IT IS BEING DONE!

Radio Record, Volume IV, Issue 15, 24 October 1930, Page 1

IT IS BEING DONE!

asserts

W. M.

DAWSON

AMIREL, MIW.T

Replying to the statement that Empire Broadcasting is Impossible

N the daily Press of October 11 there appeared a cable item to the effect that the Imperial Conference Communications Committee had discussed the question (raised by the ~} New Zealand. delegation) of Britain establishing an Empire broadcasting station near London. Opinions were expressed that this was practically impossible at present, for financial and technical reasons, and it was stated that the cast of a station capable af broadcasting reliably over the whole Empire would ‘be enormously expensive. The colonies were quoted as having only three thousand listeners. This latter state-

ment is obviously far from accurate, even if intended to apply only. to short-wave enthusiasts. The New Zealand license figures are neatly 60,000, Australia some 300,000, Canada. some 350, 000, India about 6000, ‘and South Africa some 25, 000. The number of, present listeners, therefore, who would probably. be interested in Empire broadcasts, either received direct or rebroadcast,’ runs into large figures. Technical and financial -aspects arc to some extent inter-dependent. I. is, of course, realised that a regular broadcast service ‘must provide greater probability of successful . reception ‘than is necessary for transmissions of an experimental nature. ‘Generally speaking, greater

reliability is taken to indicate greater radiated _ energy with soaring capital outlay and upkeep expenses. .This is not necessarily so. Also, readers may be prone to base their criticisms of short-wave tele-

phony on the results obtained by the average short-wave _listener with compara-

‘tively simple receiving apparatus, and listen‘ing to experimental broadcasting stations which radiate their energy more or less uniformly in all directions. It is very unlikely that any attempt would be: made to run an Empire broadcasting scheme on these lines. A little study of a terrestrial globe reminds us of time differences existing between different parts of the British Empire, and it will be further apparent that these countries may be classed into two,:or at the most three, groups as far as geographical great circle bearing from Britain is concerned. ‘ Canada lies away more or less due westward, and a direct line joining Britain with New Zealand conveniently passes through India and Australia on the way. Egypt and South Africa lie to the southward of this line, but a third line drawn to South Africa passes through Egypt. Considered in conjunction with time differences, this at once suggests the use of directional beam transmitting antennas, one oriented to transmit directly toward Canada, the second directed at New Zealand, and possibly a third facing South Africa. Normally only one aerial would be energised at any one time. Such directive aerials are thoroughly practicable for the short wavelengths that would be used and result in very considerable conservation of energy.. The magnitude of this saving is not generally realised among listeners. A well-designed short-wave beam antenna will radiate practically all of its energy within a 15 degree angle, and has an efficiency multiplying factor of approximately 200 when measured in terms of field strength at a distant receiving point with constant output

In this article Mr. W. M. Dawson, chief technical enengineer of the N.Z. branch of one of the world’s largest radio organisations, questions the attitude adopted by the Imperial Conference in respect of Empire broadcasting.

‘trom the transmitter, plain language, this means that an effective beam transmitting antenna one kilowatt of radiated energy is as effective as 200 kilowatts from an ordinary nondirectional antenna. ven then, this is not the whole story, for we may apply the same principles to the receiving. antenna with practically the same increase in. effi-ciency-i.e., 200 times. Consequently, with effective beam antennas at both transmitting and receiving. ends, the over-all efficiency increases .something like 200 multiplied by 200, or 40,000 times! Again in simplest language, this means that one kilowatt radiated from and into beam antennas is about effective as 40,000 kilowatts just "snlattered" In addition, they eliminate one of the causes of periodic fadingthe out-of-phase signal arriving from "over the other end" of the world-the long way round. This periodie fading is at present one of the main troubles associated with amateur short-wave reception, and unfortunately the simple types of shortwave receiver favoured by the average short-wave listener are largely to blame for the accentuation of this surging. Special receivers are in use at the present time which give good, steady signals when the reception of the same station on an ordinary regenerative set would be surging in and out so badly that intelligibility would be reduced almost to zero. Furthermore, by what is known as multipoint reception, this fading trouble may be still further reduced. Broadly speaking, this system consists in the erection of two or more receiving sta-

tions at points some distance apart, the basic idea being that when . the signal at one receiver is fading that. at the other station is steady, or yice | versa. : By "mixing" the outputs- from’ these two (or more) receivers, the fading at one receiving location is largely balanced out by the other station, and practically steady Yreception is obtained. : Such a receiving installation is, of course, not practicable for the individual listener, but it is more than likely. that any scheme for Empire broadeasting would include the local broadeasting stations as an essential. link, and that the special short-wave transmissions would be rebroadcast on normal wavelengths. The "Hmpire"’ broadcasts could thus with advantage form an integral part of our ordinary broadcast service-possibly form a special session. Any idea of a regular 24-hour reception service is as unnecessary as it is impractical. BHven without beam transmission and multipoint reception, but using a suitable type of receiver, a long series of tests have proved that short-wave reception from 5SW (Chelmsford, England) was sufficiently steady for rebroadcasting purposes in New Zealand over long periods, and this consistency could be further improved by the methods outlined above. Numerous excellent rebroadcasts of PCJ-notably by station. 4ZL, Dunedin -have gone a long way toward showing that rapid periodic fading is not a sine qua non of short-wave rebroadeasts. This station (4ZL) some time ago regularly rebroadcast PCJ every Saturday afternoon without a break for months, and the excellence of these

