Radio Propagation Theory — Page 20, Lesson 25

I want to walk you through the space wave, because this is the propagation path that matters at VHF and above — and it's the one that governs most of the radio navigation you'll use in the aircraft. First, the name. The space wave is made up of two paths: a direct wave and a reflected wave. So when a transmitter sends energy toward a receiver at these frequencies, part of the signal travels straight from the transmitting antenna to the receiving antenna — that's the direct wave — and part of it bounces off the earth's surface and still reaches the receiver — that's the reflected wave. The two combine at the receiver, and together they form what we call the space wave. Now here's the key idea. At frequencies of VHF and above, radio waves start to behave more like visible light. Just as we have a visual horizon with light, we have a radio horizon with these radio frequencies. And that means the only atmospheric propagation at these frequencies is line of sight. In other words, the signal can't bend around the curvature of the earth the way lower frequencies can — it travels essentially in a straight line, so the receiver has to be within sight of the transmitter, over the horizon, to pick anything up. But there's a small correction. There is some atmospheric refraction, and it causes the radio waves to bend towards the surface of the earth. That bending increases the range slightly beyond the geometric horizon — the purely straight-line horizon you'd get if the earth were a perfect sphere with no atmosphere. Because the diameter of the earth is known, and because the atmospheric refraction can be calculated, it's possible to determine the maximum theoretical range at which a transmission can be received. That's the figure we use in planning. One important detail: the amount of refraction decreases as frequency increases. So the higher you go in frequency, the less the wave bends, and the closer the radio horizon gets to the geometric horizon. But for practical purposes, for the EASA syllabus, we use a single formula to calculate the line of sight range: Range in nautical miles equals 1.23 times the square root of hTX plus the square root of hRX. Let me unpack that. The symbol hTX is the transmitter height in feet, and hRX is the receiver height in feet. So you take the square root of the transmitter height, add the square root of the receiver height, and multiply the whole sum by 1.23. The result is the maximum theoretical line of sight range in nautical miles. Notice both heights are in feet, and the constant 1.23 carries the units that turn feet into nautical miles. And here's the practical punchline, the thing that really matters for operations. At VHF and above, it does not matter how powerful the transmitter is. If the receiver is below the line of sight range, it will receive nothing. Power can't overcome the horizon. So when you're flying and you lose a VHF signal, the first thing to think about isn't the transmitter's power — it's whether you've dropped below the line of sight range for that pair of heights. That figure shows the space wave — the direct path and the reflected path between the transmitter and receiver. Keep that picture in mind: two paths, one straight, one bounced, combining at the receiver, and the whole thing limited by the radio horizon.