Questions and Answers

Radio Record, 3 June 1938, Page 65

Questions and Answers

"72.J.B." (Wawera): The effect of darkness and daylight on radio signals, in their travel through the upper atmosphere, is a rather long subject, but a brief explanation is given here, The waves travel in straight lines. unless something deflects them from their normal paths. This means that they would take the shortest path from the transmitter to the receiver. Those waves which travel in a more or less horizontal direction are absorbed by an amount which depends on the = frequeney of the waves themselves. upon the nature of the ground they travel over and, the nature of the obstacles they encounter. Hence at great distances from the transmitter these "ground" waves as they are called are practically non-existent. In the upper portions of our atmosphere there are regions of low atmospherie pressure where the sun’s rays have the opportunity to change the electrical state of the air particles. This process is known as "ionising" and ionised air has different properties, as far as the travel of waves through it is concerned, to that of normal air, If the waves at a boundary, between ionised and normal air, arrive at an acute angle, they will be reflected from the boundary at a simliar angle to their arrival, Hence these return to earth again but. naturally, ar some distance away. They may even be reflected so that they hit this reflecting layer further on and, be again reflected before reaching the earth. Thus by one or more "hops" they can reach round the earth. Such reflecting layers cannot remain constant in their amount of ionisation nor in their height above’ the earth, as there are many factors which alter these two characteristics. During the period of greatest sunshine, the layers, for there are more than one, may be very reflecting but so low that the waves are returned to earth very quiekly, Again these layers also absorb, so that the reflected wave may be weak. At night the conditions are more stable and the reflection takes place more regularly and, further, the layers are usually high so that strong signals can be obtained on the other side of the earth, The effect of frequency complicates matters further so that at certain times of the day and night we find certain bands of frequencies are received, while others are "dead."