Diffraction from the Ionosphere and the Fading of Radio Waves

WHEN a wireless wave is received partly or wholly by way of the ionosphere, it is observed to fade. Much of this fading is due to interference effects between waves which have travelled by different paths. Such waves may include the ground wave, singly or multiply reflected ionospheric waves, waves reflected from different ionospheric regions, or the two magneto-ionic components produced by the presence of the earth‘s magnetie field. It is, however, possible to isolate and observe only one downcoming wave from the ionosphere, for example, by the use of a pulse sender and a circularly polarized receiving aerial or by the use of a special aerial to suppress the ground wave under conditions when the absorption of the downcoming Wave is large enough to make second reflexions unimportant. When only one downcoming wave is observed in this way, it is still found to fade at a speed which varies from time to time. Although few detailed observations of this kind of fading have been reported, workers in the subject will probably accept the general statement that the speed of fading is roughly proportionai to the frequency of the wave and inversely proportional to the distance of the transmitter 1 .