Music from the Air

Radio Record, Volume IV, Issue 18, 14 November 1930, Page 9

Music from the Air

How a transmitter and receiver works

described by

Colin W.

Smith

from 2YA

OU have all been to the seaside when the surf was rolling in with a great long line of breakers, and no doubt very many have seen light floating objects and noticed that they do not move in with the breakers but merely drift with the tide. This shows us quite clearly that the water does not move, except, of course, where it breaks, but that the waves are Something different.. They result from a transfer of energy from one place to another. The. Waves you witness on the seashore travel only a few feet a second, often not that. Now we come to sound, which also consists of waves. Hvery time a sound is made some object vibrates and that makes the air vibrate; our ear receives the varying air-waves, and we hear. Again the air has not moved-it is that same mysterious "transfer of energy, although in this case the sound travels just over a thousand feet a second. in electricity and light the same phenomenon occurs. We still have the waves, but they travel at the remarkable pace of 186,000 miles a second. Again we must remember that it is not the medium through which they travel that is moving, but this strange nothingness called energy. Now we will return to the seashore. At different times the waves, according to the wind and tide, are different distances apart. On one day they might be separated by perhaps 10 feet, while on another 30. If we had the necessary apparatus we could measure the distance from the top of one wave to another, and we could conveniently speak of this distance as the. wave-length. We could do the same with sound and electricity but, although the wavelength could be made to vary, the rate of travel remains the same. The breakers do not come in fester because the waves are closer together, but nayre waves come in. & will now go right up the scale to the type of /ray that is used for X-ray work. That is one of the smallest we know of, and there are just 25 billion waves or ripples a second, and from this remarkable number electricity although travelling ‘at the constant pace, varies to below 20 waves every second. Your electrical supply travels in waves which leave the source at 60 to the second, and the hum you hear in your set is this turned into sound waves. ‘Between these two limits we have visible light at about 300 million waves a second, shortwave broadcasts take place on 20 million waves and broadcasting on one million. These figures are (only approximate, and of course variations take ‘place on either side. We. speak of the number of waves a second as the frequency and so my voice is now being carried .by electricity which travels at 186 thousand miles a second, and there are 720,000 waves each second, in other words 2YA’s frequency is 720,000 cycles or 720 kilo (a thousand) -cycles. Now, if the distance . between each wave were mvasured it would be found to

be 416 metres; in other words, 450 yards, so that will give you some idea of the rate at which my voice is now travelling. These waves, all of which have more than 15,000 ripples a second, are called high frequency waves,‘and you will remember they were experimented with by Hertz, and have been called Hertzian waves. They have remarkable penetrating power, for they can go underground, through space, through buildings, and through water. It ean be seen that the frequency multiplied by the wavelength must equal the speed of radio waves which always remains constant at 186,000 miles a second. ‘This is the equivalent, roughly of 300,000,000 metres. Now putting that down in simple form we have F xX W = 300,000,000 Now we can remember from our algebra that the following is the same thing stated differently : 800,000,000 F= --- Cycles WwW But we do not want cycles, so we divide the frequency and the speed of electricity each by a thousand, and we have: F = 300,000 Ww or W = _ 300,000 ¥F OW sound, as I told -you, is very much the same as electricity in its movements, but slower. The waves of audible sounds (that sounds Irish, but some sounds are inaudible) leave the object which is making that sound at a rate of between 25 and 15,000 a second. These have not the penetrating power of electricity, and consequently do not carry far. Broadcasting, in a nutshell, supplies a means of getting these high frequencies to carry the voice frequencies. If we have a coil of wire and a condenser, and induce a charge of electricity into it, this charge will surge backward and forward from one set of plates to the other, and will pass through the coil. So long as the energy is constantly supplied, this surging will be kept up. Now, if the aerial is connected to one set of plates and the ground is to the other, the surging takes place just the same, and as it ‘oscillates backward and forward it gives the aerial alternately a positive and negative charge. The number of waves leaving the aerial can be regulated by altering the value of the condenser or the coil, and the station will transmit on this frequency until either is altered. At the broadcasting stations there are many different circuits, and the final result is a strong charge going into the aerial and leaving it on a definite frequency-a definite number of waves a second. Ags I remarked before, the value of this current is varying regularly between positive and negative; in other words, it is an alternating current-perfecily regular in all

