YOU MAY NEED ONE

Evening Post, Volume CXI, Issue 106, 7 May 1931, Page 21

YOU MAY NEED ONE

Tho advent of a B class station in Wellington will bring a new pack of troubles for some listeners who arc using "local station" receivers. Some of these will probably bo unable to prevent 2YA and 2ZW from interfering with each other. Tho simplest romedy will probably be a wave-trap, which after all is a very simple and inexpensive affair, which anybody who can use his hands at all can make. It consists of a coil and a condenser. Any standard typo of variablo condenser will do; the quality does not need to be very high; it should be of .003 or .005 microfarad capacity. Tho larger is if anything tho better, and being an unpopular size can probably bo picked up cheaply. A coil made of 40 or 50 1 feet of insulated wire—enamel, silk, or cotton covered —is wound on a former from two to three inches in dianic--1 tor, and the ends are fastened, leaving a few inches for connections. Kound the middle of this coil, from 15 to 30 'turns of similar wire are wound, with 1 a strip of thin card or two or three layers of brown paper between. The ends of tho larger coil are connected to the terminals of the condenser. One end of the outer'coil'js connected to the aerial terminal of the receiver, ai d the other to tho aerial lead in; that is, the trap is connected between the aerial coil and the-receivor. If when 2ZW is being received, 2YA interferes, the trap condenser is adjusted and this should reduce 2TA to inaudibility. Conversely, if 2ZW interferes with the clear reception of 2TA, another setting of the trap will cut it out. The two settings shpuld be noted. If it is found that the condonsor has to be "all in" to reduce 2¥A, and still does not abolish it, there is probably not enough wire on the main coil, and a few more turns should'bo added. This is not likely to happen. On the other hand, if tho other station is not "abolished" till the condenser is all out, there is too much wire, and turns should be taken off, one at a time, until a fair bit of the condenser is used to bring 2ZW to minimum audibility. The efScaey of the trap depends partily on the number of turns in the smaller (or primary) coil, and this should be experimented with before the trap is finally made up on a base-board or in 'a box. It is important, if the wavetrap is to work properly, to keep it some little way from the receiver, but there ia nothing really tricky about it. Those who have dabbled in wireless for a while will have no difficulty in making up a trap. which is inconspicuous yet convenient, especially as the coils need not be of the kind described; "spider-web" or honeycomb or any other type of coils can be utilised. The average experimenter usually has the makings of two or three good wavetraps on his "junk-box."