An Easily Built Wavetrap

Radio Record, Volume IV, Issue 45, 22 May 1931, Page 11

An Easily Built Wavetrap

Discriminating Between 2YA and 2ZW

Wit the opening of the new private station 2ZW in Wellington, many local listeners who possess crystal sets of the simpler type are experiencing interference between this station and 2YA, : This trouble lies solely, however, with the receivers, and not with the stations, as these operate on frequencies sufficiently remote from each other to preclude the possibility of interference on any but the most unselective of sets. And now for a cure. Three courses are open: Firstly, adaptation of the present receiver to give greater selectivity; secondly, . installing a wavetrap; and third, building a new receiver. These three remedies will be dealt with in turn. The first, that of adopting the present receiver to give greater selectivity, is of doubtful use, for if the set is to be dismantled the listener may as well puild the specially-designed crystal set to be described later. This will sharply separate the two transmissions, bringing in either with no trace of the other. However, the effect of placing 2 variable or semi-variable compression type condenser of the order of .001 mfd. max. in the aerial may be tried. In localities where the interference is not very acute this will probably sharpen tuning sufficiently to allow of a clear .eparation of the two stations. The next solution, that of using a wavetrap, is scarcely an economic one, for with the exception of the crystal detector the receiver is duplicated. However, for those who have the parts on hand, or prefer to build a wavetrap rather than rebuild their sets, the following description of a simple but efficient trap is published. Building a Wavetrap. OND of the simplest of wavetraps consists of a coil tuned by a condenser. The whole is placed between the aerial and the set (in series with the aerial lead), and the condenser

adjusted until the wavetrap is resonating, ie, in tune, with the station it is desired to eliminate. Thus the signals of one particular frequency are absorbed, and the rest passed ‘on to be detected in the usual way by the receiver. This, in brief. is the theory of the wavetrap. The trap mentioned above is not the most efficient in operation, but it has the advantage of being cheap and extremely easy to construct, even by those who know nothing of radio.

It has been designed particularly to enable those owners of crystal sets who are experiencing difficulty in separating 2YA and 2ZW, to elimimate completely one or the other. The materials needed are one variable condenser, capacity .0005, tb No. 26 enamelled copper wire, 1 three-inch length of cardboard former, 2in. in diameter, 2 terminals, 1 crocodile clip, baseboard 4in. x 5in., and panel (wood or ebonite) 4in. x 5in. Winding the Coil. "THE coil consists of 70 turns of No. 26 enamelled wire, close wound, and tapped every tenth turn, making six tappings in all. Commence winding operations by piercing two small holes, about jin. apart, with a bradawl or knitting needle, about 4in. from the end of the eoil. Thread: the wire through these holes, leaving two or three inches for connecting purposes, and wind on ten turns. At the tenth, keeping the wire taut, make a twist in the wire, forming a small loop about in. long. Proceed thus, making a tapping at every tenth turn, until the 70 turns are wound on. The seventieth turn is, of course, not tapped, but the wire is threaded through two small holes similar to those used at the beginning, and broken off, again leaving two or three inches for connecting purposes. At each tapping scrape the loop of wire bare of enamel. The coil is now complete, and may be mounted on the baseboard, either upright by means of brackets or on its side, fastened with two wood screws to the baseboard. Completing the Trap. HH condenser and terminals may now be mounted on the panel, and the latter screwed to the baseboard. Bverything is now ready for wiring up. This may be performed with the aid of the layout diagram, or by following these instructions. A piece of flexible wire is attached to the terminal on the left, and a crocodile clip fastened to its free end. This clip igs taken to the most suitable tapping, to be determined by experiment. One end of the coil is taken to one rs

terminal of the variable condenser, and the other end to the other terminal. A wire is then run from one of the condenser terminals-either is ‘suitableto the terminal mounted on the right of the panel. The trap is now ready for operation. . Using the Wavetrap. AK® the aerial off the receiver ‘and attach to the left-hand terminal of the trap. Join with a length of wire the right-hand terminal and the aerial terminal of the receiver. Now’ tune to

the loudest point of the station it is desired to eliminate. Then, by varying the wavetrap condenser and attaching the clip to different tappings in turn, a position will be found where the station signals fade into jnaudiblity. The trap is then left so adjusted, and the wanted station tuned in.