We're starting a brand-new chapter today: Radio Propagation Theory — Page 20, Lesson 18

We're starting a brand-new chapter today: Radio Propagation Theory. This is the foundation for everything else in radio navigation, so I want to build it carefully with you. Let's begin with the introduction. In the context of radio waves, the term propagation simply means how the radio waves travel through the atmosphere. That's the core definition. Now, here's the key idea: different frequency bands use different propagation paths through the atmosphere. And that propagation path often determines the uses to which a particular frequency band can be put, either in communication or in navigation systems. So the path a wave takes isn't just a physics curiosity—it decides whether that frequency is useful for talking to ATC or for navigating. The different propagation paths associated with particular frequencies can also impose limitations on the use of those frequencies. So the path gives you capability, but it also gives you restrictions. Now let's look at the factors that affect propagation. There are several, and we need to consider them when discussing the propagation paths. The first one 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 moves away, it gets weaker. There are two distinct aspects to attenuation, and I want you to keep them separate. The first aspect is absorption. As the radio wave travels outwards from a transmitter, the energy is absorbed and scattered by the molecules of air and water vapour, dust particles, water droplets, vegetation, the surface of the earth, and the ionosphere. So the wave's energy gets eaten up by everything it passes through. Now here's the crucial relationship: the effect of this absorption—except for ionospheric absorption—increases as frequency increases. And it becomes a very significant factor above about 1000 MHz. So above roughly one gigahertz, absorption really starts to bite. Remember that exception: ionospheric absorption behaves differently, and we'll come back to that when we discuss ionospheric propagation later in the chapter. The second aspect of attenuation is the Inverse Square Law. This is a geometric effect, not an absorption effect. The electromagnetic radiation from an aerial spreads out as the surface of a sphere, so the power available decreases with increasing distance from the transmitter. Let me give you the example from the text. If, at a certain distance from a transmitter, the field intensity is 4 watts per square metre, then at double the distance, that energy will be spread over an area of 4 square metres, and the field intensity will be 1 watt per square metre. So you doubled the distance, and the power dropped by a factor of four. That's the inverse square relationship: power available is proportional to the inverse of the square of the range. So if range doubles, power drops to one quarter; if range triples, power drops to one ninth. That's why the law is called inverse square. Let me show you this with a diagram. Look at that figure. At distance R, you have a 1-metre by 1-metre area receiving 4 watts. At distance 2R, the same energy is spread over a 2-metre by 2-metre area—that's 4 square metres—and each square metre now only gets 1 watt. So the same total energy, but spread thinner. That's the essence of the inverse square law. So to summarise what we've covered: propagation is how radio waves travel through the atmosphere, different frequencies use different paths, and those paths determine both the uses and the limitations of each band. Then we have attenuation—the loss of signal strength—which comes from two sources: absorption by the medium, which gets worse with frequency above about 1000 MHz, and the inverse square law, where power falls off as the inverse square of the range. Both of these are going to matter constantly as we work through the propagation paths in the rest of this chapter.