History of wireless telegraphy and broadcasting in Australia/Topical/Publications/Australasian Radio World/Issues/1941 07

P.03 - Editorial Notes

Editorial Notes . . .

Nil

P.03 - Contents Banner

The Australasian Radio World

Incorporating the

All-Wave All-World DX News

Vol. 6 - JULY, 1941 - No. 2

P.03 - Contents

CONTENTS:

CONSTRUCTIONAL —

"Vibra 4" for Countryman . . . . 5

5-Band Modulated Oscillator . . . . 11

Thirteen-watt Amplifier . . . . 15

Power Oscillator . . . . 29

TECHNICAL —

How to Use an Oscillator . . . . 32

Modern Sound for Modern Hotels . . . . 33

Speedy Query Service . . . . 34

SHORT-WAVE SECTIONS —

Conditions Affecting Reception . . . . 20

Short-wave Review . . . . 23

Loggings of the Month . . . . 24

P.03 - Publication Notes

The "Australasian Radio World" is published monthly by A. G. Hull. Editorial offices, 117 Reservoir Street, Sydney, N.S.W. Telephone MA2455.

Subscription rates: 1/- per copy, 10/6 per year (12 issues) post free to Australia and New Zealand.

Printed by Bridge Printery Pty. Ltd., 117 Reservoir Street, Sydney, N.S.W., for the proprietor of the "Australasian Radio World," 117 Reservoir St., Sydney (Footnote P.36)

P.20 - Shortwave Sections - Conditions Affecting Reception

Conditions Afecting Short-Wave Reception AN AUTHORITATIVE ARTICLE FRO M THE ENGINEERS OF THE B.B.C. W HETHER or not a listener in any part of the world gets good reception of the B.B.C. short-wave programme is dependent upon conditions prevailing in the ionosphere and upon the solar cycle. An explanation of this, in language, which can be understood by the ordinary listener, is given in this article, specially prepared by the B.B.C.'s Engineering Division. The frequencies used for short-wave broadcasting have to be chosen with strict regard to the conditions prevailing in the inosphere over the particular route on which they are to be used. It is in the ionosphere — mainly in that part of it which lies about 180 miles above the earth and which is known as the F layer — that the waves are refracted or "bent round," so that they return to earth again at a distant point. And the behaviour of the wave in the ionosphere will depend upon the conditions existing there at the time, as well as upon the frequency used. • Briefly, it may be said that if the frequency is too high to suit the prevailing ionosphere conditions, the wave will pass clean through the ionised layers, and will not be bent back to earth at all. On the other hand, if the frequency used is too low, the loss of energy in the layers will be so high that a good signal will reach the receiving end of the circuit only if enormous power is used at the transmitting end. Hence it is most important that the frequency used be such that the wave will be properly dealt with by the ionised layers, i.e., that it will be well refracted and returned to earth with the minimum loss of energy in the layers. The frequency which is best suited to the prevailing ionosphere conditions is called the "optimum" frequency. Conditions in the ionosphere are, however, in a constantly changing state. At any one moment they are diferent for diferent latitudes, while at any one point above the world's surface they are subject to three changes of a periodic nature. These are, firstly, a change in conditions from day to night; secondly, a change from season to season; and, thirdly, a change which takes place over a relatively long period of time. The reason for that the condition electrification — of these changes is of ionisation — or the air in the

layers is brought about by the action of the sun. The layers are, in fact, produced mainly by the sun's radiation of ultra violet light. Hence, conditions prevailing at any particular point in the ionosphere will depend, in the main, upon the position of that point relative to the sun, and also upon the state of activity of the sun. It is because of these changes that the frequencies used by a short-wave broadcasting station have to be changed from time to time. Over a long transmission path, where the wave must pass from earth to ionosphere and back again several times, as it were in a series of "hops," the "optimum" frequency will really be the frequency which best suits ionosphere conditions at a number of widely separated points. This must necessarily be something of a compromise, as it is rare that conditions at any two consecutive "hops" are the same. Daily and Seasonal Changes As far as the daily changes in ionosphere conditions are concerned, they are such as to render necessary the use of low frequencies during the night and relatively high frequencies during the day. In summer the degree of change as between day and night is much less than in winter, the consequently the difference between daylight and darkness frequencies is least during the summer. Nevertheless, it is still necessary to use lower frequencies during the night than during the day. The highest frequencies which are usable during the summer become most suitable just before sunset. During last summer the highest frequency suitable for long distance working — by normal methods —in Lat. 50 degrees N. was about 22 Mc/s just before sunset, and about 12 Mc/s, just before sunrise — when the ionisation of the layers is at its lowest. It is in winter that the change of conditions as between day and night is greatest, and, as a result, the working frequencies for winter nights are lower than at any other time of the year, while the winter daytime frequencies are higher. The highest usable frequencies in winter become most suitable about noon. During November the highest frequency suitable for long distance working in Lat. 50 degrees N. was about 32 Mc/s at noon, and about 9 Mc/s just before sunrise. In December the night-time

