ABSTRACT
The silent zone is simply the region that exists between the extent of the coverage of the ground wave signal and the point at which the sky wave returns to earth (see Figure 1.13). Note also that, depending on local topography and soil characteristics, when a signal returns to earth from the ionosphere it is sometimes possible for it to experience a reflection from the ground, as shown in Figure 1.13. The onward reflected signal will suffer attenuation but in some circumstances may be sufficient to provide a further hop and an approximate doubling of the working range. The condition is known as multihop propagation. The skip distance is simply the distance between the point at which the sky wave is radiated and the point at which it returns to earth (see Figure 1.14). Note that where signals are received simultaneously by ground wave and sky wave paths, the signals will combine both constructively and destructively due to the different paths lengths and this, in turn, will produce an effect known as fading. This effect can often be heard during the early evening on medium wave radio signals as the D-layer weakens and sky waves first begin to appear.
