The fading of radio waves of medium and high frequencies

The fading of radio waves of medium and high frequencies returned from the ionosphere is analysed. The experimental data on high frequencies were recorded by the author at normal incidence using a pulse transmitter, and, when necessary, a circularly polarized receiving aerial. The medium-frequency records were made by the B.B.C. using continuous-wave transmissions from broadcasting stations at different distances up to 1 500 kilometres.A review is given of some theories suggested to explain the production of fading in terms of an ionosphere with varying irregularities in the horizontal plane. The theoretical nature of the fading curve, both with regard to the distribution of amplitude and the variation with time, is indicated, and is compared with experimental results.It is shown that all fading curves can be explained in terms of a wave which is the sum of a steady, specularly reflected component, and a random component with a Rayleigh distribution of amplitude. The relative magnitudes of these two components have been determined for a series of different conditions, and the results are summarized. It is also shown that the rate of variation of the amplitude of the received signal is approximately, but not exactly, in accord with the theories, and from this rate of change it is possible to calculate the “effective velocity” of the irregularities in the ionosphere. If the fading is caused by irregular “turbulent” motions of the irregularities, the “effective velocity” is the r.m.s. velocity of these motions; if it is due to a steady drift of the irregular ionosphere as a whole with a velocity vw, then the “effective velocity” is approximately equal to vw/14. The “effective velocities” deduced from 122 records made at vertical incidence, and 55 made at oblique incidence, lie between 0.3 and 8.0 m/sec and have an average value of 1.9 m/sec.