Radio Propagation Theory — Page 30, Lesson 30

Let’s pick up right where the ionosphere leaves off — the sky wave itself. I want to walk you through what happens when a radio wave actually transits one of those ionized layers, because that’s the heart of sky-wave propagation. First, the key idea: the ionization levels in a layer increase towards the centre of the layer. So as a radio wave passes through, it encounters an increasing density of ions as it moves toward the middle, and then a decreasing density as it moves out the other side. That gradient is what does the work. Now, what happens to the wave depends on the angle it hits the layer. If the radio wave travels across the layer at right angles — that is, straight up, perpendicular to the layer — it will be retarded, but it will maintain a straight path. It slows down, but it doesn’t bend. However, if the wave penetrates the layer at an angle, it will be refracted away from the normal as it enters, and then back towards the normal as it exits. That’s the classic bending behaviour, and it’s the basis of the whole sky-wave mechanism. The amount of refraction depends on two things: the frequency of the wave and the levels of ionization. Here’s the crucial condition: if the radio wave refracts to the horizontal — that is, parallel to the earth’s surface — before it reaches the centre of the layer, then it will continue to refract and will return to the surface of the earth as a sky wave. That’s what we call total internal refraction at the layer. The wave essentially gets bent so much that it curves back down instead of passing through. Now let’s build the geometry from the transmitter. Starting from the vertical at the transmitter, with a frequency that penetrates the ionosphere, as the angle between the vertical and the radio wave increases, you’ll eventually reach an angle where total internal refraction occurs and the wave returns to the surface. That first returning wave is called the first returning sky wave, and the angle — measured from the vertical — at which this occurs is known as the critical angle. So the critical angle is that minimum angle from vertical that still gets the wave bent back down. The distance from the transmitter to the point where that first returning sky wave appears at the surface is called the skip distance. And here’s the practical consequence: because sky waves occur in the LF, MF, and HF frequency bands, there will also be some surface wave present. From the point where the surface wave is totally attenuated to the point where the first returning sky wave appears, there will be no detectable signal at all. That silent gap is known as dead space. So you have the surface wave hugging the ground, then a zone of nothing, then the first sky wave landing — and that nothing zone is dead space. Now, the height at which full internal refraction occurs depends on frequency, but as a generalization: frequencies up to 2 MHz will be refracted at the E-layer, and from 2 to 50 MHz at the F-layers. Sky wave is only likely to occur above 50 MHz under abnormal ionospheric conditions — intense sunspot or solar flare activity. That’s why VHF frequencies used for navigation systems do not produce sky waves. So for your navigation work, you can rely on VHF not bouncing back at you. Finally, let’s look at the effect of a change in ionization intensity. Since the refraction is caused by the ionization of the upper atmosphere, if the ionization intensity changes, the amount of refraction changes too. At a given frequency, as ionization increases, the refractive index — and hence the amount of refraction affecting the radio waves — also increases. That means refraction will take place at a smaller critical angle, and consequently the skip distance and dead space will decrease. Conversely, a decrease in ionization results in an increase in critical angle, skip distance, and dead space. So more ionization pulls the first returning wave in closer; less ionization pushes it further out. That’s the complete sky-wave picture: the layer gradient, the angle-dependent refraction, the critical angle, skip distance, dead space, the frequency bands involved, and how ionization changes shift everything.