The Why of Wireless

Radio Record, Volume I, Issue 15, 28 October 1927, Unnumbered Page

The Why of Wireless

| Interesting Series Setting Out Scientific Facts Simply

(By

Electron

ows -_-_-_- DUAL AMPLIFICATION. PART VIII. In last week’s notes reerence was made to a reflex set capable of producing great amplification for a single valve. The method of achieving the results indicated is shown in the following diagrams which were omitted from last week’s article which should be read in conjunction with these illustrations. a

DOUBLE GRID VALVES. Now that receiving sets have been raised to such a degree of perfection and ‘‘foolproofness’ the trend of progress has been to make the set .independent of too many batteries. As has been stated the ‘‘A" battery can be kept. in a state of independence from dealers by means of the battery charger. The "B" battery can be eliminated entirely by another form of I‘leming valve designed to give a higher voltage and smaller current, but the expense of ‘this has led to the development of: special valves in which the B battery only requires to have a pressure of from 2 to 25 volts, the last being suitable for last stage amplification, that is, what an ordinary power valve would require 90 to 100 volts to do. ‘The main characteristic of these valves is that they have two grids, the new one being placed between the fitst and the filament. ‘the reason for this is that the electrons are very dense round the filament, and much less dense as they travel towards the plate. The large pressure of the normal valve’s plate is necessary to attract the electrons towards it, but the electrons in the neighbourhood of the filament only get a small proportion of this force. It is something similar to a crowd of people leaving a theatre. The desire to get out and on a tram is great with everyone, but the people in the vestibule cannot get out because of the people loitering about the entrance. If now some means were available to a ~ rr re

force these pcople away from the entrance as soon as they got to it there would be a much easier and quicker flow from the hall. Similarly with the valve. If we place a smiall positive rressure close to the filament this will be used up in speeding the electrons through the space and the second grid will behave as before. ‘igure 1 shows a double grid valve or tctrode as it is celled, connected for use as a detector with reaction. A strange thing about these tetrodes is that the first grid and the plate may be at the s»me voltage, the first grid apparently giving the electron such a speed that they can get to the plate without any further incentive. These valves are becoming very popular in Europe, and should before long displace the triode. ‘The external appearance of these valves is exactly the same as the triode or single grid valve with the exception that the inner grid is connected to the metal ring of the cap to which is fixed a screw. These valyes therefore can be used in an existing set without altering any of the wiring. A valve of this type with a dull emitter filament has the following astounding properties :-Tl‘ilament voltage 2.0 voits, filament current .06 amp, plate voltage 2 to 20 volts! Surely this is going a long way to solve the upkeep cost of broadcasting! VALVE TRANSMISSION. It may be as well at this stage w give an idea of how wireless telephony may be broadcast. It should be cleart understood this is not the way that a modern broadcasting transmitter operates, but it will give the average reader an insight into methods of producing modulated carrier waves, and it may serve to reduce the feeling of hopeless wonder with which most people seem to regard such a station. T‘irst of all we haye to produce a carrier wave and that may be done as has been explained under the heading of reaction and oscillation. ‘The high frequency currents thus gencrated are induced into a neighbouring coil connected to the transmitting aerial, figure 2. In this aerial circnit is placed a microphone, which, as has been explained before, increases or decreases in resistance as the diaphragm moves outwards or inwards. This will obviously decrease or increase the current in the aerial and we will therefore obtain the modulated wave which is required OTHER USES OF THE THREE ELECTRODE TUBE. It may be thorght that the various uses already detailed would be enough, hut the triode has been of such wonderful use in wireless work that it has been adopted for various other uses. ‘Truly the lamp of Aladdin could hardtae" Fw ee, SO in

ly have been more wonderful than this. The iirst application is the so-cu®ed wire-wireless or the sending of wireless messages ‘along a wire. This sounds strange, but what is meant is that a telephone wire instead of carrying the usual low-frequency currents straight from the microphone is made to carry these currents after they have been transformed into a high frequency mod‘ulated currents. The advantage of this is that instead of one wire carrying one conversation as it does in ordinary commercal telephones, it can carry as many of these carrier waves (all at different frequencies of course) as may be desired. ‘Twenty or thirty conversations all going on at one time will not cause ‘any confusion since each of the conversations is timed at each end for its own frequency. There is one of these

systems operating between Melbourne and Sydney at present with every success. What is more these wires can be used for post office telegraphs at the same time without causing any interference, so that the cost of the copper Wires is quite small and long distance telephone calls will thus be made cheaper if the demand warrants the installation of a carrier wave telephone service. Another use of the amplifying valve is in the sister industry of the gramophone. Up until mot so long ago the recording for gramophone records had to be done in a specially constructed studio, because of the bulky and delicately constructed recording appara-

