Show OCR
.'NOV. 10, 1932\SCIENCEBY DAVID DIETZ-I 3Radio Used for Long Trans-Oceanic Jumps in World'Telephone, Systems.'r | TECHNICAL difficulties in tyieway of world-wide telephony arise not only from the fact that long distances must be covered, but from the fact that many of these distances consist of seas or undeveloped land. . mThis fact is pointed out by Ban-' croft Gherhardi, vice-* esident and chief engineer of the American Telephone and Telegraph Company, and Frank B. Jewett, president ofthe Bell telephone laboratories.‘‘For years prior to establishment of the first intercontinental circuit.commercial service was given oncontinental telephone networks overcomparable distances, for example, in North America up to about six thousand miles,” they say.“The technique oi transmission over long continental distances,4 however, includes as one fundamen-ta. element the use of intermedate amplification at frequ it intervals of from forty-five to 240 miles, depending upon the electrical characteristics of the circuit.”This means, it will be observed, that in the line reaching from New York to San Fffincisco, there are at intervals amplifying apparatus which step up the diminishing voice currents.Obviously, this is not possiblewhen two continents are linked.All of the existing intercontinental circuits make use of radio for~the long trans-oceanic jumps. Thefirst technical problem to be met,therefore, are the problems of radio telephony.RmayIso.men-Iwayspinalphoidntes-id of en-rvousare ercu-dssue pref-: theones,the•hillisRange of WavesADIO waves, as all radio fans know, cover a much larger range than those used for ordinary broadcasting.The complete range runs from waves whose length is measured in fractions of a meter to waves more than 20,000 meters long.The frequency of a wave varies indirectly as the length, that is the shortest waves have the highest frequencies and the longest waves th* lowest frequencies.Only two portions of the radio range or spectrum are suitable for intercontinental telephony, according to GJierardi and Jewett.One, in the long wave range, extends from 20,000 meters down to 3,000 meters, or from about 15 kilocycles to about 100 kilocycles.The other, in the short wave range, extends from 50 meters to 13 meters, or from about 6,000 kilocycles to 23,000 kilocycles.“The transmission of these two wave ranges exhibits interesting differences in characteristics,” the two experts say. Y“For the short wave lengths the transmission is frequently referred to as being in the form of ‘sky waves.’ This Is for the reason that mils a^ ^ntermedfate distances the waves and 1 Pra?ticalIy may disappear near the surface of the ground, but reappear at greater distances.They appear to have been carried around the curvature of the earth’s surface by refraction or reflection from ionized atmospheric layers.“While the action in the longwave range at great distance appears to be also conditioned partly by the ionized layers, the fi^ld at the surface of the earth falls off continuously as the distance from the transmitter is increased.”MPd) of ter-pre-heirnpnt t offirst^ausethoseande be-busi-lovedain’s curi-^ for ;hink ds of soVery Short WavesGHERARDI and Jewett also say that interesting results are being obtained in the use for communication purposes of very short radio waves having frequencies above 30,000 kilocycles.Such waves have lengths of lessel as than 13 meters* %“The w'ork done to date indicatesthat these frequencies are not sufficiently deflected from their paths by the atmosphere to follow the earth’s curvature,” they say.“This characteristic appears toprevent the use of these very short radio waves for direct transmission over long distances and limits direct transmission to distances so short that the earth’s curvature is not a large factor.“Where the route Is over land, it is possible that such rays may find practical use in forming links in intercontinental circuits, particularly where the topography of the country affords advantageous elevated locations for intermediate repeater points.“Too little now is known of these very short waves to make their discussion other than speculative. ^ “An important requirement for radio for overseas telephone circuits is the avoidance of overhearing.“A number of these circuits now in operation, including the trans-Atlantic group, are equipped with privacy arrangements which so modify the frequency disposition of the voice weaves as to prevent overhearing of the conversations by Other radio stations not equipped with similar arrangements. ' “Experimentshave been made with more elaborate arrangements for obtaining an even higher degree of privacy than that now provided.”rhereieathitage3 me any rs oflounttnow and at a gen-and”hild-days» badihool ; de-7 be-trea-r all, irime man can th attingoureatermess.mrthnowlads
Your clip comment has been saved and pending for approval before it displayed.