Radio Broadcast (May-Oct 1922)

424 Radio Broadcast Feed-Back Connection. Now, the progressive radio-fan takes full advantage of this reactionary movement. The feed-back coil is so placed with respect to the tuner coil that the kick-back produced in the tuner coil by the feed-back or "tickler" electrons will force even more electrons to run down the tuning coil. This in turn takes more electrons away from the grid as before. At the grid, this speeds up the plate electrons again and the feed-back principle is carried as far as wise. From this it would seem as though the electrons were lifting themselves by their boot straps in this quickly rising process, and, with the batteries supplying the necessary power, that is exactly the case. The electrons are merely the agents and the radio-fan is the director. Naturally this process must not be carried too far. If it were, the reaction would carry the rise to the limits of the number of electrons given off by the filament in the vacuum tube, irrespective of the strength of the received signals. It is absolutely essential that the output of the vacuum tube should be proportional to the strength of the incoming signals, in order that they may represent the variations which constitute speech and music at the transmitter. So the feed-back must only be carried to a point just below the complete swing. Continuous Oscillations. When this critical position is reached, the electrons will act as though they were swinging to the limits of the pendulum-like action. When the limit is reached, the pendulum begins its return and makes a vigorous swing in the reverse direction, aided again by the feed-back principle which forces a swing started in the downward direction just as well as it forces one upward. As a result, the receiver will continue to oscillate electrically, even though no signals may be coming in. As the set is connected to an aerial, these oscillations will be sent out exactly as radio transmission. They will cause interference on all the receivers in the neighborhood that are trying to receive signals near the same frequency as the oscillations. The result is the familiar squeak. For his own sake as well as his neighbor's, the radio fan will do well to promote "courtesy of the -ether" by keeping his set from reaching this oscillating stage which may be recognized by the mushy noise in the telephone receivers when oscillating occurs. Feed-Back Design. As a matter of design, the radio fan will have two ways of controlling the amount of feed-back. One consists in having a fixed number of turns in the feed-back coil (their number depending upon the wavelength to be received), and in adjusting its position with respect to the tuning coil. If the feed-back coil is directly inside or next to the tuner coil with the turns in the same direction in both coils, the effect will be at a maximum. If the feed-back coil is moved away from the tuner coil or turned with respect to it, the effect will be decreased accordingly. Another way is to have the feed-back coil sectionalized with taps so that more or less turns may be used. With more turns, the feed-back will naturally be greater. It is also to be observed that more "tickler" action, as this feeding back is frequently called, is necessary for the longer wavelengths than for the shorter. The feed-back may also be accomplished by a condenser connection from the plate, back to the grid. Still another way is to allow the condenser action of the plate of this tube to react back on the grid inside the tube. In the latter case, it is necessary to put a tuning coil called a "variometer" in the plate circuit of the tube, so that it will tune this circuit of the tube to the frequency of the desired signals. These methods of course require more adjustment. Operation. The radio fan should tune his set and adjust the feed-back or "tickler" at the same time. When the desired signals are heard, the tuner should be carefully adjusted for maximum signal strength and then the " tickler" should be carefully brought up to the point just below which "howling" occurs. If the howling starts, the "tickler" should be reduced rapidly and then brought up to a position just below the critical point. Distant Signals. It is possible to find distant weak signals by using a little more "tickler" than usual. This will produce a singing note when the set is nearly tuned to the desired signal. By careful tuning, this note will be made to decrease in pitch to a vanishing point or "zero beat" position as it is called. The "tickler" action should then be reduced until articulate speech is heard. It must be remembered, however, that all the stations in the vicinity will receive the benefit of this experiment in the form of the squealing above referred to, so it should not be done when they may be listening to broadcasting on nearly the same wavelength. Major Armstrong. Two little points of his