SHORT WAVE RECEPTION

Evening Star, Issue 25224, 11 July 1944, Page 2

SHORT WAVE RECEPTION

A DIFFICULT SEASON FOR LISTENERS INFLUENCE OF THE SUN The present time is a difficult one -• for short wave broadcasters, and hence also for short wave listeners. Those living in North America—roughly north • of aline running diagonally across the Continent through Boston and Mexico City—are in a particularly poor situation'for receiving short wave broadcasts from Great Britain at this time, and have no doubt noticed that their reception is not as steady and reliable as it was, for example, four years ago. This is not to say that good reception cannot still be obtained; indeed, the 8.8. C services provide good reeeptifn in most parte of the world for • many hours of the day But the circuits are now more liable to disturbance and interruption than they -weresome much more so thau depending on the geographical location of the transmission path. ' Why, then, should short wave con-ary-either for i better or tor worse—with each succeeding year ilo find the answer to.this qua torn we must look to that prime agent in longdistance short wave communication, the eun For it is'the sun's radiations that Produce the "electrified" layers of air in the ionosphere, and so render euch communication at all possible. If these la vers-did not exist ihe waves would Sly J on travelling outwards into would not return to earth at all.* But as it is, the waves go obliquely upwards-till, high m the atmosnhere, they enter the ionosphere EJSTVb air- then acts upon the wave in such a Wjiat it is diverted" in its course ™ longer continuing on m a straight line, but bending r.ound in a wide sweep so Jiliat it eventually emerges from the layer, travelling in the direction of. the ground. Upon arrival there it duly performs the work for which it was produced—rfamely, that of actuating the'distant short wave listeners re- '' * ■ ""■ '■ ■■■ v When we consider that it is the sun s ravs that set up the electrification of the ionosphere air, then; it is very evident that its value will not remain constant, but will continually vary in accordance with the: amount ■of solar radiation affecting the.air. Tirus, at. any one point above, the earths surface,' we should espect.the amount of a electrification to vary from day to night and also from summer to winter. Such variations 'do indeed occurthough they are not of quite such, a simple nature as the foregoing might lead one to it is because of these variations that -we must always be changing the frequencies we use for short wave broadcasting. _ Another factor is the change in the activity of the sun itself, whereby those of its Tays that.produce the electrification vary in intensity oyer ; - a. period of about 11 years. Thus during one year they may be of minimum intensity,' and they then begin 1 - to increase, and go on doing so tor about five years. Then the intensity of the rays begins to decrease again, and after about six more years a period of minimum intensity is again reached; The'electrification of the air in the layers will, in a general way, follow tbe : variations in its producing agent, and thus we get a- maximum of electrification when the intensity of the lays is at a maximum, and vice versa. So, when the Tays are of "maximum intensity we can use higher frequencies (shorter wavelengths) than at any other time in the 11 years, but when thev are of minimum intensity iv.e are compelled to use lower -frequencies (longer wavelengths) than at any other time. SUtfSPOTS. The most easily observed visual evidence of the variations in the sun's activity is the variation in the niimiber and sifce of the sunspots which appear on its surface. These are regularly recorded, and it is found that the value of the sunspot "relative numbers '' does rise and fall over ,a period of about 11 years, which period is known as tho solar " cycle." The year 1933 was a year of minimum solar activity; Activity then increased and reached a peak in 1937. since which time it has been decreasing. If the.values for the past few years are' taken as a guide, there is.strong evidence that in 1944 a minimum will again be reached. And here it may be mentioned that the . astronomers have already secured reliable additional evidence that this year will be one of minimum activity.' As has been said, the intensity of tho rays that electrify the ionosphere air appear to follow the variations in the sunsipot numlbers fairly closely, and • heiice the highest frequencies of use for short wave, working rise and fall in the same way. A much lower frequency (longer wavelength) is necessary at'the minimum than at the maximum of the cycle. » _ ..... There is one further point; to observe, applying in some measure to all circuits, but again more particularly to those to North America. "Fairly frequently there occur disturbances in the ionosphere that cause the electrifi.cation further to decrease below its normal value for the time of day, season, and epoch of the sunspot cycle. At a • time when the electrification is already high, .we can afford to lose sonie of it during these disturbances, without unduly affecting our working frequencies. Thus, during the day the effect of the disturbances—in causing interruptions to tho short-wave transmissions—is less severe than during the night. At the • present epoch of the cycle the electrification of tho ionosphere air over, an all-dark path is in any event so low that a further slight reduction during a disturbance almost always results;in reception becoming difficult. Thus, for the time heing, we. must accept the fact that interruptions to the services are likely to be prevalent, though, of course, every .possible step .is taken to mitigate, their effects.