working frequencies were even lower than in November — about 6 Mc/s. Summarising the daily and seasonal change in working frequencies, we have a change from high to low frequencies from day to night, a change from low to still lower frequencies from summer night to winter night, and a change from high to still higher frequencies from summer day to winter day. The Solar Cycle In addition to the above changes — which are brought about by the varying position of the sun with regard to any particular point in the ionosphere — there is a further change in conditions brought about by a variation of activity within the sun itself. This is such that periods of maximum activity are reached about every eleven years, with periods of minimum activity about half-way between the maxima. The rise and fall in activity is not quite regular, though it is possible to estimate the degree of activity which will exist some time ahead with a fair amount of accuracy. The degree of solar activity is evidenced by the number of sunspots which appear on the sun's surface, and also by the size of the areas of flocculi, or clouds of gases, which can be observed around the sun. The point is that the amount of ionising radiation emitted by the sun varies according to its general degree of activity, and therefore it decreases considerably from the maximum to the minimum period of activity in the eleven year cycle. It is this radiation which produces the layers of the ionosphere, and so the amount of ionisation existing in the layers rises and falls in sympathy with the elevenyear solar cycle. During the year 1937 a period of maximum activity was reached and since then the general level of ionisation in the layers has been falling. The next period of minimum activity is expected to occur about 1945, so that ionisation is expected to occur about 1945, so that ionisation is expected to continue to fall towards that year. When ionisation in the layers is low we are obliged to use lower frequencies for short-wave communication, so it appears that we shall have to make more use of the lower broadcasting frequencies as year succeeds year until 1945. We must remember, however, that the decrease in work-

ing frequency is not a steady one and that, superimposed upon the gradual fall of ionisation, we have the seasonal changes already mentioned. Actually, there is a far bigger decrease in working frequency as between maximum and minimum periods of activity during the winter day than at any other time, a smaller one during the summer day and summer night, and a still smaller decrease during the winter night. During last summer the highest daytime frequency suitable for longdistance working in Lat. 50 degrees N. was about 12 per cent. lower than it was in the summer of 1937, while the highest night-time frequency was about 14 per cent. lower. How the Changes Afect the Frequencies Used So we have now passed the midwinter of the third year after the solar maximum, and the use of lower frequencies will have been noted by listeners. What has been said about the changes in ionisation and the consequent change in working frequency applies mainly to one particular point in the ionosphere. It must be remembered, however, that long transmission routes often pass from day into night or from night into day, and sometimes from midwinter into midsummer; so that ionosphere conditions will vary widely over the route, and the frequency used must be one that will suit the worst conditions encountered over the whole transmission route. In the case of very long routes conditions vary so widely at certain times of day and year that it sometimes becomes impossible to communicate over the route, for any frequency which will escape severe loss at one end of the route will penetrate the ionosphere of th other. Fortunately, this does not apply to many routes, nor at all times and seasons to the others. In fact, such a variety of conditions are encountered over a number of transmission routes that each one has to be separately examined for all seasons and times of day. Conditions of the Empire Routes We may now consider a few of the routes over which the B.B.C. regularly ,transmits, and see how the frequencies are likely to be afected during the coming summer in the northern hemisphere. Let us first take the route to India. The period during which it is most desired to serve India is during the evening hours in that country, and this will be from about 1.00 a.m. to 6.15 p.m. G.M.T. The transmission route will lie entirely within the summer hemisphere, and, as darkness falls in India several hours before it does in England, we shall be transmitting, for some part of the time, from day into night. We can therefore begin the transmission on summer day frequencies — 17 Mc/s au 15 Mc, s — and, in this connection, we may mention that the frequency of 21 lvic/s, which was used with success during former summers, has now become too high to provicL a reliable service, although it may again become serviceable during the autumn. Soon after darkness falls in India there will be a gradual fall in ionisation, and lower frequencies will become necessary. It is expected that between about 3 p.m. and about '7 p.m. ; G.M.T., the optimum frequency will fall from 17 Mc/s to 12 Mc/s, so that we shall probably finish the period on frequencies of 12 Mc/s and 1 Mc/s. Compare this with the recent midwinter period, when, between about noon and 7.0 p.m. G.M.T., the optimum frequency fell from 21 Mc/s to about 6 Mc/s. This was met by the successive introduction of lower frequencies during the English afternoon. Similar considerations apply to the routes to the Far East and to Malaya. In the case of the route to South Africa diferent conditions exist. In the first place we shall have summer conditions at the English end of the circuit and winter conditions at the South African end. A few months ago this position was reversed. Secondly, there is very little time difference over the route, so that, generally speaking, we transmit from day into day or night into night. Both these facts help to account for the fact that there is relatively little change in the optimum frequency from winter to summer. The working frequency usually has to be a compromise that will suit both summer and winter conditions, and its choice is rendered easier by the fact that the time difference over the route is small. Up to the present, the higher frequencies have been found to suit the summer and winter conditions during the day, and this applies also to the equinoctial periods. This year, however, owing to the fall in ionisation caused by the efect of the solar cycle, it is possible that conditions at the English end of the circuit may be such as to render 21 Mc/s too high a frequency. A good dayti me service in Africa, however, should continue to be given by 17 Mc/s. At night conditions are likely to be somewhat dificult, for then, during the northern summer, the extreme south of South Africa requires a lower frequency than is suited to more northerly areas. It is expected, therefore, that, in the case of the extreme south, the optimum frequency will fall from 17 Mc/s to 9 Mc/s — possibly even lower — between 4 p.m. and 10 p.m. G.M.T. In the case of the more northerly parts of Africa 1 Mc/s should still be a serviceable frequency at 10 p.m. G.M.T.

"American-style Communications" Receiver

To obtain best short-wave results it is deÉrable to be able to switch from cne wave-band to another quickly, os with the "American-style Com munications" receiver which was described in our August, 1940, issue. It had bond-switching for five bands.