to the F layer. This may have split into two as the F1 and F2 layers. When the signals at a frequency of f3 reach the first of the layers they are again reflected back to earth. Then as the frequency rises to f4 they pass on to the F2 layer where they are reflected. As the frequency rises still further to f5 the signals pass straight through all the layers, travelling on into outer space. During the day at the peak in the sunspot cycle it is possible for signals as high as 50 MHz and more to be reflected by the ionosphere. However, this figure falls to below 20 MHz at very low points in the cycle. To achieve the longest distances it is best to use the highest layers. This is achieved by using a frequency that is high enough to pass through the lower layers. From this it can be seen that frequencies higher in the short wave spectrum tend to give the longer distance signals. Even SO it is still possible for signals to travel from one side of the globe to the other on low frequencies at the right time of day. But for this to happen good antennas are needed at the transmitter and receiver and high powers are generally required at the transmitter. Distances and the angle of radiation The distance that a signal travels if it is reflected by the ionosphere is dependent upon a number of factors. One is the height at which it is reflected, and in turn this is dependent upon the layer used for reflection. It is found that the maximum distance for a signal reflected by the E layer is about 2000 km whereas the maximum for a signal reflected by the F layer is about 4000 km. Signals leave the transmitting antenna at a variety of angles to the earth. This is known as the angle of radiation, and it is defined as the angle between the earth and the path the signal is taking. 38 Newnes Guide to Radio and Communications Technology Low angle signals can be heard over a much greater distance Figure 2.19 Effect of the angle of radiation on the distance achieved It is found that those that have a higher angle of radiation and travel up