Radio age research, manufacturing, communications, broadcasting, television (1941)

groups, having a short-lived, but fixt'd, solar location, are synchro- nized with the rotation of the sun. These two distinct and separate types of short wave interruptions are well known to radio traffic men as "drop-outs," and "magnetic blankets." When short wave radio signals transmitted from the earth's sur- face encounter the under side of the ionosphere, the peculiarities of pen- etration, reflection and absorption are again in evidence. The wave Buffers repulsion and bending when it enters the ionosphere. The bend- ing reaches a maximum for the low frequency long waves. It becomes progressively less to the point of penetration as the wave length shortens. Microwaves (extremely short) consequently are useless for direct long distance communication, since they are not turned back by the "radio ceiling." In crossing the Atlantic, a short wave signal is bounced between the radio ceiling and the sea. The num- ber of hops required varies with the distance between terminals, with wave length and with radio ceiling height. The signal strength varies with the turbulence of the ceiling and its absorbing properties. Solar radiations account for nearly all these variations, which simultane- ously are reflected in terrestrial magnetic activity. A flow of ions, whether in a wire, or in the thin upper atmosphere, is an electric current. The upper at- mospheric currents produce meas- urable eff'ects in the earth's mag- netism. These effects are concen- trated at the magnetic poles. Mag- netic reactions between the agitated magnetic poles and the upper at- mospheric currents cause the radio ceiling to be more turbulent, hence less useful as a reflecting medium over the poles than in lower lati- tudes. During a magnetic storm, a signal path over the magnetic pole may be opaque for radio communi- cation. The blanketing effect in- creases as the circuit path ap- proache.s the magnetic pole. It has consequently been considered sound practice not to lay out a circuit path closer than .30° to the magnetic pole. The magnetic storm is accompa- nied by an after-effect, which like- wise is more pronounced and of longer duration near the poles. The lower latitudes, on the other hand, are affected only by the most severe of storms, and for relatively short periods. The after-effect within 30° of the magnetic pole may last four or five days. It may be more ditli- cult to get a signal through, at a high latitude, three days after a storm has passed than at low lati- tude during the height of the storm. Considerable study and research has been applied to the causes of "drop-outs" by many organizations. Noteworthy contributions have been made by Dr. J. H. Dellinger of the National Bureau of Standards, by the Carnegie Institution of Washington, which has sponsored annual conferences for ionospheric study, and by Communications Re- search, RCA Laboratories. The gen- erally accepted theory is that the solar flare sets up radiations that pierce the normal layers of the earth's ceiling. These penetrations establish temporary absorption re- gions in the normal path of the radio wave. The temporary absorp- tion may produce a complete, or partial, drop-out depending upon the violence of the solar eruption. When the flare subsides, the absorb- ing screen dissipates, and normal conditions return. The entire drop- out may last from two to thirty minutes, seldom longer. Agreement is not so general upon the "magnetic blanket" type of dis- turbance. It is probable that a sim- ilar process is involved, over a longer time, in which the source is the slower radiation from a sun spot group passing the center of the sun's disc. The longer interval would permit the above noted re- actions from the earth's magnetic poles. Fading was one of the first ob- served characteristics of short wave signals. The range varied from a slow drift to the extremely rapid type called "flutter fading." As far back as 1925, a relationship was noted between fading and terres- trial magnetism. Poor circuit con- ditions soon became synonomous with "magnetics." At this early date, the radio ceiling was just be- ginning to be explored and studied. There soon followed the develop- ment of radio echo technique, per- mitting heights of distinct ionized »>• - A PHOTOGRAPH OF THE GREAT SUN SPOT OF JULY, 1905, TAKEN FROM THE YERKES OBSERVATORY AT WILLIAM BAY, WISCONSIN. layers to be measured. The occur- rence of a sun spot maximum in 1928 further interrelated fading, circuit interruptions and terrestrial magnetism. The tools were evi- dently available for a considerable degree of success in "radio weath- er" forecasting. Magnetics could not be fully understood, however, until an extended correlation could be undertaken on typical radio cir- cuits all over the earth. Through the agency of the Re- search Department of RCA Com- munications Inc., this study was undertaken on its world wide com- mercial radiotelegraph circuits. It involved the development of a meas- uring unit for magnetic variability, as well as long period signal record- ings on many great circle paths. A circuit disturbance rating num- ber system supplemented signal re- cordings to make the project more general. The investigation indicated that, on a given circuit, a decrease in the signal was proportional to an in- crease in magnetic variability. It also became evident that general radio circuit conditions could be monitored by a magnetic observa- tory located near the mid-point of the circuit. The mid-points of trans- Atlantic circuits lie in mid-ocean. The practical compromise was therefore selection of an observa- tory nearest the terminals of the most disturbed high latitude cir- cuits. The U. S. Coast & Geodetic Survey Magnetic Observatory at Cheltenham, Maryland was the se- lection. Daily "magnetograms" from Cheltenham are analyzed, and [RADIO AGE 19]