Construction Continued

Radio Record, Volume I, Issue 42, 4 May 1928, Page 13

Construction Continued

The Production of Reaction

NSIDE a valve. when in action, we ‘have the filament heated by the A battery, which causes a stream of electrons to be shot off or "emitted" from the filament. These electrons, being negative, are attracted to the plate, which is always positive on account of the B battery current which flows from plate to filament and back to the B battery. The voltage impulses of signals that come to the grid have a great effect upon the conductivity of the space between , the filament and grid with regard to the flow of electrons, so that as the grid becomes more or less negative, the flow of electrons decreases or increases.. In this way the small impulses on the grid:-are able to produce ‘larger’:changés in the plate circuit, and thus amplification is ob--tained.: 2 ; aa ROM ‘this it is. seen that. there is more energy flowing in the plate eircuit than: in the grid circuit, so that-if we:ean feed back a little of the plate circuit energy in such a way "as .to increase that which is ‘already ‘in the’ grid citcuit, this increase will be’ amplified’ and appéar asa still ‘greater increase ‘in currents flowing. in the plate circuit. ~ oa ' ¢ " 3 : THERE is always. a: certain amount of "damping" in a wireless circuit, which tends to make signals die down. We can’ prolong the’ dyingdown’ process by supplying ‘energy to compensate for’ that-‘which is lost, so in an oscillatory circuit’ we ‘can feed in energy and wipe out the effects‘of damping. If we use reaction to feed energy back into the grid circuit of a valve we ¢an so adjust matters that the oscillatidns’ produced .by passing: waves are prolonged and die away gradually, or we can’ feed back a Jatger amount, so that oneé ‘osci}lations are started; they are maintained indefinitely. This latter conditions is called "self-oscillation," and oceurs whén the’ energy fed. back balances that lost in. overcoming resistance, and so on. °° This process is .equivalent to reducing the damping of the circuit, and it is most beneficial’ in increasing the sharpness of tuning of the circuit, since the lower the losses by damping, the sharper the tuning, and this holds good up to the point at _- panrenpweusnnesenemeneen se

which self-oscillation commences, when the damping is said to be reduced to zero. THE actual method of producing reaction is quite simple. It is usually done by including in the plate circuit of the valve a coil which is arranged to act inductively upon the tuning coil in the grid circuit, by one of the usual methods of inductive coupling. When coils are sco placed, currents flowing in one coil cause similar currents to flow in the other by the familiar process of elec-tro-magnetic induction. Hence, this arrangement is often called reaction coupling. . So much for the good points of reaction. We must not forget that reaction wrongly used is a eurse instead of a blessing, not only to the user, but to all. his neighbours as well. It is quite easy to see how this may be if we remember that reaction is capable of producing continuous oscillations in the grid cireuit of a valve. If that grid cireuit is connected to an aerial, it is obvious that oscillations . will flow-in the aerial, and the whole becomes a miniature transmitter radiating rather Weal: | continuous waves. ° : }

This radiavion takes place upon whatever wavelength the user of the set is receiving, and so if he is trying to receive broadcasting, the radiation will be right on top of the waves from the broadeasting station and will produce howls and whistles in the receivers of everyone within a couple of :ailes or more, who may happen to be listening to the same stution. Moreover, the speech heard in the oscillating receiver will be more or less distorted and spoiled, although a little louder than when reaction is properly adjusted. O make praper use of reaction when receiving broadcast, it should be adjusted so that the circuit is. almost oscillating, but not quite. This is in the case of weak signals. When strong signals are being received, reaction may be very much reduced or brought to zero, full volume being still obtained. Modern circuits are designed with a view to preventing detector oscillation from reaching the aerial, so that interference may be. reduced to a minimum. a oe