 an HF communication or relying on ionospheric propagation, dawn and dusk are your problem windows. That's a real operational consideration, not just theory. Let me just recap the numbers so they stick: D-layer 75 km, day only. E-layer 125 km, always present. F-layer 225 km, splits at sunrise. F2 can exceed 400 km in summer. Ionization peaks at midday, fluctuates with solar output, and dawn/dusk bring electrical turbulence and interference. That's the structure of the ionosphere. Next, we'll look at how a wave actually bends through these layers — that's where the propagation mechanics come in.