Radio Propagation Theory — Page 30, Lesson 33

Let’s pick this up right where the physics of the ionosphere left off. We’ve already seen that the ionosphere has layers that refract radio waves back to Earth, and that the amount of ionization in those layers changes with the sun. Now I want to walk you through what happens when we change the frequency of the wave, and then what happens when we change the height of the layer. These two effects control the whole geometry of sky wave propagation. First, the effect of changing frequency. For a given ionization intensity — so, holding the layer’s electron density constant — the amount of refraction of a radio wave decreases as frequency increases. Here’s the physical reason: as frequency increases, the energy contained in the radio wave increases, and that higher-energy wave is less easily bent by the ionosphere. So refraction decreases. Now, follow that chain of consequences, because it’s a cause-and-effect sequence you need to be able to recite. If refraction decreases, the wave is bent less. That means the critical angle — the steepest angle at which the wave can still be returned to Earth — will increase. And when the critical angle increases, both the skip distance and the dead space will also increase. Skip distance is the distance along the Earth’s surface between the transmitter and the point where the sky wave first returns. Dead space is the gap between the end of the surface wave coverage and that first return point — the zone where you get neither surface wave nor sky wave. There’s a second contributor to dead space here. As frequency increases, the surface wave range will decrease. So dead space grows from both directions at once: the skip distance pushes the first sky wave return further out, and the surface wave range pulls the reliable ground coverage closer in. The dead space widens from both sides. Now the converse. If we decrease frequency, we get the exact opposite: a decrease in critical angle, a decrease in skip distance, and a decrease in dead space. So the rule is simple — higher frequency, bigger skip and bigger dead space; lower frequency, smaller skip and smaller dead space. Next, the height of the layers. The skip distance is also affected by the altitude of the refracting layers. As the altitude of the layer increases, the skip distance also increases, and greater ranges are experienced by refraction at the F-layer than at the E-layer. Think about the geometry: a wave refracted from a higher layer leaves the Earth at a shallower angle and travels further before it comes back down. That’s why the F-layer, sitting higher, gives you longer hops than the E-layer. Now let’s move to a very practical case — LF and MF sky wave propagation. This is where the D-layer becomes the star of the story. During the day, the D-layer absorbs radio energy at frequencies below about 2 MHz — that covers the LF band, Low Frequency, and the MF band, Medium Frequency. So during daylight, those low-frequency sky waves are simply eaten by the D-layer. But at night, the D-layer is effectively non-existent. The sun is gone, so the ionization that creates the D-layer collapses. With the D-layer gone, at these same frequencies below about 2 MHz, sky waves refracted at the E-layer are now present. So the E-layer takes over at night and returns those waves to Earth. Here’s the operational consequence, and it’s a big one for you as a pilot. This means the sky waves at LF and MF are not reliable for continuous long-range use. You can’t count on them as a steady long-haul propagation path, because they only exist at night. And there’s a second problem: the presence of sky waves at night, at the relatively short ranges associated with these lower frequencies, will cause interference with short-range navigation and broadcasting systems that rely on surface wave reception. In other words, at night you get a sky wave arriving at a receiver that was designed to use the surface wave, and the two signals clash. This directly affects ADF — the Automatic Direction Finder — and we’ll discuss that in detail in Chapter 7. So the picture is: by day, the D-layer blocks LF and MF sky waves, and you rely on the surface wave. By night, the D-layer disappears, the E-layer returns sky waves, and those sky waves interfere with the surface wave systems at short range. That day/night contrast is the whole story of LF and MF propagation. Let me show you the geometry of that day/night difference.