ASTATIC TUNING COILS

Sun (Auckland), Volume II, Issue 563, 16 January 1929, Page 14

ASTATIC TUNING COILS

OVERCOMING OSCILLATION DIFFERENT METHOD OF WINDING In home-made multivalve receivers intended for long-distance reception, probably the most common and most troublesome operating difficulty met with is the tendency for the receiver to oscillate uncontrolledly. In any receiver employing tuned radio frequency amplifying circuits, the tendency to oscillate will alway tend to appear. In the better type of commercially produced receivers, much research has been employed to find effective means of preventing unwanted oscillation. Many different methods of checking it have been designed, but in the majority of cases, amateurs find it difficult to employ them satisfactorily. Carelessness of layout, due to the fact that the biulder does not understand the cause of unwanted oscillation in a receiver, often aggravates the truble. The reason that the radio frequency amplifying valves, and detector valve, may be set in oscillation accidentally, is not difficult to understand. If a receiver incorporates one stage of radio frequency amplification, and a detector valve, it will contain at least two tuned circuits. One of these will control the grid circuit, or input circuit of the radio frequency amplifying valve. The other will be in either the output circuit of the radio frequency amplifying valve, or in the input circuit of the detector valve. Impulses impressed on the first of the two tuned circuits will be amplified by the radio frequency amplifying valve, and strongly repeated in the second tuned circuit. In many circumstances, the two circuits can be so placed that a portion of the energy in the second circuit is re-transferred to the first tuned circuit. It then operates the first valve, is amplified into the second circuit again, and part is repeated back into the first circuit. The cycle is repeated indefinitely, and it has the effect of maintaining a series of pulsating currents in both circuits. In other words, the valve is oscillating. It is worth noticing that this is the very effect deliberately used for signal amplification in all forms of regenerative receivers. In these receivers, a portion of the amplified energy from the plate circuit of the detector valve is deliberately transferred back to the grid circuit, and as a result, a substantial amplification of signals becomes possible. OSCILLATION CONTROLLED. In the ordinary regenerative receiver, however, oscillation is easily controller It will be clear that if a radio frequency amplifying receiver is to be prevented from oscillating, it is necessary that care must be taken to see that there is no possibility of transference of energy between the various tuned circuits it contains. Since the electrical field round a tuning coil is usually extensive, it is often necessary to check this interaction effect. It is seldom possible to do it by separating the coils, because the space limitations of most cabinets require that all components must be assembled fairly close together. It is, however, possible to protect the coils from each other by shielding with metal shields. Most of the better modern receivers are completely shielded in this way. In the well-known circuits of the neutrodyne type another method of preventing interaction is adopted. The coils are so arranged, with respect to each other that although fairly close together, their fields do not interact. This is effected by placing them parallel, and at an angle to the baseboard on which they are mounted. Since the angle at which the coils must be placed varies with the physical dimensions of each coil, it is seldom easy for home-made neutrodyne coils to be adjusted without difficulty. NEW WINDING METHOD In the last few months a method of coil winding, which practically eliminates the stray field round tuning coils, has been employed in home-made receivers with considerable success. The principle, which is known as the astatic principle, has been widely used for other purposes for many years. In effect, it has the effect of causing each coil to neutralise its own field, so that it cannot interfere with other coils. The method of winding the coils is simple, and the system provides probably the simplest method which can be adopted by amateurs to stabilise radio frequency receivers. It is of special value in receivers employing valves of the screened grid type. The coils must be wound on cardboard cylinders in the usual way, but they are wound in two sections instead of one section. The direction of winding in one section is opposite to that in the other, and, as the two sections are placed on the same tube fairly close together, the stray field of one section neutralises that from the other, and practically eliminates any field outside the coils. Assuming that No. 24 gauge doublecotton covered wire is uifed for the windings, each coil for the ordinary broadcast wave lengths should consist of about 60 or 70 turns on a former three and a-half inches in diameter. The winding is begun in the usual way, and half of the total number of turns for the tuner are wound on to the tube. At that point the winding is stopped, and resumed in the opposite direction about half an inch farther along the tube If the coil is to operate satisfactorily, it is essential that each half should contain the same number of turns, and that the space occupied by each section should be the same. REVERSE WINDING It is most important, also, that the direction of winding should be reversed after the first section has been placed on the tube. If tuners of the transformer type, that is, coils containing a primary and secondary winding, are required, each winding is made in the same way. The most effective method of arranging the coils is to wind the primary and secondary windings on separate cardboard tubes, one of which is sufficiently smaller in diameter than the other to slip inside it. Each coil is wound in two oppositely directed sections, so that the centres of the two sections on each coil are the same distance apart. When the inner tube is slipped inside the outer one each half-winding on the inner coil should lie directly beneath the corresponding half-winding on the outer one.