Communication E V a

In the directional apparatus so far described the radiation or absorption is symmetrical about the vertical axis of the apparatus. This fact gives rise to an ambiguity of 18o° in azimuth; for in stance, after finding the direction of a station the operator may still be unable to say whether it is in front or behind him unless assisted by geographical or other considerations. This ambiguity may, however, be removed electrically by adding an ordinary straight antenna and the necessary tuning equipment to the exist ing direction finding apparatus. It can be shown that, by adjust ing the phase and magnitude of the oscillatory current produced in this single wire, the radiation or absorption, as the case may be, in one direction can be annulled and in the other increased. In other words the direction finder is made unidirectional.

Long Distance Communication.

Wireless communication across great distances has progressed greatly during the past few years. An important step was taken in 1918 by the United States army in erecting the Lafayette station at Crois d'Hins near Bordeaux for transatlantic communication. This station was more than double the strength of any predecessor and achieved world ranges with a wave-length greater than 20,000 metres; the original plant comprised two Federal-Poulsen arcs rated at r,000 kilo watts and supplied with current at goo volts. Somewhat smaller equipments were installed at Nauen near Berlin, at St. Assise near Paris, at Rocky Point near New York, and also in many other countries, during the succeeding four years, nearly all of them using high frequency alternators of German, French or American design. In 1922 to 1923 the British Government com pleted arrangements for the establishment of an Empire wire less service under which the Post Office would erect at Rugby a station capable of direct communication with all parts of the Empire, and the Dominions would erect corresponding stations. Under these arrangements the Marconi Company in 1923 planned to handle extra-Empire communications from a group of even larger stations in Wiltshire. A little later the Government plans were modified to permit of Rugby conducting this work.

Meanwhile a few American amateurs, using waves shorter than 30o metres, succeeded in communicating across the Atlantic with the expenditure of very small power in December 1921; and by December 1922, American, British and French amateurs suc ceeded by hundreds in similar transmissions. In the early months of 1923 amateurs telephoned from New York to California by aid of small stations using loo metre waves, and it was becoming clear that waves less than ioo metres in length were likely to be useful in practice. This was unexpected because the very thorough pioneer work of C. S. Franklin in 192o and 1921, published in 1922, seemed to show that waves of 15 metres length, and even of loo metres, were only suitable for distances less than 200 miles. But the surprising results obtained by the amateurs com

pelled professional attention, and early in 1923 the Radio Cor poration of America erected a short-wave equipment in Maine, and the Marconi Company independently installed a similar plant at Poldhu, for experimental work. The first commercial long distance message by short wave was on September II, 1923, when a ringside account of a prize fight was sent from Maine to Buenos Aires. By the end of the year experimenters were at work all over the world to such good effect that in the spring of 1924 the Radio Corporation erected five more short-wave commercial trans mitters. In July 1924 the British Government ordered from the Marconi Company a number of short-wave stations for the pur pose of completing the Empire scheme of communications. These stations were to be provided with reflectors of the type developed by C. S. Franklin in order to direct the radiation mainly towards corresponding stations to be erected in Canada, Australia, South Africa and India. All these Government stations were in opera tion by 1928 and are considered to be the most efficient short wave stations extant.

The use of reflectors at both the transmitting and receiving stations ensures that the signals, under favourable conditions, are ro to 15 times as strong as they otherwise would be. In all cases, with or without reflectors, it is necessary to provide that each station shall be capable of operation on one of two or three wave-lengths, for across long distances waves shorter than, say, 3o metres are required for daylight transmission, and longer than 3o metres for night transmission. Four years' experience with short-wave transoceanic telegraphy and telephony has shown that it may suffer erratic and lengthy periods of fading which cause loss of signals or distortion of speech and, therefore, for such work as transatlantic telephony the long-wave plant is gen erally more trustworthy. Best of all is the collaboration of long wave and short-wave telephony now being practised between America and Britain (and through Rugby with large areas of Europe). On the other hand short-waves have two great advan tages over long waves—they are capable of much greater tele graphic speeds and the plant they require costs less to erect and operate. The relation between short- and long-wave systems is analogous to that between motor car and railway transport ; the car is for many purposes better than the railway, but it is more subject to interruption and accident; for a long time to come progressive communities will require both forms of service.