Radio-Frequency Chokes

Radio Record, Volume II, Issue 9, 14 September 1928, Page 27

Radio-Frequency Chokes

HEN in circuits such as the ReinA" * artz we alter the value of a conde*vser in series with an inductance so as to vary the amplitude of the radiofrequency currents passing through that inductance, the condenser itself prevents the passage of the direct current to the plate circuit. It is necessary, therefore, to feed the plate circuit with direct current. from the B battery by a parallel path, and if this path allows any radio-frequency current of apprecable magnitude to pass, the variable condenser designed for reaction control becomes useless, as the radio-frequency current will simply pass round the alternative path. For this reason, in the Reinartz circuit it is essential to take steps to prevent any radio-frequency currents passing through the telephone or transformer circuit, and thus we introduce a coil of wire of sufficiently high in-

ductance.und tow self-capacity to obstruct the passage.’ Unfortunately, it is impossible to design any coil to have no self-capacity, and every radio-fre-quency choke possesses this property in some measure. Bearing this in mind. it will be seen that any radio-frequency choke consists of an inductance shunt: ed by a capacity (Which latter, however, may be very small indeed), and is thus a circuit capable of resonance. provided the ohmic resistance is not too high. The d.c. resistance must be reasonably low, otherwise it would unduly restrict the passage of the direct current to the plate from the B battery. ww good r.f. choke can be made on a -gSlluloid frame constructed from _f Strong motor-hood celluloid joined ‘with cement made by dissolving chips / of the celluloid in liquid acetone which may be obtained from a chemist. The cement should be of a creamy consistency, and is best made in a small

-_~ bottle, the bulk of small chips being about half that of the acetone liquid. The cement is applied to joints with a small sharpened stick, and _ dries, solid in two or three hours if not too thickly applied. | Two strips of celluloid are cut din. wide by 4in. long, and one strip lin. wide by 4in. long. For a base a piece is cut the shape shown, lin. wide and 14in, from point to point. The diagram of parts is given full size so that the celluloid may be placed over it and the lines scratched in with a sharp point. The corners of the notches should be rounded as shown so that the wire will not catch on the edge when winding. Two holes are shown in the base through which Hin. screws can. be placed to hold the choke in position. The position of the upright strips is shown by dotted lines. The two din. strips are to be cemented to the centre of the inch strip, one on each side, cement being placed along each side of the joint. To do this the #in. strip can be supported on edge by a strip of wood each side,

the top edge cemented and the inch strip put in place, supported level by strips of cardboard packed underneath. The other in. piece is then cemented on and similarly propped up. When the cement is partly set. the supports may be taken away and the upright cemented to the base and left to dry. A useful choke for broadcast wave: lengths is made by winding 200 turns in each set of notches. making 1000 turns in all. Thicker wire may be soldered on at the ends for connecting up. A useful gauge of wire is 34’s or 36's. For short-wave a good choke can be made by omitting the notches and space-winding 30’s enamelled wire about 30 turns to the inch, so that 100 turns occupy nearly the whole length of the former. If desired, the turns can be secured by a little cement applied where the turns cross the edge of the celluloid.