WIRELESS IN THE HOME.

Otago Witness, Issue 3595, 6 February 1923, Page 63

WIRELESS IN THE HOME.

WHY DO RADIO WAVES HUG THE EARTH? In the early day of radio, even after messages had been sent across the English Channel, physicists of authority mostly believed that the field of utility of the new art must be very limited because it was assumed that the Hertzian waves travel in straight lines, and therefore must quickly leave the earth on a tangential path, making distant communication impossible. To-day long-distance radio is a commonplace, but- physicists are by no means agreed as to how the radio waves get beyond the horizon lino and over the bulging earth and still are manifest at the earth’s surface. To cite the extreme case, radio signals have been heard at the Antipodes —half-way round the world. The waves that .carried these signals either passed directly through the globe itself or else hugged the surface of a hemisphere equivalent to a mountain four thousand miles high. No one speaking with authority appears to champion the idea of direct transit of the waves through the earth’s substance in a right line between sending and receiving' stations. Yet radio waves are believed to start- out from the transmitting antenna in straight lines, radiating in all directions as light radiates from an incandescent point. How does it happen that some of the waves are bent presently, so that they follow the curvature of the earth? There are two rival theories. The one that is most commonly cited is known as the, “Heaviside-Layer Theory.’’ It assumes that there is an upper layer of the atmosphere that- is ionized, perhaps by electrons from the sun, and from which the radio waves are reflected, as light is reflected from a mirror. According to this theory, radio waves do travel in straight lines, and attempt to escape tangentially from the earth, but are perpetually reflected back and thus made effectively to travel in a circuitous course, just as light may be—and in laboratory experiments sometimes is—made to travel circuitously by a series of reflectors. 'The alternative theory, called the “Gliding Wave Theory,” takes a totally different view. A prominent advocate of this theory is Dr Elihu Thomson, who gives in Popular Radio (new York) a brief' but emphatic statement of the explanation of the anomalous action of radio waves that appears to him most valid. Every thoughtful radio user should be interested in the explanation. Says Dr Thomson: “When Marconi brought out his system of wireless telegraphy about 1896, it was at first thought by most scientists or physicists of the time that it was a plain case of the sending out of waves of the Hertzian type, which Dr Heinrich Hertz had so ably investigated ten years before. If such were the case, the transmission was necessarily in straight lines from the oscillator ; necessarily, also, such waves could not follow the curvature of the earth’s surface, but they must leave the earth as if they were light beams —another case of electromagnetic waves moving in a ■ straight course. “There were some of us, however, who, taking into account the grounding at the base of the antenna, recognised the fact that the Marconi transmission was not made hy real Hertzian waves, hut on account ’ of the grounding, by half-Hert-zian waves only, and that the Marconi oscillator or antenna system was a halfoscillator only. From this it followed that the waves were in reality attached to and guided by the earth’s surface, and particularly by the sea surface, more conductive than the land. ‘ ‘lt followed that there would be electric currents in the sea and earth-surfaces accompanying these half-Hertzian waves, and magnetic fields overlying the currents in the space above the earth’s surface. “When it was announced by Marconi a few years later that he had received signals across the Atlantic ocean by Hying a kite, the cord of which constituted an antenna with the usual ground, many regarded him as something of a faker. At least, they believed that he was mistaken in his observations. Among these doubters weTe not a few of the leading scientific men and engineers of the day. It followed that if the waves were of true Hertzian type and were propagated in straight lines, they could not in any probability curve round and over a mountain of water nearly two hundred miles high, as they would have had to do if they crossed the Atlantic close enough to the earth’s surface to be detected. “As it was soon demonstrated that Marconi was right, and that the signals did come around the curve of the earth’s surlace, those scientists who failed to recognise (and some of them even yet seem so to fail) that there was a fundamental difference between the waves in their propagation and in their generation as regards true Hertzian waves, had been mistaken —and not Marconi. “Then a singular thing happened. “When confronted with the facts, this assumption pure and simple was made, which unfortunately lives and has character even to-day: that there was an electric mirror of ionized gas, or conducting gas, say fifty or sixty miles up in our atmosphere, the under surface of which was so definite as to reflect the waves without diffusing or mixing them up, and so send them around the earth by successive reflections from above. “I think that anyone who reflects for a moment on the requirements in such a case must predict that such an assumption is not only unnecessary, but that it strains the imagination too far, and plainly will not work. In order to work, it would have to be something like a metal surface, confined to a certain smooth regularity and of sifeh a nature that the wave fronts could not penetrate it to any considerable depth without being turned hack. It must he without swellings or wavy contour, and it must reflect the waves in such a way as not to interfere with those

that are more directly transmitted, and so keep the waves in phase. It would have to be, as it were, Nature’s gigantic whispering gallery for electric waves. “According to what for many years has been known as the ‘gliding wave’ tlieorv. there never was and never could have been any doubt of the waves used by Marconi (the half-Hertzian) following the rotundity of the earth’s surface. “Experience shows that transmission over the sea is far better than over the land. Direction-finding discloses that the direction of transmission favours the sea. Experience shows that when the land surface between two stations has been wetted by rains, great improvement in the transmission follows, to be again lost when the land surface is once more dried by evaporation. A good ground for the transmitting system or an ample condenser counterpoise is shown to favour greatly the launching of the waves. That the waves above the earth's surface tend to johow closely that surface, or may even be said to cling thereto, accords with tile results obtained from aerial antennae, ground antennae and loops or coils used as antennae. ‘There never has been anv occasion for the existence of the assumption of an upper conducting layer of such a nature as to reflect the waves without confusing th ™ or diffusing them, and it is regrettable that such an assumption, having once received the sanction of great names, thereby continues to have a support and recognition which should never have been given and was never needed.”