Loose Coupled Crystal Circuits.

Radio Record, Volume I, Issue 42, 4 May 1928, Page 12

Loose Coupled Crystal Circuits.

SELECTIVE AID GOOD FOR DX

N February 17 was described a ‘simple method of making spiderweb coils on cardboard formers, and at the same time the construction of a one-coil non-tuning crystal set was described. In the following issue, the 24th, a crystal set with two-coil tuning was described. A simple but ‘more selective type of circuit is now to be described. With the: use of a secondary tuning circuit it is often possible to obtain an increase in volume of signals over that obtained from a single circuit. _.-The diagram: of the circuit. is "shown with ‘a variable condenser for each coil, primary or aerial, and secondary. If only one station is t- be received, the primary condenser can be dispensed with by tuning the coil to the station’s wavelength by fnding the exact number of turns. By adjusting another coil to suit another station this could also be tuned in without .a condenser, by changing the coil. | WLUG-IN coils can now be pur- »« Ghased very: cheaply, and failing the, usual type of holder, two plug-in bases ,could be procured, one to be fixed, the other to slide backwards ‘and ‘forwards on a small square of wood running in grooved strips. » Another method of obtaining fine ‘tuning on erystal receivers is by using coils with taps connected to a switch, and a small capacity or vernier, condenser to complete the tuning. There are at present on the. markét a number of five-plate vari-: able condensers’ of good make, com‘plete ‘with knob and dial, that have’ been taken out of altered factory receivers, These are being ietailed at a very low price, and are very suitable for tuning any coil that can be adjusted to. be only a little short of.the required wavelength. Spiderweb coils similarly adjusted could also be used, a separate coil being provided for each station to be tuned in. "THE primarr could conveniently be a.60-turn spider-web coil with taps ag shown. When winding, a smali loop is twisted in the wire at each tap and tagged with a piece of stamp edging to show the number of turns. A connecting wire is afterwards soldered to each turn to form.the taps which are connected to the appropriate switch studs, which may be the heads of short brass screws in the baseboard. A switch arm is made from a strip of 18’s brass sheet, drilled at one end to take a screw for pivot. The tapped coil is the fixed one, so that the taps present no difficulty. ‘The secondary coil may have 35 turns and may be tuned by a .0005 mfd. variable condenser. This coil mounting must be so arranged that

its distance from the primary may be varied from close up or "tight coupled" to one or two inches away or "loose coupled." This may be accomplished either by a hinge arrangement or a sliding device of some kind. The connecting wires must not be too stiff or the coil may not "stay put.’"’ The gauge of wire for the coils may be 24’s or 26’s 3.w.g., the former preferred. THE Winding of the tapped coil is carried out by putting on 40 turns with a tap at every ten turns, and to complete the 60 turns, 12

turns are put on, then a tap, and a tap at every second turn to the end. By connecting these taps to switches as shown, any even number of turns that may be required is easily obtained. With such an arrangement it has been possible to dispense with a condenser, but a vernier is advisable for best results, and is connected "in parallel’? across the two extreme ends of the coil. It is an advantage in crystal circuits to do as much tuning as possible with the inductance and employ condensers of as small capacity as practicable. ITHER of the circuits shown will give good results on long distance if carefully constructed. By trial it is possible to adjust the number of secondary turns to suit stations near in wavelength so that orly a vernier need be used for the secondary tuning, but the .0005 will be found more satisfactory, as it will cover the whole broadcast band.