FOR RADIO LISTENERS.

Hawera Star, Volume XLVI, 6 October 1926, Page 2

FOR RADIO LISTENERS.

MORE ABOUT REACTION. CAUSES OF FADING. (By “Senex.”) As mentioned last week, reaction to improve the efficiency of a radio receiver must be used with care and gentleness in order to give satisfactory reception of broadcasting. Tiie post office authorities have very wisely prohibited the use of the twocoil circuit, where reaction is direct on to the aerial coil. In any arrangement of regeneration it is difficult to prevent self-oscillation • from “feedback,” even in the three-coil circuit, where reaction is in connection with the secondary coil and the aerial turning coil is moved quite a distance away from the others. The great need of suppressing “feed-back” has exercised the ingenuity of many inventors. Scott-Taggart, in England, and Professor Heseltine in America, brought out almost simultaneously the principle of neutralisation.

The Neutrodyne receiver is manufactured under Heseltine patents, the radio co-operation having control of the patients previusly acquiring the Scott-Taggart ones. The principles underlying this interesting type of receiver will be explained in a subsequent article. The best reception is obtained when the reaotion control is just short of the receiver breaking into oscillation. Oscillation is evidence of over-reaction and is productive of squeals and howls, distorting the reception of music or speech, and, at the same time, interfering with other reception in the vicinity.

Other methods of getting a receiver to operate smobthly as regards regeneration are as follow: i

1. Adjusting the value of the gridleak on the detector or rectifying valve. This is generally of two megohms (2,000,000 ohms) resistance, various sizes of leaks from one to five megohms should be tried. The correct one will depend principally on the type of detector valve used.

Variable grid-leaks are sometimes used, where uhe adjustment is obtained by screwing a knob out and in. It must 'bo remembered that the ordinary lead made of graphite will alter in value as time goes on, owing to fine changes taking place in the material used. The newer type of wire-wound lead is free from this defect.

2. By altering the “B” battery supply to the plate of the detector valve. The higher the voltage the more tendency to oscillate. All depends again on the type of valve. A soft valve may require 18 to 24 volts, whereas a harcl valve may have 45 to 60 volts. Any valve of a particular type may require a different setting from another valve of the same class. By a moderate clip on the “B” battery connection, different voltages can 'be tried out until the correct one is obtained by testing for clearness and loudness of signals without oscillation taking place —the golden rule being to do with as little “B” battery as possible. 3. Poor connections in the receiver itself and outside it. All battery connections should be looked to~ and tightened up. With a piece of stick — a pencil would do —tap all the connecting wires inside the apparatus and see if any are loose or unsoldered. Do'not Use a screw-driver, or else the metal will cause a short-circuit between wires carrying the low tension or “A” battery and the high tension or “B” battery, with the result of burntout valves—an expensive experiment. 4. Where the oscillations are impos-

sible to control, a high resistance may be put in the ground wire to the set. One end of a potentiometer of 400 ohms is connected to the ground post oT the receiver, and the slider to the earth itself. By carying the amount of resistance, a point will be reached when oscillations in the receiver will cease. FADING.

When listening to a broadcasting station it' may be found that the

signals get fainter as if receding in the distance until they may go out altogether, gradually coming back again. The phenomenon of fading may be due to several causes. At the transmitting station a swinging aerial or a variation in current supply are the chief causes. Certain atmospheric conditions will produce it. Again there are certain localities practically prone to this annoying feature. At the receiving end a swinging aerial, faulty ground connection, a fault in the receiver such as imperfect grid-lead, and the running down of the batteries may .all be contributary causes. To prevent swinging the aerial wire must’ be kept tight, so as to stop it swaying with the wind and thus altering its capacity to the earth. Means of attaining this will readily suggest itself to the reader. Where a rope is used as a halyard, a heavy weight can be suspended at the end so as to keep it taught. Rope or sash-cord will contract when it is wet with rain and slacken when it is dry. The weight will make up for this variation due to climatic conditions.

The earth, or ground connection, may be made of wire, pipe, or copper sheet buried at least two feet in the ground, and where the ground is damp. The cold water pipes of a house make a good earthing system. It is very important to see that the wire running from the earth terminal of the receiver to the grounding material be made of well-insulated wire of heavy gauge, and a well .soldered joint made. A bare wire will come in contact with grounded objects e.g. wall or floor of house, on its way to the aqtual ground —a condition of “partial” earth which must be avoided. The batteries must be tested from time to time and recharged or replaced if they are getting run-down. Finally, fading may be caused by intereference from neighbouring oscillating valves. So we are back again to our old hobby-horse, apd, although we do not want to over-ride it, let us stress the point again. To have comfort in one’s own reception and to let others enjoy the excellent entertainment provided by modern means, let those with receiving apparatus using regeneration exercise considerate care. Where a broadcasting station begins to fade, do not immediately jump at the receiver and twirl the dials; but wait patiently. Within half a minute or so in all probability the station will come back again.