Heaviside Layer Theory

Radio Record, Volume III, Issue 8, 6 September 1929, Page 10

Heaviside Layer Theory

‘Research by Professor E. V. Appleton.

T’ is announced that Professor BH. V. Appleton, F.R.S8., has been awarded the Morris "Liebmann _-memorial prize for 1929 by the American: Iustitute of Radio Engineers, for: making the most important contribution to radio science: during the last twelve months. : ‘Professor Appleton occupies the chair of Physics at King’s College, and has for the last five years been associated with the Radio Research Board. in investigating the nature of the Heaviside Layer and its effect on long-. distance transmission. . . In the early days of wireless if was thought that signal waves could not. travel freely through the ether except in a straight line. Other scientists held that, for some peculiar reason,’ the speed of propagation increased considerably as the waves travelled upwards, with the result that they were automatically bent downwards again towards the earth’s surface. — . The Bending Effect. T is now definitely known that they" " ‘are prevented from escaping into interstellar space. by) the action of the so-called Heavisid2 Layer, which is an area where the rarefied atmosphere is strongly ionized. — An ionized gas, containing a large proportion of free electrons, acts -upon the waves in the same way as a metal-. lie conductor, In other words it presents’ an almost impenetrable barrier in the outward cirection, at the same time it. reflects the waves back to the earth, more or less as a mirror reflects light. 7 The reaction of the ionized layer to the impact of a wireless wave may be briefly explained as follows: Owing to the extreme tenuity of the air in these regions, the free path of the active electrons is comparatively long. In fact, it

may extend over many alternations of the electric force o. the field of radiation, ie, of the wireless wave. _ Such electrons will therefor be -vibrated or oscillated by the incoming wave, without any loss or dissipation of energy, thus acting to retransmit the waves back in the direction of least resistance, ie., downwards towards the earth, According to o ; authority the action of free electrons should bé regarded as one tending to increase the velo-

city of the upper-part of the wave as it enters the Heaviside Layer, relatively to the lower part of the wave. This results in bénding the whole wave forward, until it changes direction and emerges downwards, in much the same way as a ray of light is refracted or pent when passing from one medium to another of different density. Whether the process is actually one of reflection or refraction is of little consequence. We know br observation that the result is. to throw back the wave to earth, and so prevent its escape into outer. space.

Professor Appleton has devoted considerable"time and ingenuity to the problem of "measuring the effective height of the Heaviside Layer above ground. "Experiments with 6 BM. One method that has been successfully applied depends upon: an interference effect which occurs between the upper or space wave and the lower or earth-bound wave, owing to the differ-~ ent distances. traversed by- the two. ° ‘Suppose, for instance, that station T is transmitting to a receiver at R, The earth-bound or direct wave from T to R obviously travels a shorter distance than the space wave T L R received after reflection from the. Heaviside Layer. : oo For a given working wave-length the difference between the lengths of the two paths may correspond to a certain number of complete waves, sa that both arrive at the receiver 10 phase. In actual practice this is seldom the ease. There is usually a fraction of a wavelength over, and this gives rise’ ‘to out-of-phase conditions at the receiver. Sometimes the two waves add together and sometimes they oppose each other, with the result that, the signals fade periodically. Now if the working wavelength is deliberatély and periodically varied between an upper and lower limit of frequency at the transmitter T, a "beat" effect is produced in the receiver R which depends solely upon (a) the difference between e two wav "2ngth limits used, and (b) the difference in distance between th path of the space wave and that of the earth-bound wave. The "beat" frequency can be ascertained by direct observations, whilst the difference_hbetween the two wavelengths transmitted is known, as is the listance T R, ie, the direct path of the earth-bound wave. By equating these known factors the length of the path T L R is first calculated, and the height of the Heaviside Layer is then determined by a simple process of triangulation. , Day and Night Altitudes. ‘A series of eayeriments on these lines has been carried out between the Bournemout B.B.C. Station and a receiver at Oxford, corresponding to a distance T R between transmitter and receiver of approximately 160 kilometres. It was found thai the difference between the two wave-paths T R and T L R amounted to 80 kilometres at night, corresponding to a height above ground of 85 kilometres for the Heaviside Layer. During the daytime, owing to the ionizing effect of the sun’s rays, the layer expands downwards until it reaches a level of between forty and fifty kilometres above the earth’s surface.