t gives the minimum number of reflections as shown in Figure 2.21. Lower frequencies are more likely to use the E layer and as the maximum distance for each reflection is less, it is likely to give lower signal strengths than a higher frequency using the F layer to give fewer reflections. Not all reflections around the world occur in exactly the ways described. It is possible to calculate the path that would be taken, the number of reflections, and hence the path loss and signal strength expected. Sometimes signal strengths appear higher than would be expected. In conditions like these it is possible that a propagation mode called chordal hop is being experienced. When this happens it is found that the signal travels to the ionosphere where it is reflected, but instead Stronger signal from single reflection Figure 2.21 The minimum number of reflections usually gives the best signal 40 Newnes Guide to Radio and Communications Technology of returning to the earth it takes a path which intersects with the ionosphere again, only then being reflected back to earth. Fewer reflections are needed to cover a given distance. As a result signal strengths are higher when this mode of propagation is used. Critical frequency When a signal reaches a layer in the ionosphere it undergoes refraction and often it will be reflected back to earth. The steeper the angle at which the signal hits the layer the greater the degree of refraction is required. If a signal is sent directly upwards this is known as vertical incidence as shown in Figure 2.22. Figure 2.22 Vertical incidence For vertical incidence there is a maximum frequency for which the signals will be returned to earth. This frequency is known as the critical frequency. Any frequencies higher than this will penetrate the layer and pass right through it on to the next layer or into outer space. When a signal is transmitted over a long-distance path it penetrates further into the reflecting layer as the frequency increases. Eventually it passes straight through. This means that