We're starting a brand-new chapter together — Chapter 2, Radio Propagation… — Page 20, Lesson 18

We're starting a brand-new chapter together — Chapter 2, Radio Propagation Theory. So let's set the stage properly, because this whole chapter is the foundation for everything else in radio navigation. First, the word "propagation." In the context of radio waves, propagation simply means how the radio waves travel through the atmosphere. That's the core definition I want you to hold onto. Now, here's the key idea: different frequency bands use different propagation paths through the atmosphere. And that propagation path — the route the wave takes — often determines what a particular frequency band can actually be used for, whether that's in a communication system or a navigation system. The different paths associated with particular frequencies can also impose limitations on the use of those frequencies. So the path isn't just trivia — it decides capability and it decides restriction. Now, before we talk about the paths themselves, we need to look at the factors that affect propagation. There are several, and the first one we meet is attenuation. Attenuation is the term given to the loss of signal strength in a radio wave as it travels outward from the transmitter. So as the wave leaves the transmitter and moves away, it gets weaker. That's attenuation. And there are two distinct aspects to it, and I want you to keep them separate in your mind. The first aspect is absorption. As the radio wave travels outwards from the transmitter, its energy is absorbed and scattered by the molecules of air and water vapour, by dust particles, by water droplets, by vegetation, by the surface of the earth, and by the ionosphere. So the wave is literally losing energy to the stuff it passes through. Now here's the important relationship: the effect of this absorption — except for the ionospheric part — increases as frequency increases. And it becomes a very significant factor above about 1000 MHz. So above roughly 1000 megahertz, absorption really starts to bite. That's a number worth remembering. The second aspect of attenuation is the inverse square law. This one is geometric rather than about the medium. The electromagnetic radiation from an aerial spreads out as the surface of a sphere. So imagine the energy radiating in all directions, forming an ever-expanding sphere. Because the energy is spread over that growing spherical surface, the power available decreases with increasing distance from the transmitter. The book gives a concrete example. Suppose at a certain distance from the transmitter, the field intensity is 4 watts per metre squared. At double that distance, the energy will be spread over an area of 4 square metres, and the field intensity will drop to 1 watt per metre squared. So the power available is proportional to the inverse of the square of the range. Double the range, and you get a quarter of the power. That's the inverse square law in action. Let me show you this with a diagram, because it's much clearer visually. So to summarise where we are: propagation is how radio waves travel through the atmosphere; different frequencies take different paths, which determines their use and their limitations; and attenuation — the loss of signal strength — comes from two sources, absorption by the environment, which grows with frequency and becomes significant above about 1000 MHz, and the inverse square law, where power falls off as the inverse square of the range. That's our foundation. Next we'll move into the actual propagation paths themselves.