Radio Propagation Theory — Page 20, Lesson 25

Let’s start with the space wave, because this is the propagation path that matters at VHF and above. The space wave is actually made up of two distinct paths: a direct wave, which travels straight from the transmitter to the receiver, and a reflected wave, which bounces off the earth’s surface before reaching the receiver. So when we talk about the space wave, we’re really talking about those two components combined. Now, here’s the key idea. At VHF frequencies 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. That means the only atmospheric propagation at these frequencies is line of sight. In other words, if the receiver is beyond the horizon, the direct wave simply cannot get there. But there’s a subtlety. There is some atmospheric refraction, and this causes the radio waves to bend towards the surface of the earth. That bending increases the range slightly beyond the geometric horizon. So the radio horizon is actually a little farther out than the visual horizon, because the wave curves down toward the earth instead of flying straight off into space. Now, because the diameter of the earth is known, and because the atmospheric refraction can be calculated, it becomes possible to determine the maximum theoretical range at which a transmission can be received. That’s what we call the maximum theoretical range. One important detail: the amount of refraction decreases as frequency increases. So at higher frequencies, the bending is less, and the range extension is smaller. But for practical purposes, for the EASA syllabus, we use a simple formula to calculate the line of sight range. Here it is: Range in nautical miles equals 1.23 times the square root of hTX plus the square root of hRX. Let me break that down. hTX is the transmitter height in feet. hRX is the receiver height in feet. So you take the square root of each height, add them together, and multiply by 1.23. The result is the range in nautical miles. Now, the critical operational point. 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 does not help you get over the horizon. The only thing that extends the range is height — raising either the transmitter or the receiver. That’s why aircraft fly high, and why ground stations are often on high terrain or tall towers. So to tie it together: the space wave is direct plus reflected, it’s line of sight at VHF and above, refraction bends it slightly toward the earth to extend range a bit, and the maximum theoretical range is set by the heights of both antennas through that 1.23 formula. And once you’re beyond that line of sight, no amount of transmitter power will save you.