A CRYSTAL RECEIVER WITH TWO-COIL TUNING

Radio Record, Volume I, Issue 32, 24 February 1928, Page 10

A CRYSTAL RECEIVER WITH TWO-COIL TUNING

SIMPLE TO MAKE, EFFICIENT IN USE

AST week the making of basket or spider-web coils was dealt with, and it was also shown how one coil could be made to permanently tune-in the local station by finding the correct nuniber of turtis to give maximum signals. It wili-now be shown how two coils may be made with a suitable number of turus and provided with a simple means of varying their distance apart or "coupling," so that any broadcast wavelength may be tutied in, A coil with 35 turns and another with 25 turns will tune-in 2YA, and will no doubt tune-in 1YA and 3YA also. Possibly for 4YA it might be necessary to increase the 25-turn coil to 80 or so. The correct wire is 26’s s.w.g, enamelled. , ‘Khe circuit and other arrangements will be the same as for the one-coil arratigement described last week, only instead of screwing one coil to the back: edge of the board we sliall eniploy two coils and miake them both moveable, as by the simple method shown it is easier to make both move than to make one move and have the other fixed. Four pieces of wood 2} by 1 by jin. will be required, the amount of finish given te them being determined by the constructor. Two of these are drilled 18 in. for two screws as shown (A). ‘The ether two, which will support the coils, are to have holes bored at the ends with a thin awl 1-8in. from the lower edge, and central with the two sides (RB). The blocks with the screw-holes are now taken, and 1t-8in. below the top edge two lin. nails are driven in part way, jin. apart, a shade more, rather than less. The heads are now cut off these nails, leaving less than fin. projecting. The next operaticn is screwing the coils to the moving strips, and this is best done by means of a washer, under which the connecting wires can be clipped. When making basket coils thes should all be commenced, wound, and fittished in the same way, and a lares uumber indicating the turns marked on the side on which the beginning of the coil comes through the central pertion amd crosses to commence thie turns. Be careful to clean all euamel eff top surface of turns of wire round edxes of the securing slots before fastening down with the washers. The twe coils ate to be fastened to the two moving pieces, so that when the coils are put close together the numbered sige of each is facing the operator. Then the left-hand of the back coil connects to aerial and crystal; the right hand is connected by two or three thin Wires (for flexibility) to the lef, hand of front coil, and right hand to earth and "phones. The other end of crystal is cotinected to the remaining ’phone clip, and all is ready when the earth eud aerial are connected. Tuning is eftected by varying the distance between the two coils until the miaxitium volume is obtained. The

higher the wavelength the closer will the coils couple. If the coils are separated as far as possible, and yet a station does not appear to be at maximum, then one of the coils should be exchanged fur one of fewer turns, High waveletgths require most turns to tutiein. This is a simple two-coil outfit, but a regular two-coil holder and plug-in coils can be purchased ready-made if

desired. If thicker wire than 26’s is used, more turns will be required on the coils, as increasing the gauge decreases the wavelength for a given number of turns.