Radio Propagation Theory — Page 30, Lesson 30

Let’s pick up right where the sky wave story gets interesting — inside the ionosphere itself. I want you to picture the ionosphere not as a single flat sheet, but as a series of layers, each with a centre. The key fact is this: the ionization levels in each layer increase towards the centre of the layer. So as a radio wave travels through a layer, it encounters an increasing density of ions as it moves towards the centre, and then a decreasing density as it moves out the other side. Now, what happens to the wave depends entirely on the angle at which it hits the layer. If the radio wave travels across the layer at right angles — that is, straight through, perpendicular to the layer — it will be retarded, meaning it slows down, but it will maintain a straight path. It just gets delayed, not bent. But if the wave penetrates the layer at an angle, something different happens. It gets refracted — bent — away from the normal as it enters the layer, and then back towards the normal as it exits. Remember, the normal is the imaginary line perpendicular to the layer’s surface. So the wave bends one way going in, and the opposite way coming out. Here’s the crucial part. The amount of refraction the wave experiences depends on two things: the frequency of the wave, and the levels of ionization in the layer. Now, if the wave refracts so much that it bends to the horizontal — parallel to the earth’s surface — before it reaches the centre of the layer, then it will continue to refract, and it will eventually return to the surface of the earth as a sky wave. This is what we call total internal refraction at the layer. The wave is bent so sharply it never escapes the layer — it gets turned back down towards the ground. Let me tie this to a practical picture. Imagine a transmitter sending a wave straight up, vertically. At that vertical angle, with a frequency that penetrates the ionosphere, the wave just goes straight through and out into space. Now, as we tilt the wave away from the vertical — increasing the angle between the vertical and the radio wave — we eventually reach a point where total internal refraction occurs, and the wave returns to the surface. That first wave that manages to come back down is called the first returning sky wave. And the angle, measured from the vertical, at which this happens is called the critical angle. Now, follow the geometry. From the transmitter, the first returning sky wave comes down at some distance away. That distance — from the transmitter to the point where the first returning sky wave appears at the surface — is called the skip distance. But here’s the thing: sky waves occur in the LF, MF, and HF frequency bands. That’s low frequency, medium frequency, and high frequency. And in those bands, there will also be some surface wave present — a wave that travels along the ground. So you have two signals: the surface wave hugging the earth, and the sky wave bouncing back down. Between them, there’s a gap. From the point where the surface wave is totally attenuated — completely died out — to the point where the first returning sky wave appears, there will be no detectable signal at all. That silent zone is called dead space. Let me give you the numbers that govern where this refraction happens. The height at which full internal refraction occurs depends on frequency. As a generalization: frequencies up to 2 MHz will be refracted at the E-layer, and frequencies from 2 to 50 MHz will be refracted at the F-layers. Above 50 MHz, sky wave is only likely to occur under abnormal ionospheric conditions — specifically, intense sunspot or solar flare activity. That’s why VHF frequencies, which are very high frequency and used for navigation systems, do not produce sky waves. They’re simply too high in frequency to be bent back down under normal conditions. Now, the last piece — and this is the dynamic part. The whole reason refraction happens is the ionization of the upper atmosphere. So if the ionization intensity changes, the amount of refraction changes too. Let’s think it through. At a given frequency, as ionization increases, the refractive index increases, and hence the amount of refraction affecting the radio waves also increases. More ionization means more bending. And what does more bending do? It means refraction takes place at a smaller critical angle — the wave gets turned back sooner. And that means the skip distance and the dead space both decrease. The wave comes back closer to the transmitter. Conversely, a decrease in ionization results in an increase in critical angle, skip distance, and dead space. Less bending means the wave travels further before it comes back down, so the silent gap grows. So the whole system is a balance: ionization up, everything shrinks; ionization down, everything stretches. That’s the core of sky wave propagation — the critical angle, the skip distance, and the dead space all dance together with the sun’s activity.