The Why of Wireless

Radio Record, Volume I, Issue 14, 21 October 1927, Unnumbered Page

The Why of Wireless

{nteresting Series Setting Out Scientific Facts Simply |

‘By

Electron

PART VU.-HETERODYNING, ) The cause of the atmoying whistle an be easily explained. We have got wo transmitting stations, the legitinate ‘one and the oscillator, sending ut. waves of mearly the same freuency. ‘Lhe effect on a third set cau ie éxplained as follows :- Imagine a bicycle pump fitted with wo, plungers, as shown in Fig. I, with little third one midway between nem and perpendicular to the tube. vet: A be the broadcasting station, B he oscillator, and C some other lisener. If A is transmitting, and B s behaving himself, then both ~ and > will respond to the to-and-fro moion of A. Suppose, now, B uses too nuch reaction, then he will begin to nove to and fro independently of A. fo make things easier to explain, supiosé. A is on the air, but no one is spedking, then A is sending out a lighi-frequency wave of constant ampliude . B is doing the same, but the frequency is slightly different, say, slightly greater than A. At a cer‘ain instant both plungers will move inwards together and cause an increase of current in C’s aerial, A little. while later B will be moving outwards when A is moving inwards, so that the movement of C is decreased. fhe effect on C, therefore, is that instead. of responding to A alone, and moviig in and out a constant distance, he will find that his plunger is first rising, then falling, in ampli-tude-exactly the same «as if someone were -plaving the flute in A’s studio aud causing the waves to increase and decrease in amplitude, The number af theee increases and decreases in the

rectified current of C’s set will be seen to be eyual to the difference in frequency between A and 8. So that B can alter the note by moving his vatiable condenser, and so "tering his tuning. We have, therefore, an casy way of. telling when we are the culprits. Tf we vaty our variable condensers and the note changes in pilch, then we are the offenders, whereas if the note merely weakens, but does not rise and fall, then someone else is {to blame. One method of tuning is to make the set oscillate, and then search about (by moving the dials) for a whistle. The dials are then turned till the note gets lower and lower, till it becomes inaudible. When this state of affairs is reached, then the receiving set is oscillating at the same frequency as the transmitter. Dam-

a age is being done, however, because the quality of the reception is very much reduced, and also the B_ batteries are being run down quickly, because it is from them that all this extra power comes. This, however, explains many of the freak crystal receptions about which one lteats. If a crystal set is being operated close to an oscillator, then the oscillator, in acting like a small transmitter, will re-radiate part of the waves which he is receiving, and the crystal user says he got a station several hundred miles away. The older types of valve receiver had movable or hinged coils, so that the reaction could be varied by moving them to and fro till oscillation stopped. While quite useful and comparatively harmless in the hands of an experienced user, these sets were a curse when manipulated by 1 tyro, and any set capable of oscillating is now rigidly iNegal in New Zealand. Notice that reaction is not illegalit is used deliberately in practically all valye sets which have a_ high-fre-quency valve; but it must not be pushed to the point of causing the set. to oscillate. |

NEUTRALISATION, ‘he modern set, therefore, differs from the earlier models in having some form of device which prevents reaction from causing oscillations. .\ set with this construction is said to be neutralised. In a neutralised set the first valve has a connection from the plate to its grid, but in such a direetion as to prevent oscillation, This is shown in Fig. 2. The first valve is connected in the usual way, but its plate is connected to the primary of a radio frequency transformer. The advantage of the transformer method of coupliny two yalyes together was explained last week, since the voltage of the secondary can be made to be greater than the primary. ‘The second valve is connected up with the secondary across the filament and erid, as beerwees Rt eal :

fore. The new departure is, however, a wire is brought from the transformer secondary through a small variable condenser, and thence to the grid of the first valve. It was mentioned in the second article of this series that when we had a transformer the induced current always flowed in the opposite direction to the primary current, and so by connecting the grid of the first valve to the transformer secondary, the result will be to put a "brake"? on the. tendency to oscillate. The treason for the neutralising condenser is to vary the amount of this neutralising feed back until the set just ceases to oscillate. When the adjustment is made the neutralising condenser should then be left severely alone. Reaction is made use of in the second, or detector, valve, but the neutralising

device on the first will effectually prevent any trouble being caused. Some people, especially those with home-made sets, may have had trouble with oscillation, although they do not use reaction consciously. The reason is that we do not need two coils to have reaction. ‘wo straight wires, one from the grid and one from the plate, if they are allowed to run parallel, and close to each other, will cause trouble. ‘This is especially the case when a small panel is used, consequently the components are cramped for room. If any of the modern sets are examined it will be found that the high-frequency valves are placed alternately with the low-tre-quency ones, so as to have them as far as possible away from each other. Also, the mounting of the coils theinselves ig most interesting, as it will

— be seen in many to Le impossible fort the magnetism of any coil from passing through either of the other two. DUAL AMPLIFICATION, ‘fhe diverse uses of the three electrode tube are not yet complete, as the. next iritriguing application is to make it do two things at the same time. It is possible for us to receive signals in the aerial, amplify thent by means of a valve, then pass them — through a crystal to rectify them, and then bring them back and amplify them again by passing them through the same valve. A set arranged in ~ this way is called a reflex set, and can . produce great amplification for a single valve. ‘wo main methods ate possible, one with a high frequency transformer and | the other with a low frequency transformer. ‘The first is shown in Tig. 8, and should be fairly clear to those who have followed these notes from the beginning. The high frequency currents enter the aerial and operate the grid in the usual way. ‘The little condenser "fa" in the aerial will not, of course, affect these high frequency curents in any way. ‘The amplified currents flow from the plate, as before, through the tuning condenser and coil, and on through the condenser "bh" through the B battcrv, and thence hack to the filament. ‘These currents induce corresponding currents in_ the secondary of the transformer. These currents are rectified by «he crystal, and from now are Jow frequency currents unable to flow through condensers, but able to pass round coils. These low frequency currents cannot pass through condenser "a," so have to flow backwards up the aerial coil and into the grid. ‘The corresponding amplified current flows from the plate through the transformer primary, and, being unable to flow through ‘hb,’ passes round the coiis of the ’phones or loudspeaker. With this explanation the other type should now in its turn be understood. There is very little trouble from oscillation with these sets, as the crystal has a stabilising tendency. ‘The saving in cost is not so great as might appear at first glance, since the components are practically those necessary for a two-valve set, with the exception of the second valve, end its fittings, and the grid leak and condenser, but still, the set is a very useful one, and exceedingly easy io use.