Station Interference Overcome

Radio Record, Volume IV, Issue 28, 23 January 1931, Page 9

Station Interference Overcome

‘In a recent number of the:"Record" advance particulars of a revolutionary radio invention, the Stenode Radiostat, were given. This set was designed to relieve the serious. congestion of the ether im America and Europe-a condition of affairs which happily will not obtain for many years in New Zealand. N everthe less, we believe the majority of. our readers will be interested im the inventor's own explanation of ithe working 'of ‘his new apparetus, and ‘in \his : discussion ‘of the difficulties encountered during its evolution.

INH of the greatest radio 4, problems at present facing | broadcasting authorities in »] America and on the OonZ24 tinent is that of overcrowdhae ing of the ether. In the ¥ %} broadcast band only 90 or 100: channels are available for radio stations, and yet in the United States alone there are over 600 stations operating. The apparent anomaly of the last statement is exained when it is realised that many 'f these stations-in one case over 50 -operate on the same frequency. These are of course as geographically remote from one another as possible, yet imagine the resulting babel from the speaker of.a sensitive set’ situated in the centre of such a group of transmitters. This, briefly, was the serious problem facing the American Federal Radio Commission a short time ago. And then preliminary announcements were made in the Wnglish daily press concerning a new and sensational invention called the "Stenode Radiostat." Imagine a system of reception which enables over a thousand stations to operate on a band where previously one hundred was the recognised limit. This was the claim made by Dr. James Robinson, M.B.B., Ph.D., a noted HEnglish scientist and radio engineer.

Successful Demonstrations. F¥ Dr. Robinson’s invention would truly allow the multiplication of possible transmissions by even two, then the word "revolutionary". could be applied without exaggeration, and the commercial importance of. the invention would be immense. Dr. Robinson and his associates, however, claimed more than this; they said that under their system there was theoretically almost no limit to the multiplication of stations, and a multiplication of ten or even twenty would be comparatively simple. . No wonder the scientific world was full of doubt and was inclined to suspend judgment until a further demonstration and a disclosure of the technique could be given. Such a disclosure and . demonstra- _ tion have now very effectively been fe both in Hngland and America, , radio engineers are unanimous in eir agreement with the somewhat ‘Startling claims made. . ‘ In the following extract from "Radio News" Dr. Robinson himself explains the working of his invention, and discusses the various difficulties which had to be overcome before success was achieved. The Inventor Explains. "[t has often been said that in order to solve a problem the best way is first of all to define that problem clearly. Just what did we want to do? Obviously we had to clear the ether and make room for more sta- \, tions. , "The main reason why, for example, : @ minimum space of 10 kilocycles was laid down as the minimum separation between broadcasting stations, was that according ‘to the side-band theory taf

a station modulated by frequencies up to 5000 cycles automatically produced wave lengths which had to be receiverl up to: 5000 cycles of each side band carrier. .It is, of course, essential that the receiver should go as far as possible and faithfully copy all the sounds impressed upon the broadcast transmitter microphone. In order to get any selectivity, the phenomenon of resonance or tuning had to be recognised. If we made our receivers too sharp then we received the carrier wave and practically nothing on the side waves. At the same time, the quality was thoroughly bad. high notes being cut off and music and speech made unrecognisable. It was considered necessary that the resonance curve of the receiver should be sufficiently wide to embrace all of the side-bands without appreciable loss, and receivers which we made flat enough in tuning to embrace all, these side-bands inevitably received some of those of the next channel. "This was particularly the case when the strength of the station on the next channel was of a high order, or to put it more simply, when we wanted to receive a comparatively weak transmission through the interference of.a local station but one channel away, the interference from the local station was then noticeably present. "The fact that the cutting of the side bands was accompanied by a sacrifice of quality led everyone to believe that this was a case of cause and effect, and one of my first important discoveries was that a loss of quality was due to an entirely different cause. "A thorough mathematical investigation of the principles of modulation, of the form of the

modulated wave transmitted and of its effect on receiving circuits: of various degrees of damping, gave me my first clue to the Stenode Radiostat. I soon realised that no matter how selective the receiver may be, it can be made to reproduce faithfully all the modulation frequencies impressed upon the transngitting microphone. Laboratory Research. "THE next step was to prepare in the laboratory, experimental apparatus to give a degree of sharpness of tuning hitherto considered useless. "My theory, at this point, had developed far enough for me to see that we could cut off all the side bands, leaving only the carrier frequency without loss of any modulation frequency. The quartz crystal resonator at once suggested itself as almest the ideal sharp-ly-tuned circuit, for’ a properly prepared quartz crystal will resonate freely at one frequency, and scarcely at all on the frequencies more than two cycles on either side, even on such high frequencies as are used on the broadeast band. "A receiver made up in this way with a quartz crystal ground accurately to the frequency of a broadcast transmission gave reproduction so bad that. it was useless in any form of reception of speech or music, and the result would appear to confirm the de ductions from the side band theory. "T know, however, that the reason for this bad quality was that, the frequencies in the audio spectrum were being disproportionately magnified, and we could say that if all frequencies from 100 to 5000 were impressed at equal strength upon the transmitting microphone, the output from our

highly selective receiver would be expressed as inversely proportional to frequency. "Thus notes of 100- -cycle frequency were twice as strong as those of 200 cycles, those in turn being twice as strong as notes of 400 cycles and so on. It then remained to design a special audio frequency amplifier having a characteristic curve directly proportional to frequency, notes at 200 cycles being magnified twice as much as those of 100, and so on. In this way the overall response curve of the re ceiver could be ‘made substantially uniform over the whole scale, giving first-class quality without sacrifice of the abnormal selectivity which charac: terised the radio frequency portion.

