The Construction of Tuning-Coils

Radio Record, Volume II, Issue 51, 5 July 1929, Page 28

The Construction of Tuning-Coils

For all Purposes

By

MEGOHM

UNING coils for radio receivers are now seldom made greater than three inches in diameter, whilst -for valve sets particularly there is now a great ten‘dency toward 2% and 2-inch coils. In short-wave receiverers the diam~*er may be as little as idin, as illustrated by the valvebase coils now so much in nse, and from which high efficiency is obtained. Short-wave coils have few turns of wire, even when of small diameter, but as reducing diameter increases the number of turns necessary to tune to a given wave-length, too great a reduction in the diameter of broadcast coils is not always desirable, because in order to avoid a great number of extra turns, a wire of comparatively thin gauge must be used, Radio-frequency currents travel only on the surface of conductors, so that a wire giving the most outside surface presents least resistance to such currents. A hollow metal tube is just as good a conductor for r.f. currents as a solid conductor of the same diameter. In order to increase the "skin" surface of wire it is sometimes made up of strands of thin enamelled wire, the whole being then silk-covered. For radio-frequency coil construction this wire may be composed of, say, twenty strands of 36’s wire, and is known as "Litzendraht" wire. This wire is expensive, and though efficient on the broadcast band, but on account of the increased capacity introduced by the strands, it is not suitable for use on short-wave coils. For the average aerial or secondary tuning-coil, wire ranging from 20 to 24's, 8.w.g., Is suitable. Where it is desired to get the most from a erystal set, 2 low-loss coil wound with 20’s wire is suitable, but for local reception up

to a few miles, and where selectivity is not required, cotton-covered or enamelled wire of any gauge above-mentioned, and wound without spacing, may be used. The use of cotton-covered wire has the advantage of automatically spacing the conductor by virtue of the two thicknesses of covering intervening between the turns, although close wound. The inductance of 4 coil is stated in "microhenrys," or millionths of a henry, which latter is the unit of inductance, The average constructor does not need to deal with this measurement, so suffice to say that the inductance of the average broadcast eoil is between 150 and 325 microhenrys. Types of Induction Coils. "THE type of coil now most used in radio receivers is that known as a "solenoid," which consists of turns of insulated wire wound in one layer upon a cylindrical former, the turns being wound either close together, or slightly spaced apart, from a half to the full diameter of the wire. The honeycomb coil, once popular, is now little used. It is a compact type of coil, made to plug into a suitable holder; the turns are wound in-and-out on spokes radiating from a central hub, the spokes and hub being afterwards withdrawn and a plug-in block attached. Spider-web coils are a simpler but rather less efficient type of compact coil. They are wound in slots around a flat celluloid or cardboard former. These are also called "basket-coils." Formers of cardboard

or other material upon which to wind these coils may be obtained at the dealers for a small sum. In the construction of any coils wound in slots or over pins or spokes, there must always be an odd number of slots or spokes.

In following specifications given in descriptions of valve receivers, constructors should keep as nearly as possible to the measurements given, as to diameter, turns to the inch, and the total number of turns. If the winding is spaced a greater amount than specified, the range of the coil and associated condenser will be reduced, with the consequence that 7ZL, 3AR, and even 2FC, all high wavelengths, may not be received. The usual’ remedy in such a case is to add turns to the coil or coils. The spacing of turns makes the recevier more "selective," which means that it will tune sharply, making it an easier matter to tune out stations on a. wavelengh not many degrees away from that being received. Actual Construction. [HH simplest coil for a beginner making up a crystal or small valve set is the solenoid of double cottoncovered copper wire on a cardboard former. Cylindrical or ribbed formers made of various materials may be purchased at any radio dealer’s, the required length being supplied. Any of these are suitable for coil construction. If the constructor desires to make his own former, this may easily be done. Thin white cardboard that will roll up easily should be procured from a

