Radio Propagation Theory — Page 37, Lesson 35

Right, let's get into the meat of sky wave propagation. We've already covered how a single hop works, so now we need to look at the practical limits and the real-world application of this. First, let's talk about the maximum range you can achieve with a single sky wave. The absolute maximum range is achieved when the path of the radio wave is tangential at the surface of the earth at both the transmitter and the receiver. In other words, the wave just grazes the earth's surface at both ends. Now, a simple calculation shows that the average maximum range for refraction from the E-layer, which sits at about 125 km, is 1350 nautical miles. And the average maximum range from the F-layer, at about 225 km, is 2200 nautical miles. These are the averages, and they will obviously change as the height of the ionized layers changes. Now, what if you need to go further than that? That's where multi-hop sky wave comes in. This occurs when the wave is refracted at the ionosphere, then the sky wave is reflected back from the surface of the earth up to the ionosphere, and so on. It bounces. Multi-hop sky wave can achieve ranges of up to half the diameter of the earth. That's a huge distance, and you can see that illustrated in Figure 2.13. Now, let's move on to how this is actually used in practice, specifically for HF communications. Over inhabited land areas, VHF communications are ideal for all communications between aircraft and ground. But over oceans and uninhabited land areas, you need long-range systems. Satellite Communications, or SatCom, are not yet the norm, so long-range communication must be provided by either surface wave or sky wave propagation. Let's consider surface wave for a moment. To achieve ranges of 2000 to 3000 nautical miles using surface wave propagation would require low frequencies, either from the lower end of the LF band or the upper end of the VLF band. But there are serious drawbacks. Communication systems utilizing these frequencies would require relatively complex equipment with an associated weight penalty. Lower frequencies are also subject to greater static interference than higher frequencies, making such systems somewhat tedious to use. Furthermore, data rates associated with low frequencies are notoriously low. So, surface wave is not a practical solution for this job. Currently, therefore, the only practical solution is HF Communications utilizing sky wave propagation. In the future, no doubt, SatCom will become commonplace, but for now, HF sky wave is it. Now, here's a critical concept: the maximum usable frequency, or MUF. The MUF for a given range will be that of the first returning sky wave. This is the ideal frequency for that range because it will have had the shortest path through the ionosphere, and therefore, will have experienced less attenuation and contain less static interference. However, there's a catch. Since the ionization intensity fluctuates, a decrease in ionization would result in an increase in skip distance and hence loss of signal. So, you can't just use the MUF. Instead, a compromise frequency is used, known as the optimum working frequency, or OWF. By decades of experimentation and experience, it has been determined to be 0.85 times the MUF. So, OWF equals 0.85 times MUF. Finally, let's consider the effect of day and night. Since ionization levels are lower by night than by day, it follows that the frequency required for use at a particular range by night will of necessity be less than the frequency required for use by day. A good rule of thumb is that the frequency required at night is roughly half that required by day. So, if you're using 12 MHz by day, you'd expect to need around 6 MHz by night for the same range. That's exactly the situation you'd face on a mid-Atlantic crossing, as shown in Figure 2.15.