THE CRYSTAL

Radio Record, Volume I, Issue 3, 5 August 1927, Unnumbered Page

THE CRYSTAL

SETTING LONG DISTANCE

With the advent of a powerful broadcast station such as YY¥A, good crystal reception will be possible on both sides of Cook Strait, aid for a considerable distance inland both north and south, Crystal listeners situated fifty miles or more from Wellington will be able to put in much interesting experiment in the way of increasing the volume of their reception; firstly, by the testing of different combinations of crystal and cat’s whisker; and secondly, by the addition of an amplifier as is found necessary. But the first item in getting distance with the crystal is to provide the very best aerial, for it is necessary to collect as much as possible of the infinitesimal voltage that comes through the ether. LVorty feet is not too high, and a hundred feet or more will be a suitable length. Good insulators are essential, and great care must be taken to prevent the smallest leakage at the lead-in. The position chosen should be as far from trees as possible, as trees close to the aerial on the side from which the wireless waves arrive may easily reduce the strength of signals to one-half. However, the screening effect of trees can only be determined by actual trial at the par ticula1 spot, and if they are not too numerous, and are situated not less than fifty yards from the aerial, their effect will probably not be ct all seri ous Whilst permanent detectors are good for short distances, they would, unless particularly sensitive, be superseded by the cat’s whisker contact for long distance. The same rules apply to purchasing crystals ag to the purchase of components-it pays to pay for a good one. Just recently the writer purchased a couple of permanent detectors to try out. One of them cost 3s. 3d. and the other Is. 6d.; the former gives just double the volume of the latter. Galena is the crystal most recommended for long distance, and, instead of the usual contact with the end of a fine copper wire, a small pointed portion of graphite from a B grade leadpencil may be bound to the end of the cat’s whisker with a fine wire and used to contact the face of tlie crvstal. This combination is very sensitive, and its adjustment very critical, as only a very light contact is necessary. On this account a less sensitive combination may be preferred on account of its greater ease of operation, especially whiere. valve amplification is employed. The efficiency of a whisker depends upon its freedom from any tendency to oxidise. In this respect it is said that there is nothing quite so good as ‘platinum, which will be found to give ‘surprising and consistently good results, says a writer in an Iinglish journal. It would only be necessary to bind a short fragment of platinum wire to the end of the cat’s-whisker in order to experiment. Gold is also said to be a close runner-up to platinum. Adjustment Essential. Tine adjustment is an essential factor for long distance, and to obtain this it is recommended that the usual adjustment is provided on the cat’swhisker holder for finding a sensitive spot, the whisker itself being a delicate spring of about four effective inches of No. 86 copper wire, The crystal cup is mounted upon a strip of brass at a distance of about one inch from one end. At this extreme end, which is bent downwards to meet the panel, a hole is drilled and the end of the strip bolted to the panel, with the main portion of the strip slightly raised above the panel. At the end farthest from the crystal a hole is drilled and a screw or bolt passed through it and the panel in such a way that turning the screw will raise or lower the end of the brass strip. This provities a very fine rise and fall for the crystal, and after the usual

adjustment has been made with the whisker, pressure can be regulated by turning the screw. Different varieties of ctystals vary to an enormous extent as regards the actual pressure of contact which should be made with them by the cat’s:whisker, For instance, a sensitive crystal of iron pyrites works most efficiently when it is used with a rigid steel contact (such as an ordinary sewing needle), applied with the pressure of its own weight. On the other hand, however, a silicon erystal requires only a very light pres‘gure of a brass wire to produce its maximum volume of reception. With galena crystals, the finer the texture the finer will be the required pressure of the whisker. A solenoid coil tuned by a variable condenser will give the smoothest control and sharper tuning than is usually obtained from a moveable two-coil arrangement, 0000000000000000000000005,