A Dual Wavetrap

Radio Record, Volume V, Issue 8, 4 September 1931, Page 15

A Dual Wavetrap

Will Keep Both 2YA and 2ZW from Spreading

INCH "B" class stations have commenced to operate in the cities owners of unselective sets have been troubled with not only the separation of those stations, but also with the clearing of them from the ° remainder of the dial. It has been found that a simple wave trap. will reject one station and allow the alternative programme to be listened ce without interference. But owners three and four-valve sets have found the blotting of a large portion

of the dial by the untrapped station a serious handicap to-D.X. work. The function of the dual wave trap is to cut out both stations simultaneously. Simplicity of control and cheapness have been attained by the use,of | semi-variable condensers, or, as they | are better known, Formodensers, Once these are adjusted, the wave trap is set for all time, conditional upon the aerial and the set remaining unchanged. It can be used in conjunc: ~ tion with any set, and it is easily made and quite inexpensive. -It will be noticed that there are two coils and two condensers, the function of each circuit being to reject one station. Constructing the Trap. WE shall first deal with the construction of the wave trap and then explain how it is to be-worked. The parts necessary for the standard trap are shown:in a panel, t First build up the coil by taking gabout four inches of three-inch dia-

meter former and winding upon it the two coils. Use 26 dse wire. Li, for trapping out 2YA, consists of 50 turns tapped at the 45th and 35th turns, while the smaller coil L2 consists of 30 turns tapped at the twentieth and twentyfifth turns. It is not altogether neces-

sary to put in these taps for the numbers of turns given have been arrived at after a great deal of experimenting and are correct for 2YA and 2ZW Taps can be put in and connected up as shown in the optional circuit when conditions demand some deviation from the standard. This arrangement will be of use in cities other than Wellingon, It is necessary to separate the coils by a matter of some two inches or s0, the exact distance not being critical.

Taps may be made by twisting portion of the wire on the desired turn and afterward cleaning and soldering it, or by slipping a piece of non-inflammable material under the desired turns when the job is finished, cleaning the portion. of the wire and soldering on the leads, using heavier wire for the pu . The coil is to be mounted horizontally, and in order to keep the wire off the baseboard, two separators must be made to fit between it and the coil. A couple of fair-sized nuts will do splen- ‘ didly for this purpose. Small separa-

torg of wood could be pressed into service, Layout and Wiring. THE layout of the remainder of the components should present no difficulty, as all positions are shown clearly in the diagrams. Wiring is a simple operation, but it. must be noted that a considerable amount of it goes underneath the baseboard, which is ‘held from resting on the ground by either slats of wood or by a suitable frame. The latter makes the neater job. (Concluded on page 30.)

Parts List for the "Dual" ‘Trap ow 2 type G Formodensers _2 terminals ‘Four inches ‘of Sin, former 4lb. 26 d.s.c._ wire, Switch (single) Panel, Zin. x 4in, Three-ply base board, Tin, x 4in.

Coil Specifications Former Sin. diameter wire 26 d.s.c. Li 50 turns tapped at 45th and 35th . turns, L2 36 turns tapped at 20th and — 25th turns . Most effective results on 2¥A and 2ZW have been obtained by using the whole of both coils.

| ie (Continued from page 18.) ‘Dual Wavetraps

Operating the Rejector. put the device into. operation, connect the aerial to the appropriate terminal (marked aerial) and join the other terminal by means. of a short piece of flex to the aerial termina! of the set. Loosen the adjusting screws on both Formodensers so that there is minimum capacity across each coil, Now tune the receiver to 2YA -just off the loudest point and with the switch closed. turn down, turn down the screw on the Formodenser until the station becomes perceptibly weakened or disappears. This operation will slightly alter the setting of the tuning condenser on the set, so make the required readjustment and endeavour to reduce still further the signal strength by careful adjustment of the right-hand condenser. If complete rejection is not possible change over to different tappings and then try again. With the switch still closed repeat the performance on the next Formodenser, by tuning just off 2ZW. The use of the switch enables one of the traps to be thrown out of the circuit when one station only is operating. If both were off the air neither trap need be used and for the purpose of cutting both traps out a further switch could be used in the lead going from the bottom of the right-hand tuning condenser to the lower end of the tuning coil, In the optional circuit greater variety is allowed for, and the twoway switch makes possible the rapid changing from one trap to another.