. ~ 5 printer or elsewhere. .The white ecard known as ‘"4-sheet" or "6-sheet" is suitable, and will be in _ sheets measuring 25 by 20 inches. Irom a table get the number of turns to the inch of the wire to be used, and from this calculate the total length across the required number of turns. These tables are given in ‘Listeners’ Guide." Add to this one inch to allow a half-inch clear at each end. We shall suppose the total length of the cylindrical former is to be five inches. A strip this width and 25 inches long is to b |; from | the sheet of card. A cylit® slightjy under three inches in di.Mneter is now required upon which to build the former. A round bottle is good, if one of the correct size can be found, This should be covered with one thickness of thin paper to prevent accidental sticking of the former. The paper is only to fasten to itself ‘by its adjacent edges, and not to the bottle; be sure of this. The measurement around the bottle will be about 10 inches, so that a 25-inch strip will go round 24 times. The finished thickness should not be less than 1-16in., so unless the card is sufficiently thick a further strip must follow the first when constructing. Seccotine is a good adhesive to use. One end of the card is rolled rourid the bottle and secured with a sparing amount seccotine where it meetsY the other portion of the card. Strips of seccotine are applied at frequent intervals, rolling up the card, and when the necessary thickness is built up, a waste plece of . card should be wrapped round the outside, and the whole bound up with twine and put aside to set for a few hours. When Set, a coat of shallac, dissolved in methylated spirits, will be an improvement, though not an actual necessity. Its function is to keep dampness away. The former should be kept on the bottle during the process of winding, but should be tested to ensure that it is removable. Wire of standard gauge (s.w.g.) is now required, according to the specifications being followed. Wire is sold by the pound, or quarter and ‘halfpound, and as a guide to the quantity: likely to be required for a coil, the following ‘quantities are given. hen ordering wire it is always best to @rder more than the exact amount Gired, as any surplus will always co: ful for connections. On a 3in. former, quarter-lb. 20's d.c.c. wire will suffice for 70 turns, and enamelled a few more turns. The same

weight of 22’s d.cc. will make over 100 turns, and enamelled about 120 turns. Two ounces of d.e.c. or enamelled 24’s will make about 90 turns. On @ 2%in. former, quarter-lb. 20's d.e.c. will make 80 turns, and enamelled about 88, and for 100 turns just over Soz. of 22’s d.c.c. or under that quantity of enamelled. One ounce of 26’s d.c.c. makes over 75 turns. These figures allow a small margin over for connecting up, but if the former is even slightly over the specified diameter, more wire will be required ----

Winding on the Turns. AVING the wire as specified, two small holes are now made half an inch from one end of the former, sliding it partly off the bottle for the purpose. Through these holes the end of the wire is passed, leaving six inches spare for connecting. The former can now be placed on the knees, with the spool of wire on the floor. Commence winding at the left-hand end, turning the coil towards you, and guiding the wire with the left thumb, using suffi-

cient pull to straighten out ordinary unevenness. Sharp kinks should be straightened by special treatment. When the requisite number of turns has been wound,.pass the end of the wire through two holes as at the beginning, and leave a sufficient length over for connecting. ‘Low-Loss Coils. HE usual low-loss coil is made of plain tinned.or enamelled wire wound with a space between each turn, half the diameter of the wire being a good proportion for the spacing. For short-wave reception heavy wire is used, spaced its own diameter apart when necessary. Whether the wire is tinned or enamelled matters little, but the latter ensures that turns will not

"short" if accidentally touched owing to being misplaced by a knock. Suitable gauges of wire are 20’s to 24’s, and the best method of spacing is to wind an enamelled wire of several sizes smaller, alongside the wire composing the coil: This thinner wire is afterwards carefully unwound, leaving the coil turns ready to be secured to celluloid strips with celluloid cement. These strips, about half an inch wide, may be three or four in number. When the winding is complete, the permanent wire is secured through two holes at the end of the former, and the spacing wire is then carefully unwound without disturbing the position of the permanent turns. Thin celluloid cement is now liberally applied to the turns where they cross the celluloid strips, and when this has been done, a duplicate strip of celluloid is pressed into place above the turns and underlying strip. Put aside to set overnight. The cardboard former, may be shaped upon a suitable jar or bottle as al-_ ready described, or made by the alternative method now to be outlined. By | this system, formers of any exact dimension may be easily constructed, '

