MAKING SPIDER-WEB TUNING COILS

Radio Record, Volume I, Issue 31, 17 February 1928, Page 12

MAKING SPIDER-WEB TUNING COILS

HANDY AND COMPACT FOR CRYSTAL RECEPTION

HIS is a simple method of making " tuning-coils having a similar effect to the honeycomb type. Owing to the style of construction, this type of coil is most suited to turns numbering not more than a hundred, However, this is no hardship, as one hundred turns have a minimum wavelength of over 800 metres-much above the broadcast level, and the coils most required have not more than 50 or 60 turns.. |

MAKING THE COILS, A support for the wire has to be provided, and this may be either motor-hood celluloid, or hard, thin eard, such as Bristol board. The former is first cut to shape by marking out a circle of suitable diameter for the number of turns to be put on the coil, 26’s s.w.g. wire being a suitable gauge. If there is any doubt about size, mgke the circle oversize, as it can easily be trimmed down afterwards. Fifty turns of 26's enamelled wire occupy one inch, and 75 turns one and a half inches, so sizes may easily be reckoned. Now the making of a fifty-turn coil will be described as an example. With a pair of dividers or compasses, radius 1fin., describe a circle A, 3zin. diameter. Then with radius 5/8in., describe the circle B. Now mark off the square portion, from circle to edge, C, 4in., and length D, 2in. Now the two slots G, H, are cut, their centres being 11/8in. apart, 3/8in. deep, and just over 1/8in. wide. Nine slots are to be cut as shown, equally spaced, each 1/8in_ wide. Note that the slot at C is to be marked first, and others will come in the right positions. It is an essential feature that there should be an odd number of slots. Winding is commenced by piercing a hole at E, passing four inches of wire through, then proceeding to slot F, passing the wire through that, under the former, up through the next slot, down through the next, in the direction of the arrow, always pushing the wires towards the centre. This winding is continued until the requisite number of turns has been wound. It will be seen that the odd number of slots causes the turns to come on opposite sides of the former, automatically spacing them out. Counting across the turns on one side of the coil gives half the totaJ number -of turns actually wound. The end of the wire is cut off with a few inches to spare, and is passed down through one of the holes at H and up through the other, then threaded in and out a few times through holes’ in the edge of the slot, then cut off close and the enamel taken off with glass-paper round the slot H. The beginning of the wire is then finished off in the same way. Such a coil may then be secured under two washers held by two screws the correct distance apart. .

WAVELENGTH OF COILS. The approximate number of turns for a given wavelength is given to enable constructors to provide a suitable number of turns for the station they wish to receive. For fine tuning it is necessary either to connect a variable condenser across the coil or to have two coils so that their distance can be varied to "tune-in’’ the station. A 40-and-30-turn makes a good combination, but depends to

some extent upon the wavelengths to be received. These wavelengths only refer to 26’s wire on a former as specified. | 20 turns......185 metres 80 turns,.....250 metres 40 turns......530 metres 50 turns......405 metres 60 turns......490 metres 70 turns......580 metres. By adding a few turns to the above in order to get above the wavelength required, it is possible to either twist taps into the last few turns of the coil, or tune-in and take turns off until maximum signals are obtained, and thus have a coil that tunes in the nearby local station without the use of a condenser or other tuning arrangement. This constitutes a very simple method of crystal reception, and might be fixed up as shown be-

sieeantememanenaendionsnsnaraemraateero ee low. A piece of board 5 by 3 inches will hold the complete outfit. The coil is secured by washers and screws into the pack edge of the board, the two connecting wires being also held under the washers. The crystal is clamped by its edge under a slip of 18’s brass sheet secured by a screw into the wood, the connecting wire being trapped underneath, and continued to the aerial. The other side of the coil connect to the ’phone clip and then to earth. The ’Phone clips are made of thin brass curled up as shown and held by a small brass screw. | About 50 to 54 turns brings in 2YA_ at short range of a few miles without any critical tuning, but at a distance careful tuning becomes necessary, and a tuning condenser essential. Even then if the number of turns on the coil is almost correct, but just below the wavelength of the desired station a vernier condenser of only two or three plates would accomplish the exact tuning. FACTS ABOUT TUNING COILS Whilst on the subject a few hints affecting the construction of coils generally will be of interest. A coil, whether spider, solenoid, or honeycomb, has a wavelength of its own, without any tuning arrangement entering in to the question. A condenser connected in "parallel" across the two ends of the coil increases the wavelength, but a condenser connected in "series," with one end to the coil and the other end to aerial or earth, reduces the wavelength of the coil to an amount depending upon the capacity of the condenser. If this condenser is variable, then we | have a means of altering the wavelength of the coil to any wavelength within a certain range. If a condenser is connected to a coil in ‘Yarallel," we cannot -une to a wavelength below that of the coil alone, and in fact with the condenser set at minimum, there would be a slight extra capacity added to that of the coil, so that the minimum wavelength tunable would be slightly in excess of the coil alone, Now it will | be seen that if we wish to tune over a certain range of metres with a coil and variable condenser, the natural wavelength of the coil must be a few metres below the minimum wavelength required, and the condenser must be of sufficient capacity to enable the combined effect to reach the maximum wavelength desired. If tuning is to be accomplished with a slider, then the coil must have a wavelength slightly in excess of the required maximum, and lower wavelengths are then tuned in by tapping a reduced number of turns with the slider. In deciding the details of construetion for a_ coil, the chief factors governing the wavelength are: 1. Diameter of coil, 2. Gauge of wire. 8. Number of turns.

4. Amount of space between turns. Increasing the diameter of the coil, keeping the same number of turns, increases the wavelength, so that we can increase the diameter and reduce the number of turns. to keep the same wavelength. Reducing the ciameter lowers the wavelength, so adding turns would keep it the same. Increasing the diameter does not effect a proportional increase in wat elereth. Doubling the diameter of a 50-turn, 2-inch coil with a maximum wavelength of 315 metres would only mcrease the wavelength to 505 metres. The gauge of wire being reduced, increases the wavelength. If a Sin. solenoid is wound wtih 50 turns of 20’s wire its wavelength will be 365 metres, whilst if the gauge of wire is reduced to 22’s it will be 390, 24’s 410, 26’s 485, 28’s 450, 30’s 470 metres. The increase of the number of turns increases wavelength. A 3in. solenoid wound with 20 turns of 24’s wire has a maximum wavelength of 215 metres, and if the turns are increased to 40, the wavelength is 355 metres: to 80 turns, 575 metres. Spacing the turns slightly apart instead of winding each one close to the preceding one, is known as "lowloss" winding. This spacing reduves loss by causing the radio-frequency currents to traverse the whole length of the coil instead of allowing an appreciable amount of the energy to take a short cut by skipping from turn to turn across the short path \between closely-wound turns. Spaceing turns slightly reduces wavelength but only a few extra turns are required to compensate for this. Spacing makes tuning more selective, that is, makes it less easy for a station to spread over a large number of de. grees on the dial.