RADIO AND ITS RECEIVERS

Sun (Auckland), Volume I, Issue 255, 18 January 1928, Page 14

RADIO AND ITS RECEIVERS

Conducted for THE SUN by < THE TASMAN FLIGHT Radio might have been of considerable use in this gallant attempt if' a little more information had been given. The statements that the aviators knew little or no Morse is hardly credible, as the writer remembers a 15-words-a-minute test early in the war. Both men held commissions in the R.A.F., and a denial of their ignorance of Morse code comes from both Mrs. Hood- and Mrs. Moncrief. The trouble seems to be mainly that no one knew the wave-length to be used, and if, as is stated in Australia, a code of signals was to be employed, no one knew of that code. Many amateurs would have listened enthusiastically, and possibly have picked up messages which the skip distance rendered inaudible at those station that knew of the code. N.Z. AS AREA OF RECEPTION One is always struck by the test reports of British sets, and the relative poorness of these tests has probably retarded the sales of British sets in New Zealand. The average New Zealand listener turns up his nose at a two-valve set that fails to give him two or three Australian A class stations at moderate speaker strength. Yet the popularity of the Polar Twin as a British set giving good results here must astonish the British ifidio public, if the following extract from the London “Observer” is to be taken seriously. In addition, there are many amateur and locally-built two and three-valve sets, such as t&s Aerola. Crossley Tridyn, Raleigh Tridyn, Courier and N.2S. Radio which would astonish the English listener. Replying to a correspondent last week, I said “no three-valve set is suitable for loud-speaker work at a distance of 40 miles from the transmitter.” .That is a matter of fact, and not of opinion—dealing with the ordinary and not high-power station—but a gentleman living in Oxford begs leave to disagree with me. He 40 miles from Daventry and 50 from London, and he tells me that, using a high frequency amplifier, a detector, and a stage of audio frequency amplification, he gets good loud-speaker results from 2LO. I tell him, frankly and flatly, that he doesn’t. If he is using a horn type loud-speaker, he is out of court at once; because there isrh’t a horn instrument made that will give good results. If he is using a cone type reproducer, the minimum energy across the grid and filament of the output valve, for really good results, must be such that it can just be controlled by such a valve as the DE5a with 200 volts anode pressure and about 30 volts negative grid bias. Passable results may be had when the grid swing on the output valve is such that it just can be controlled with a DE5a valve, 120 anode volts and 20 volts negative bias. I am quite prepared to believe that my friend in Oxford could get that amount of energy from Daventry, using three valves; but if he is getting it from London he is performing a miracle, and I should very much like to know how he does it. If he troubles to write to me again I expect I shall find out that he is using a 6,000 ohms impedance valve as his output, and again I shall be right, but disappointed.

!. M. TAYLOR, B.Sc., M.I.R.E. AUCKLAND PROGRAMMES The powers that be are getting themselves into print again with regard to the type of programme. The quality is undoubtedly very much improved, but to the writer is appears that the “highbrow” and the jazz lovers are the ones who are catered for, to the neglect of the man in the street, who can enjoy a little of these, but whose main line of musical enjoyment is that in-between class, to which the popular comedy music belongs. Good stirring marches never fail to win applause. If this type were not almost excluded, as at present, there would be many more satisfied listeners. In North Taranaki IYA has been the main source of broadcast, but in a recent letter from New Plymouth it is said that 2YA has become the premier station, in spite of its fading and comparative weakness. IYA is known there as the station of the “awful organ.” AMPLIFICATION DIFFICULTIES Save only in the case of the crystal or single-valve detector, the purity of the speech and music that is received depends on the means employed in amplifying or magnifying the rectified energy. To visualise the waves of energy sent out from the transmitting station is an extremely difficult matter, but we can get an idea that is quite good enough to work with if we think of the transmitter and receiver as connected by tube with very flexible sides. This represents the carrier wave, actually extremely rapid vibratlo&a in the ether which pervade all space. We can think of this tube as being formed immediately the power is put on to the aerial by the transmitter. As soon as the microphone is agitated by sound, that sound is pumped into the tube, and the sides expand and contract with the varying intensity of the sound of the sound which pass along it. These waves are very complicated in form even when a single note is sounded into the microphone. When a full orchestra is at work, the complications are enormous Fortunately these complications in wave-form are almost the least of our difficulties. What we have to do is to get those waves into the air of our room, or into the air between our ear drums and the telephone diaphragm, with exactly the same shape as they had when they hit the moving part of the microphone. In the air audible vibrations range between alternations at a speed of 16 times a second and those with a speed of something over 20,000 times a second. To give absolutely true reproduction, both microphone and reproducer should be capable of handling equally well all vibrations between these two extremes. At present this is an impossibility. The machinery of transmission and reproduction falls considerably short of both extremes. The whole thing has to be a compromise, and, luckily, our ears are very accommodating. We can get results that sound very good indeed, though the scientist knows that he is hearing something very unlike that which is being performed before the microphone. We have to design our amplifiers to make the best of that which comes through the ether.