tus. These studios had to be carefully made sound tight, and were always padded to prevent echoes from confus-_ ing the recorded sound. ‘The result, ns though clear and clean cut was somiewhat ‘dead’ because of the lack’ of. "‘atmosphere."?’ When we listen to a band or orchestra in a hall, the echoes | are mingled with the music and give a. pleasing effect. These echoes cannot. be satisfactorily produced in a small studio, and hence had to be removed entirely. Also instruments which are not portable, for example a pipe organ, could not be recorded at all. But with the advent of the amplifying valve, it became possible to place a microphone anywhere, bring the telephone wires into the studio, amplify the sounds till they were able to operate the instruments of the gramophone recorder, and thus give us the "electrically recorded’? pieces which are in such demand because they capture all the ‘"‘atmosphere" of the concert room. Still other important uses of the valye which cannot be fully explained at present are radio television, the talking kinematograph or phonofilm and the public address system in which a speaker addressing a large crowd has a microphone in front of him, which when amplified is fed to loudspeakers distributed among the crowd. This was largely used at Wembley, when the crowds at all parts of the grounds lieard the speeches of the King and other speakers. ECHOES. Reference has already been made to echoes in the transmitting studio. If the studio, as in the case of 2YA, is a large one, a certain amount of echo may be retained, as it gives a feeling of naturalness to the sound as broadcast, but if the studio is small it is advisable to damp out echoes completely, as the result is displeasing. Even in the case of 2YA’s grand studio a certain amount of padding of felt, etc., is essential to take the place of the clothing of the audience in a wellfilled hall. Everyone knows the ‘‘empty"? sound of speaking in an empty house, and the different sound when it has been furnished, and in the same way an empty studio without padded walls or heavily draped curtains would produce an effect far from pleasing. Some people might think there is no echo in 2YA’s studio, but if one listens intelligently many little points will be noticed, especially when people speak from some distance*from the microphone, when, of course, the echoes are magnified as ‘well as the voice, which is now weaker. This frequently takes place during the children’s hour, when "Uncles" are saying goodbye from a

FS SS distance, the echo, as if uncle were speaking through a corridor, is quite distinct. FADING. When reception is obtained from a distance two difficulties are met with in tairly considerable proportions-tading and static. ‘The former is a peculiar weakening and strengthening of the received signals without any interference or alteration on the part of either transmitter or receiver. Several suggestions have been put forward to explain this phenomenon, but one of the most feasible is one suggested by Oliver Heaviside, of England. Ordinary air under. normal conditions is practically a perfect insulator, and electricity cannot flow through it. If, however, the air is reduced in pressure, that is, if it is exhausted gradually from a glass vessel by means of an air pump, it will be found that after a certain degree of vacuum has been reached the air becomes a pretty fair conductor. Most people know that the pressure of the atmosphere becomes less and less as we climb, and, as a matter of fact, it was this rarifying of the air at extreme altitudes which caused the failure of the Mount Everest expedition. At even higher altitudes the air pressure becomes so sinall that it becomes conducting. ‘This layer of conducting air surrounding the earth several miles up is called the Heaviside layer, atid when wireless waves strike it electric currents are induced in it. These electric currents have a magnetic effect which acts like a choking coil-the waves are unable to penetrate the layer -and if they hit the layer at an angle they will glance off it, that is, they will be reflected. ‘This state of affairs is shown in fig. 8, when the path of a wireless wave is shown leaving the broadcasting station, striking the Heaviside layer, and being reflected to a receiving set at X. Wireless waves always travel in straight lines like light, and since the surface of the earth rs round, it is clear that the only way in which a distant station can receive signals is by means of one (or more) reflections from the Heaviside Layer. This is quite straightforward if the lay is smooth and even, but if there is a "swell" on the surface it will mean that the reflection will be broken up. This gives a pretty fair explanation of fading of both kinds, as the trouble may take more than one form. We may have the "regular" fading, it which the signals rise and fall in strength regularly, either every few sec onds, or at intervals of half an hour or so. ‘Then there is the kind when the strength of signals varies in a most erratic way, due probably to a storn instead of a heavy swell. The Heaviside layer can also explain the existence of ‘‘blind spots,’’ that is, areas where a particular station is practically inaudible even with the best sets, while other stations come in witli ease. ‘The explanation of this, however, necessitates a knowledge of the "‘interference’? phenomenon of light, which is beyond the scope of these notes.