ways. It was this current that Marconi used ' in his early experiments, and which is now used’ for Morse, but it is altered for ordinary broadcast, | [THE microphone is a wonderful little instrument. At the present moment it is picking up the. slow vibrations in the air caused by my voice. They are being strengthened by a valve which is tucked away in the microphone housing and taken into another amplifier until it is a very big voice indeed. At this stage it is taken to the oscillating current which goes into the aerial, and in a very. remarkable way it is super-imposed on it, and the. wave now leaving the aerial is said to be modu- . lated. , Your receiver merely reverses the procedure that is taking place in the broadcast station. You have a coil and a condenser, perhaps a coil only in the very simple sets. When the value of the condenser and the coil are at a certain figure, and this is the same as at the transmitter, your set and the transmitter are in resonance, and you can receive the waves from it. But it offers a very high: resistance to all waves that it is not in resonance ' with, and that is why you cannot hear the other stations. It is really the case of a lock and key. From your point of view the key is the transmitting station and cannot be altered. But you have the power to alter the lock, so that you can admit one key and keep out all the rest. [HH power you receive from a transmitting station is very small. It can be measured only in thousandths of a volt. If you connected a pair of telephones (or earphones, as they are generally called) to this tuned coil and condenser you would hear nothing, though the energy is undoubtedly there. No it is travelling through space at a rate too fast for the diaphragm, that is that thin piece of metal in the earphones, to respond to. The slow speech vibrations which are imposed upon the faster ones must be stripped away, and this is the function of the detector. Now I told you last time the interesting times some of the earlier experimenters had in trying tu find a sensitive detector, but when you come to consider there are only thousandths . of a volt of energy obtainable, you need something fairly sensitive to receive them. " The erystal is one of the best detectors. It will" strip away high frequency and leave the low, but» the sound waves have been converted to electrical . waves, and before we can hear them they must be changed back to sound waves. ‘This is the func-. tion of the phones or loudspeaker. The crystal has * no means of strengthening up the weak currents, ~ and so you can only listen to the station on the phones. If you wish to use a loudspeaker, valves must be added. Now the valves, as we noted in. talking about the birth of radio, has made radio: as we understand it, possible. If a wire is arranged in a vacuum and heated to a white heat it will give off electrons (small (Concluded on page 29.).

Broadcasting Theory (Continued from page 7.) negative charges of electricity), and if a plate also in a vacuum is close by | this, and positively charged, it will attract those electrons. Now if the third wire is placed between them and electrical surges imposed upon it, they will be magnified. This is how the valve amplifies. The electron emitting wire in the valve is now coated with thorium, and gives off plenty of electrons when heated only to a dull red. It is thus called a dull emitter. To heat the filament a battery or a constant source of electricity is needed, and so we build up our circuit. We can step up the half volt received from the broadcasting station before the high frequency current has been stripped away and -afterwards. If we step it up before, we speak of the amplification as being radio or high frequency. If afterwards, it is known as audio frequency amplification. The latter part of the set is concerned, not so much with amplifying at a high ratio as it is with handling the amplified current, adding a little amplification of its own and yet not , 4 4 4 : 4 : ‘ ‘ : r 4 4 r : ‘ 4 ‘ : : r ‘ ‘ ‘ ‘ 4 r 4 4 r twisting or distorting the signal. The’ amount of amplification that can take place in a set is really remarkable. Modern screen grid valves are capable of amplifiying about 300 times, and if you have three stages of screen grid you are getting a theoretical amplification of 300 times 300 times 300. That is a total of 27 million... Now you wonder why when you turn your set on full you get static and parasitic noises. The audio side also aniplifies. The first may amplify by eight and the last two by three. If you have a power detector that will also increase the signals, so you see that if you are close to a transmitting station and turn on your set full there is going to be a little noise. Of course there are losses in a set, and this theoretical figure is. not nearly reached, but the amplification is very great indeed. The fundamental principles underlying transmission and reception should now be realised, and the different types of set can to a certain degree be appreciated. There is first of all the crystal set, which has no amplification of its own, the one-valve set, which is a detector, and, to a very small extent, an amplifier. And so by adding valves, on either radio or audio sides we build up our signal until we have them just as strong as ever we want them. Of course, with apparatus that is in the ordinary household there is a limit to the amount your valves will handle without distortion, but this is the subject of another talk. Then, there is the superhetrodyne, a receiver which works on a_ different principle. The first valve is usually an ordinary amplifying one and the next an ordinary detector. Following these is not the audio amplifier but an oscillator which, generating another frequency, carries. the signal on that like the transmitting station does. Like station, too, it is amplified by the intermediate stages and then passed to the second detector. Following that is the audio amplifier which is similar to that of an ordinary set. This type of set is now becoming very popular.