Inductance Coil Design

Radio Record, Volume III, Issue 18, 15 November 1929, Page 30

Inductance Coil Design

. Effects of Screening

(Continued from last week.)HE problems encountered in. a re- ‘ ceiver without screening having been treated fairly fully, it remains to deal shortly with the special problems introduced by metal screens. It is fairly well known that introducing any metal object within the field of an inductance coil increases its losses, the loss appearing in the form of an inereased high-frequency resistance. What is not so well known, is that the inductance is also reduced by the presence of a metallic screen or other object in the field of the coil. Some readers will remember the "Peridyne" receiver once introduced in America in which this phenomenon was utilised for matching inductances so as to. permit ‘of single-control tuning. Older veaders will’ remember the once popular "spade-tuning" wherein tuning was effected by bringing a metal sheet nearer to or further from. the inductance coil, thus varying its effective inductance. The metal acts as.a shorteircuited secondary, the coil being the: primary, and the result that of loading the coil with a resistive load. From every point of view, then, it is imperative that the screening must be kept clear of the denser parts of the eoil’s field. Hxperiment has indicated that a sound working rule is to have all screening distant from the ends of the coils at least the diameter of the coil, and distant from the sides of the

coil at least half that diameter; thus a coil of diameter 3 inches and winding length 3 inches, mounted vertieally, would demand a.screening compartment of height 9 inches; if the same coil were mounted horizontally, a height. of 6.inches. would. sufiice. Where iv is desired to screen only a coil, rather than a complete high‘frequency stage, "Swan" cylindrical hot-water bottles (aluminium) with the top: cut off may conventiently be inverted over a 2-inch diameter coil, this being disposed. centrally within the adapted bottle; the coil must, of course, be mounted on a dise of aluminium to ~gereen the underside-it is hardly necessary to add that all screening must be connected to the filament cireuit. For winding broadcast coils,.a solid former should be used rather than 2 skeleton one of celluloid, as no appreciable loss will be introduced. The former should, however, be of high-quality insulating material, and cardboard should be avoided; hard rubber is

really best, but formica, bakelite,.’ or paxolin are almost equally good. A ribbed former may be’ used if desired, but it confers no particular advantages, contrary to popular opinion. We have now treated all the considerations which should gnter into coil design; a summary of the procedure of design is appended :- (1) From the considerations outlined‘in this article, determine the most suitable diameter and ratio of winding length to diameter to suit the particu‘lar circumstances. (2) Determine what value of tuning condenser is to be used, and from this the required value of inductance. (3) From the appropriate ‘chart read off the number of turns required to reach this inductance with the diameter and shape already decided upon. (4) Read off the optimum diameter of wire and ascertain the nearest standard gauge to this; this gauge should be used. ¢ (5) If the coil is to be close-wound, ascertain what class of insulating covering will distribute the required number of turns of the chosen gauge of wire over the allotted winding length. A close wound coil will show some loss of efficiency as compared with a space wound coil of the same dimensions and using the same gauge of wire. Silk-covered wire can usually be space-wound without a threaded former by winding a thread of. suitable thickness along with the wire.and between the turns; enamelled’ wire merely slips out of place if the former is not grooved, and even the silk wire must be wound very tightly indeed to avoid trouble. Any garage possessing a screw-cutting lathe will usually groove a former with the required number of grooves per inch. In eases where this can not be done a fine wire (uninsulated) can be. interwound with a thread to distribute the necessary number of turns over the _ allotted space, and a. heavy current passed through the wire to heat it sufficiently to burn a shallow groove -in the former; this little dodge will only work with a hard rubber former, however.