Radio Propagation Theory — Page 30, Lesson 26

Let’s start with the tail end of the space-wave formula, because that’s where this excerpt opens, and then we’ll move into the ionosphere. We’ve got a worked example: a receiver at 1600 feet, a transmitter at 1024 feet. The range formula is: Range = 1.23 × (√1600 + √1024) So the square root of 1600 is 40, the square root of 1024 is 32. Add those, you get 72. Multiply by 1.23, and you get 88.6 nautical miles. That’s the maximum VHF range for that geometry. Now, the key note here — and I want you to hold onto this — is that regardless of the possible propagation paths, if a receiver is in line of sight with a transmitter, then the space wave will be received. So the space wave is the direct, line-of-sight component. Even if other paths exist, line of sight guarantees the space wave arrives. Now we shift into ionospheric propagation. Before we can study how the ionosphere bends radio waves, we need to understand what produces the ionization in the upper atmosphere, and what properties of the ionosphere give rise to the sky wave. Let’s define the ionosphere itself. It extends upward from about 60 km altitude to the limits of the atmosphere, notionally 1500 km. In this region, pressures are very low — at 60 km, atmospheric pressure is only 0.22 hPa. Because the pressure is so low, the gaseous atoms are widely dispersed. Here’s the mechanism. Incoming solar radiation at ultra-violet and shorter wavelengths interacts with these atoms. It raises their energy levels and causes electrons to be ejected from the shells of the atoms. Now, an atom is electrically neutral. So when it loses an electron, you get two things: a negatively charged free electron, and a positively charged particle left behind. That positively charged particle is called an ion. That process — the ejection of electrons by solar radiation — is ionization. Now, these electrons are continually trying to reunite with the ions. So the highest level of ionization occurs shortly after midday, around 1400 local time. That’s when there’s a balance between the rate of ionization and the rate of decay, where electrons rejoin ions. The lowest ionization is just before sunrise at the surface. Seasonally, summer gives higher ionization than winter. And ionization increases as latitude decreases — meaning closer to the equator — again because the intensity of solar radiation is greater there. Now, solar flares. Increased radiation from solar flares is unpredictable, but it can produce exceptionally high levels of ionization. That, in turn, can cause severe disruption of communication and navigation systems, particularly space-based ones. It’s not unusual for communication and other satellites to be shut down during intense solar flare activity, just to avoid damage. One more important point about how ionization varies with altitude. As incoming solar energy is absorbed by the gaseous atoms, the amount of energy available to ionize atoms at lower levels reduces. So the levels of ionization increase with altitude. But — and this is the crucial part — because the normal atmospheric mixing processes of the lower atmosphere are absent at higher levels, gravitation and terrestrial magnetism affect the distribution of gases. That means the increase in ionization is not linear. Instead, the ionized particles form into discrete layers. So that’s the setup: the ionosphere is a layered region, and those discrete layers are what produce the sky wave. We’ll look at those layers next.