wards more steeply cover a relatively small distance. Those that leave the antenna almost parallel to the earth travel a much greater distance before they reach the ionosphere, after which they return to the earth almost parallel to the surface. In this way these signals travel a much greater distance. To illustrate the difference this makes, changing the angle of radiation from 0 degrees to 20 degrees reduces the distance for E layer signals from 2000 km to just 400 km. Similarly using the F layer distances are reduced from 4000 km to 1000 km. For signals that need to travel the maximum distance this shows that it is imperative to have a low angle of radiation. However, broadcast stations often need to make their antennas directive to ensure the signal reaches the correct area. Not only do they ensure they are radiated with the correct azimuth, they also ensure they have the correct angle of elevation or radiation SO that they are beamed to the correct area. This is achieved by altering the antenna parameters. Multiple reflections The maximum distance for a signal that is reflected by the F2 layer is about 4000 km. However, radio waves travel much greater distances than this around the world. This cannot be achieved using a single reflection, but instead several are used as shown in Figure 2.20. To achieve this, the signals travel to the ionosphere and are reflected back to earth in the normal way. Here they can be picked up by a receiver. However, the earth also acts as a reflector because it is conductive and the signals are reflected back to the ionosphere. In fact it is found that areas which are more conductive act as better reflectors. Not surprisingly the Radio waves and propagation 39 Figure 2.20 Several reflections used to give greater distances sea acts as an excellent reflector. Once reflected at the earth's surface the signals travel towards the ionosphere where they are again reflected back to earth. At each reflection the signal suffers some attenuation. This means that it is best to use a path tha