transmissions .was a byword in the Southern city. The effectiveness of reception is largely a function of seasonal and hourly time against -wavelength ‘used. . Jf the transmitting wavelength is fixed, the daily period of best reception varies with the season, and there should be no insuperable bar to making slight adjustments to the time of the "Hmpire session" if it proved impracticable to alter the transmission wavelength at seasonable intervals. A point worthy of note is that none of the countries to be serzod lie within the initial skip distance of the wavelength ranges, that would be used. The writer’s. idea has not been to attempt a detailed exposition of the factors involved, but to point out some of the lesser appreciated points. The'scheme is about as feasible as a commercial telephony system (already. operating between Hngland and Australia). . Surely, if private enterprise can initiate and. maintain for years without fee.or tangible remuneration such stations as the Philipsowned PCJ and PHI, the financial and technical objections to an "Empire scheme" are bogeys, ‘ yather: than actualities, PCJ Short-waver. S the. fame of PCJ is world-wide, and as it was the pioneer regular short-wave experimental station, some brief particulars of it will no doubt interest "Record" readers. PCJ came into being in March, 1927, for tthe specific purpose of studying, experimentally, the problems associated with and peculiar to short-wave telephony transmission. It had previously been ‘demonstrated that the band of wavelengths lying below 100 metres made possible phenomenal distance telegraphic communication with very small output powers-previous standards were revolutionised. The part played by world amateurs in this development is well: known and appreciated. Short-wave telephony, however, had proved until that time practically im‘possible; still, if telephony could be made practicable on these short waves, long-distance broadcasting would change from a dream into a reality. The Philips Laboratories, at BHindhoven (Holland), with their innumerable resources, were certainly the best equipped to conduct such costly experiments. Not only was thought given to p ssible two-way communication with the Dutch Bast Indies, but also of a possible world-wide broadcast servicereally a much more ambitious objective than the establishment of an "Empire" scheme.’ Accordingly the wavelength was chosen with a view to obtaining good universal reception. The object of the experimental transmitter was definitely not record-breaking--~in such case, a less carefully-designed transmitter would have sufficed-but to find out whether a really reliable broadcast transmission over very long distances was possible. In the transmission and propagation of -the ultra-short waves, several very stranre phenomena, that often can only be partly overcome, occur. ee , There is, for instance, the so-called fading effect familiar to all broadcast listeners. In the reception of the ultrashort waves, however, this effect is much more pronounced so that the

strength of reception may vary several times within the space of a minute, or even of a second. There is further, the influence of the hour of. recepti on the signal strength. The most favourable time of recep tion depends upon the wavelength employed, an on the season of the year. The bad influence of fading, too, depends to a large extent on the wavelength employed, on the distance _between transmitter and receiver, and on some other hitherto not fully known fe tors. The propagation of the very short waves is entirely different from that of the long waves, and is of such @ complicated nature that only recently scientists have got an idea of ‘what really happens between transmitter and receiver. Constancy of the transmitted wavelength is of vital importanc# in ultra-short-wave telephony transmissions# ; With the usual modulating systems the speech currents delivered by the microphone are intended solely to vary the .amplitude, and not the frequency of the oscillations generated by~the transmitter; unless special precautions, so-called "frequency modulation," takes place, causing acute distorOn, In the case of POJ, frequency stability is obtained by the use of an oscillating. quartz crystal-this is a thin wafer of quartz held between two metal plates.: Depending upon the thickness of this quartz wafer it may be excited to expand and contract mechanically at a definite frequency and with great constancy. These minute mechanical oscillations are accompanied by minute voltages, developed, and these control the i valve in the transmitter, which valve, though it be only a small receiying type, controls the transmitted frequ of the entire transmitter, which may be radiating many kilowatts of energy. The oscillations from the initial val. are then multiplied in frequency in several succeeding amplifying stages until the desired transmitting frequency is obtained, by which time the valve proportions. have grown from that of the humble Philips miniwatt to the huge 20-kilowatt water-cooled types having an input of 25 to 27 kilowatts and requiring a plate voltage of the order of 8000 to 12,000. The transmissions of PCJ have, of course, been reported on from every corner of the globe, and the careful analysis, coordinating and filing, of these reports has been a work of no small magnitude. As a result, the Philips laboratys have accumulated a veritable weali ‘of data of particular significance to the establishment of a British Bmpire broadcasting scheme. The idea of Hmpire broadcasting is, of course, not new. The following extract from the English paper, "Wireless World," dated as far back as April 27, 1927, is headed "Hmpire Broadcasting." "For the second time in the short history of broadcasting it has been left to Holland to steal a march on this country, and in fact on Hurope generally.... Now again Holland sets the example by leading the way, in short-wave broadcasting. . y¥. We congratulate Holland, andy ‘the Philips Company in particular} on the enterprise shown in establishing a broadcasting record, more especially as the purpose is apparently (Continued on page 29.)

Empire Broadcasting (Concluded from page 2.) to provide a means for long-distance broadcasting, and advantage has been taken of the peculiar suitability of the 30-metre wavelength... It is a matter for regret that we should have neglected to take advantage of the wonderful opportunity which broadcasting provides for communication with the’ outlying sections of the Empire. . "When we know that wireless sets are being bought in places as far distant as India and Iraq in the hope of being able to pick up the transmissions from Daventry once in a while for perhaps no more than 4 few minutes, we can form some idea of the enthusiasm which would be created if a short-wave station were established in this country of sufficient power to be heard more or less consistently." This was two and a-half years ago! Now with the amazing technical advances that have taken place in the meantime, surely we Britishers are not altogether so unprogressive as to sit by with folded hands, and a resigned expression, feebly murmuring, "It can’t be done," while someone else grasps the opportunity and again leads the way. Let us hope that the next cabled report on the subject after the conference with the B.B.C. experts will not be in the same pessimistie vein as the one referred to in the beginning of this article. .