Radio Propagation Theory — Page 37, Lesson 35

Let’s pick up with the idea of achievable ranges for sky wave propagation, because that sets up everything about HF communications. The maximum range for a sky wave 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 each end of its path. That geometry gives you the absolute farthest you can reach with a single refraction. A simple calculation shows that the average maximum range for refraction from the E-layer at 125 km is 1350 NM, and the average maximum range from the F-layer at 225 km is 2200 NM. So the higher the ionized layer, the longer the hop. And these ranges will obviously change as the height of the ionized layers changes — they’re not fixed numbers, they’re averages tied to those layer heights. Now, when you need to go even farther, you use multi-hop sky wave. That’s when the wave is refracted at the ionosphere, then reflected back from the surface of the earth up to the ionosphere again, and so on — bouncing between the ionosphere and the ground. Multi-hop sky wave can achieve ranges of half the diameter of the earth. That’s the practical limit of this technique. Now let’s move to HF communications, because that’s where this all gets applied. 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 — SatCom — are not yet the norm, so long range communication must be provided by surface wave or sky wave propagation. To achieve ranges of 2000 to 3000 NM 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 communication systems using those 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 answer for those long ranges. Currently, therefore, the only practical solution is HF Communications utilizing sky wave propagation. In the future, no doubt, SatCom will become commonplace. Now, the key concept here is the maximum usable frequency, or MUF. The MUF for a given range will be that of the first returning sky wave. That’s the highest frequency that still comes back to you at that range. And 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, since the ionization intensity fluctuates, a decrease in ionization would result in an increase in skip distance and hence loss of signal. So you don’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, the OWF has been determined to be 0.85 times the MUF. So you take the MUF and multiply by 0.85 to get the frequency you actually operate on. Finally, 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 planning an HF frequency, you’d pick one about half the daytime value for nighttime operations. That’s the core of HF sky wave planning: the MUF as the ideal, the OWF at 0.85 times that as the practical choice, and the day/night frequency split of roughly half.