Radio Propagation Theory — Page 30, Lesson 26

Let’s start with the line-of-sight calculation, because that’s the practical bit you’ll actually use in the aircraft. The formula for maximum VHF range is: Range = 1.23 × (√h₁ + √h₂) where h₁ and h₂ are the heights of the transmitter and receiver in feet, and the range comes out in nautical miles. The 1.23 is a constant that converts the geometry of the Earth’s curvature into nautical miles. So take the example: a receiver at 1600 ft and a transmitter at 1024 ft. The square root of 1600 is 40, and the square root of 1024 is 32. Add them: 40 + 32 = 72. Multiply by 1.23, and you get 88.6 NM. That’s the maximum range at which the receiver can pick up the VHF transmission. Now, one very important note here: regardless of what other propagation paths might exist, if the receiver is in line of sight with the transmitter, then the space wave will be received. That’s the key principle — line of sight guarantees reception of the space wave. Now let’s move into ionospheric propagation. Before we can understand how the ionosphere produces the sky wave, we need to understand what creates the ionization in the upper atmosphere and what the ionosphere actually is. The ionosphere extends upward from about 60 km altitude to the limits of the atmosphere, notionally 1500 km. At 60 km, the atmospheric pressure is only 0.22 hPa — extremely low. Because the pressure is so low, the gaseous atoms are widely dispersed. Here’s what happens: 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 electron and a positively charged particle called an ion. So ionization is the process of stripping electrons off atoms, creating free electrons and positive ions. Now, these electrons are constantly trying to recombine with the ions. So the highest levels of ionization occur shortly after midday — around 1400 local time — when there’s a balance between the rate of ionization and the rate of decay, where electrons rejoin ions. The lowest ionization occurs just before sunrise at the surface. Seasonally, ionization is higher in summer than in winter. And it increases as latitude decreases — meaning closer to the equator — again because the solar radiation is more intense 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 to communication and navigation systems, especially space-based ones. In fact, it’s not unusual for communication and other satellites to be shut down during intense solar flare activity just to avoid damage. Here’s the last key point about how the ionosphere forms layers. As incoming solar energy is absorbed by the gaseous atoms, the amount of energy available to ionize atoms at lower levels reduces. So ionization levels increase with altitude. But — and this is important — because the normal atmospheric mixing processes that exist in the lower atmosphere are absent at higher levels, gravitation and terrestrial magnetism affect how the gases are distributed. That means the increase in ionization is not linear. Instead, the ionized particles form into discrete layers. So that’s the foundation: the ionosphere is a layered region from 60 km to about 1500 km, created by solar radiation stripping electrons from atoms, with ionization peaking around 1400 local time, higher in summer and at lower latitudes, and disrupted unpredictably by solar flares. And those discrete layers are what produce the sky wave — which we’ll get into next.