Designing the Tuning Circuit. "FARLY ‘in the article I indicated that according to the theoretical views held by practically all scientists, the resonance curve of a receiyer musi embrace all of the side-band frequencies if proper quality is to be obtained, also indicating’ that my own theory had shown this reasoning to be fallacious. Let us now consider for a moment exactly what happens in a. Circuit of a very low impedance, one that is tuned so sharply that hitherto it has been considered useless for radio telephone communication. "Let us imagine, for example, that | a pure note of 1000 cycles is played in front of a broadcast station microphone. This note causes a rising and falling in amplitude of the carrier wave 1000 times a second, and we say that the carrier is modulated at this frequency. When the signal is picked up in a receiver tuned to the particular transmission, a resonance effect takes place and we have, roughly speaking (considering a carrier frequency of a million) 1000 radio waves to each rise and fall. If we impress upon the microphone a note of 2000. then we have only 500 waves for each rise and fall, similarly a note of 4000 will have 250 waves, and so on. "Now the intensity of current which the signal will build up in a sharply resonant circuit is a function of the number of. waves impressed upon the receiver. You will. see that in each pulsation note, of 1000 you have twice. as many waves for resonance purposes as you have in the note of 2000, there- | fore, the built-up signal will be roughly ‘twice as strong. Carry this reasoning through all the frequencies in the audio spectrum and you will see ‘very easily that the reproduction of frequencies in a very sharply tuned circuit is, as I‘have pointed out early in this article, inversely proportional -to frequency. "So far, of course, we have not found the ‘ideal sharply tuned circuit, which would be a circuit that would respond easily to one frequency to the total exclusion of all others. Bven the quartz crystal falls considerably short of this ideal. At the same time the commercial application of such a circuit would be impractical because we.would pass through 'the station so. easily when tuning as to miss it ailtogether. . "The present Stenode circuits have been worked out to be a practical com-

promise between infinite selectivity and’ easy handling, ‘The tuning is’ broad enough to be easy to handle while being at least twice as sharp as the most difficult modulated ‘conditions require; and when turning the dial we find a station, pass through it, find silence, and ‘then, sure enough, the next station will come up clearly and distinctl; without a trace of interferénce, so soon as we reach the correct degree on the dial. . Phase Reversal. 7 HAVE. already mentioned. the phase reversal method of. receiving which forms the basis of one of my Stenode receivers.. In this the effects of persistence in giving a disproportionate building up of different frequencies are counteracted in the high frequency part of the circuit without in any way sacrificing selectivity, the audio frequency amplifier being of 4 normal type. The particular circuit used for this is one of great potentiali-

ties and has already given admirable results, but at the moment is not quite so simple as that’ used in the Stenode Radiostat receiver which I have brought to this country. "The reproduced circuit diagram syill enable you to see how the Stenode is worked out in practice. The high selectivity which is the great feature of the receiver is-obtained by sending the incoming signal through a quartz resonance circuit, and in order that one quartz circuit'can be made to function for all frequencies in the broadcast band, we utilise the stperheterodyne principle, converting the incoming waves from their normal frequency to that of the quartz crystal circuit. ° "We thus have a loop receiving circuit, or tuned circuit eonnected to an aerial if necessary, an oscillator circuit for beating the local oscillations with the incoming oscillations, so as to produce the intermediate . frequency, screen grid intermediate circuits, .a

highly selective quartz circuit (or the ‘Gate’ as we sometimes call it), the second detector and a specially designed audio frequency amplifier to correct the effects of persistence. For fixed wave-length working, such as amateur radio telephone, for example, or for trans-oceanic telephone communication, it is unnecessary to utilise the supersonic superheterodyne principles, for the special quartz tuned cirenit can be tuned directly to the transmission frequency and considerable simplification effected in this way. Numerous Problems. "T HAVE not, of course, much space to deal with the thousand and one problems and difficulties which have had to be overcome in developing the Stenode receiver in its present form. "The quartz crystal, for example, which is mounted in a vacuum, had to be very carefully investigated, and the best form of mounting found; the superhetérodyne receiver. had to be restudied from the very beginning, for it was found that in the forms most

generally used the superheterodyne suffered from a number of defects which prevented me from obtaining really high-grade. quality. "Special precautions had to be taken in the design of apparatus to tune sharply enough for the Stenode. Thus, by many months of intensive work, with a laboratory staff held keenly interested in the development of something essentially new, all of the difficulties were overcome. Basis of Operation. — "Winally, I would like to point out — that the whole of the work on the. Stenode has been based on well-known mathematical:and physical principles and the fact that the apparatus works in &@ way which has hitherto been held. impossible according to the side band theory does not worry me in the "ee est. "A number of scientists of interna ‘ tional repute are now working on the theoretical basis of the Stenode, and it has already been indicated to me that at least two of them have found what may be termed the missing link in the side band theory which will enable it to accommodate the Stenode facts and also the basic. theory of side hands. What of the Future? The ultimate theory may be a year or two in appearing, but meanwhile the practical work and application of the new facts go on. In this connection I would point out that there is no general agreement on the theory of the erystal detector which was rendered obsolete by the valve, and: even Marconi’s coherer is not properly understood ! "Probably the Stenode Radiostat will ‘be in universal -use long before scientists agree on the _ theory, but. what. does that matter so long as we have cleared the ether?"

"Pros and Cons’ ONE ACT PLAY Written by Keneth Barnes Produced by J. W. Bailey & Company, from 1YA Friday, 30th. SCENES : "Man Proposes," "Woman Espouses," "God Disposes."