The chief difficulty is to get even amplification over the greatest possible sound range. Most amateurs

imagine that they can do this by employing resistance-capacity amplification. A mistake! This form of amplification can be worse than the worst possible transformer coupling, unless the couplers are correctly designed and used with the correct valves. Properly designed transformers can give very good results indeed, though • a properly designed resistance capacity coupling, with the correct valves, undoubtedly gives the very best results we can get at present. The second difficulty is to get the waves in the correct form or shape. This is partly looked after by the method of coupling, but most depends on the choice of valve and the amount of voltage in the anodes and the amount of grid bias. We want our carrier wave tube, which has been cut in half, lengthways, by the detector, to have exactly the same shape, in the half we use, as that half had When it left the transmitter. To do this we must use the best possible coupling, but the choice of valves is far more important, and most important of all is to use our valves correctly. NOTES FOR HOME CONSTRUCTOR Excepting exceptions, the would-be listener can now buy, ready made, better and cheaper receiving apparatus than he can possibly build himself. The exceptions are those highlyskilled experimenters who want some very special characteristics, in the way of quality or long range, that it. docs not pay the manufacturer to put into a commercial set —because of small demand. Despite the steadily rising level of performance in the repetition work receiver, and the steadily lowering price of the completed article, there would still seem to be. many who prefer to assepible their own apparatus. For such I have some very seriously-intended advice. First, buy or obtain a good design. Do not be led away by fancy- named circuits which are, often enough, purely advertising stunts. Despite anything anyone may tell you, you cannot have ultra-simplicity in tuning without paying for it in lack of sensitivity, or, what is probably worse, selectivity. Choose a circuit which gives separate tuning for each coil that has to be tuned. In a multivalve circuit choose one which has at least one stage of re-sistance-capacity amplification, and that, the one immediately following the detector. If you require a stage of radio-frequency amplification, use a neutralised transformer-coupled stage. And, finally, having ohosen your circuit, stick, exactly in every particular, to the circuit and the description of the designer. If you take this advice you cannot go far wrong. Almost every day, among my correspondence, is a letter—or, maybe, two—asking for a circuit that will give results that will eome up to my standard of performance. Of course, I comply with the request to the best of my ability; but always I add a solemn warning to the effect that the circuit itself is not the essential thing. Skill in design and lay-out, among other requirements, are necessary More and more am I amazed at the light-hearted way in which a man will confess that he has little or no knowledge of wireless, and yet will plunge into the very difficult work of receiver building. Of course, anybody can shuffle together a few coils and condensers, and make an arrangement that will rectify and render wireless signals.| It is the very ease with which some sort of results can be obtained that encourages the home constructor in his efforts. For the very best results by that I mean really faithful reproduction, receiver and loud-speaker must be considered as a whole. We really work backwards, so to speak. Having the loud-speaker, and knowing its characteristics, the output stage is built to fit it. The preceding stage is built to work best with the output stage, and so on, working back to the aerial coupling. The valves that are to be used are very important in the calculations that must be

made. Probably experience is just as important as knowledge gained from books in the difficult matter of obtaining perfect reproduction, and experience means a good deal of expenditure in both time and money. I feel quite sure that the average listener will do better for himself in every way by buying a receiver according to his means than by attemptting to build one. There are many men who could build a motor-cycle and some who could even build a car in their own homes; but no one would undertake the task of building something as good as a Rolls-Royce. It is, comparisons in weight aDd size eliminated, just such a task chat a home constructor undertakes when he sets out to build a receiver to give perfect reproduction. NEW APPARATUS A new aerial, especially suited for the man in a flat where there are no facilities for erecting mast&, has been brought out by the Supertron Company, well-known as valve-makers in America. The aerial is a cage with 100 feet of stranded enamel wire wound over a bakelite fermer on a low-loss principle. The cage is carried on a short mast, which has extension pieces for increasing the height, and has a foot with a universal ball joint which permits of adjustment in awkward places. The weight is only 91b., and among its many uses are as a portable, mounted on the step of a car, on a launch or yacht, on a window-sill or in the conventional way, mounted on a mast or roof, carrying, as it does, 100 feet of wire, it should give good results. Interest is being shown In the new screened grid valves for H.F. amplification, which are self-balancing. Milliard, Cosser, Marconi and Osram, are all showing this valve, and in addition Mullards have the new Robinson valve for similar purposes. American valve makers are producing with a great flourish a double grid valve, but this has been on the market from British and Continental sources for the last three or four years, and is not really new. TURN DIAL SLOWLY Every operator of a regenerative set should remember to turn the tickler dial slowly and carefully. If they are twisted quickly he will be making himself the nuisance of the neighbourhood. The dial that causes all the trouble is the one that controls the tickler coil, in the majority of sets, or the plate variometer in some of the more old-fashioned outfits. The best position of this dial is reached gradually, unlike that of the condenser tuning dial, which is .quite sharp in its setting. Now, the trick is to handle the condenser with one hand and the tickler with the other, in such a manner that the sound in the phones caused by the movement of the tickler is always a soft hush. It will be found that by turning the dial just a little more, the hush will end in a sharp click. It is when this click is heard that the set is “oscillating,” and at the same time causing a squeal to be sent out. RECEIVER TUBES It is a good plan to have the tubes, in a receiver tested after every 300 or 400 hours of service. If a receiver is in use an average of three hours a day, for instance, it will be worth while to have a service man test the tubes about once every four months and to replace any that are found to be wearing out. This is particularly important where the receiver makes use of rheostats for the adjustment of the tube filament supply because if a single tube starts to wear out there will be a tendency to make up the decreasing volume by turning the other tubes up higher and the usual result is that several tubes are prematurely worn out, whereas replacement uf the one poor tube would have saved the others.