and once made, can be used as ofteng as desired. . ‘A number of circles of cardboard are cut, the diameter being less than that of the inside of the finished coil by twice the thickness of the former. A number of small squares of wood, 3 to lin. thick, are also required. A hole is drilled in the centre of each circle and block, and the whole can then be threaded upon.a piece of twitie, which is secured at each end by twisting round a nail or otherwise. This pile of circles can be used to make the former on just as a bottle is used, but it has the advantage of being withdrawn without trouble when the coil is complete, and if the former is split lengthways it may be used repeatedly. See diagram. Some constructors -yse 2 tin for the former instead board. Stock Coil Formers. MANY types of coil former are stocked by radio dealers, a convenient form being composition tubing from which the required length may be sawn as required by the purchaser. This tubing may be plain circular or may be cast with six or more projecting ribs running lengthways. Upon these ribbed formers good low-loss coils may be wound without the preliminary labour of constructing the former. Another type of ribbed former is the six-pin coil with plug-in base. This former is cast in bakelite, with 12 small ribs, The diameter may be 2 3-8 or 2% inches, and length 2} to 5 inches. ‘Three separate coils, prim ary, secondary and tickler, may be wound upon these formers, the whole being plugged into the corresponding base with one operation. Some are provided with notches on the ribs ta facilitate spaced winding, otherwise spacing may be obtained by using double cotton-covered wire, or .by the method already given, securing the turns with cement. Celluloid Formers. YVHEN the coil is to remain upon the former it may be constructed of motor-hood celluloid, about 20 mills thick, a very neat-looking coil resulting. A former must ‘be used around which to shape the celluloid with an overlap of not more than }+ inch, A strip of paper under the joint prevents the celluloid from adhering to the former. When the joint has been made, wrap many turns of twine around the whole, and set aside until next day, Wind the wire before removing ea celluloid from the former. If the ¢#iluloid is thick there will be more difficulty in making a neat join and tendency to pull out of shape. ‘Primary and Tickler Coils. PRIMARY and tickler coils vary cousiderably in construction. In some cases these are wound on insulating strips over one end -of the secondary coil. to give the required space between coils. In the Browning-Drake the primary is invariably placed inside the first turn of the filament end of the secondary coil, and must have as little "self-capacity" as possible. Sixteen ta twenty-five turns, according to the impedance of the valve, are usual, and the capacity is kept low by winding the turns irregularly or jumbled, so that they cross and recross as much as possible. Double cotton-covered wire assists in giving separation, the usual gauge being 30’s &.w.g, . .

There are three typical ways of mak- ¥ ing this coil, the diameter in every case being such that the coil can be slipped inside the end of the secondary without leaving an appreciablé space between the two coils, though there must be proper insulation between them, otherwise a "short" of the B battery may résult.. The three methods are shown in a diagram. One is a bunch of turns of the requisite diameter, bound together in places with twine. The second method is to build up a slotted former of cardboard, the slot round the periphery measuring barely 3-16in. each way. The third method is to turn a similar former out of wood. _ Rotating ticklers for the BrowningDrake are close-wound upon a short foyter of small dimensions; the turns shbuta.not be spaced on ticklers. There is ¥'o neéd to endeavour to make the tickler as large as can be rotated in the end of the.secondary. A smaller tickler with a few more turns to compensate, will give even better results. Mounting Coils. ARIOUS methods of securing: solenoids to the baseboard are shown in a diagram. Other suitable means will be devised by the constructor to meet special cases. Small metal angle-pieves may often be bolted to thé former and fastened to the baseboard with screws. Short-Wave Coils. HIS subject would make an article to itself, and can only be briefly dealt with here. The making of valvebase coils is usually dealt with in the _Specifications of receivers. For coils £ larger diameter 18’s wire is often ‘used, on the low-loss plan, only strips of celluloid holding together the turns, which are usually few in numberfrom 8 to 20. The coil may be secured to a strip of ebonite by bolting it down through extensions of one of the celluloid strips, or by bolting across the inside of the coil, another small strip of ebonite. The tickler, of thinner wire, may be attached to the projecting celluloid strips of the secondary coil, and connected to pins in the ebonite mounting strip so that it plugs in at the same time. The aerial coil, spaced, of few turns; is placed near the grid erd of the secondary; it may be made to move on a hinge, or to be variable in some other convenient way. General Points. (OILS associated with the aerial do not require as many turns as a secondary coil, as there is the added (faprsity of the aerial. By coupling the adeygl to only a few turns of the first sta¢v@ funing-coil, as in the BrowningDrake, the added capacity of the aerial is very small, with the advantage that the coil and tuning condenser will then cover a greater range than in the case of the aerial being coupled to the full coil. Loose coupling gives a similar advantage. Coils with spaced turns require more turns than unspaced ones of the same diameter if required to cover the same range with the same capacity of variable condenser, but they give greater selectivity. -4 If theré is any uncertainty as to the mber of turns required on a coil to tuhe in certain wavelengths, the best way is to put on too many turns. When the receiver is completed, it is easy to remnove a few turns so that the highest wavelength station required is just tuned in when the condenser is full in. Coils of large diameter and short winding length are the most efficient,

theoretically, but modern practice does not always recognise the equal-dia-meter-and-length compromise, a8 compactness is often a vital consideration, especially in shielded receivers.Useful tables will be found in the Listeners’ Guide concerning the number of turns required upon coils for yarious. purposes. . Celluloid: Cement. (CELLULOID cement is easily made by dissolving chips of celluloid in a small bottle with liquid acetone, Half fill the bottle ‘or test-tube with chips and pour in acetone to about double the depth, otherwise the cement may be too stiff.. Apply with a thin stick. Keep well corked, and away from a naked light.