How to Obtain Selectivity

Radio Record, Volume III, Issue 41, 24 April 1930, Page 30

How to Obtain Selectivity

No Need to be Annoyed by the Local

(BY

ROY

KEITH

ANY listeners who live near a broadcasting station find that it is difficult to tune it out and get other stations, and usually blame the station in question. But the fault is not with the station, but usually with the set or location. This may be overcome no matter what type of set or how close ft may be to a :. tation, and there is no reason for any listener to have trouble of this description nowadays. Many crystal users are greatly troubled in this: way mainly because of the erudeness of most crystal sets which. are usually designed to receive whatever may be on the air. It is just as possible for a crystal set to be able to tune to any of a number of local stations as it is for any valve set to tune in distant ones without interference of any kind. Any of the three following methods will be found quite satisfactory if followed carefully, and in fact they may be used singly or in combination if found necessary, although in the hands of the writer any one of them alone is quite satisfactory. Three Types of Rejectors. [HE first method .o be described is most satisfactory for a crystal set, and although it may be used for a valve set it is only suitable when it is desired to tune out one local station and tune in another local station, and consists of placing a small fixed condenser between the aerial and the set, the eapacity being from .00005. to .00001, or a midget condenser may be used if you have one. Another method has been found quite satisfactory, and has the advantage of cheapness, which should appeal to crystal users. Take about 10 feet of ordinary lighting flex such as is used on electric lights. Make a knot in each end of one of the wires, making sure that you knot both ends of the same wire. Disconnect the aerial wire from your crystal or valve set and connect the wire with the knot in it to the aerial terminal taking care of course to first remove the insulation. At the-other end of the flex connect the end without a knot to the aerial wire, removing the insulation as before. Proceed to tune in either of your locals when they are both in operation: sou will find that the tuning of your set has become much sharper. However, you still get both at once, and cannot separate them. Proceed to untwist the remaining knotted end of the flex from the other, readjusting ihe set each time you have unwound a few turns until you will find a point where you can tune in either of them separately.

Now fasten the ends of the flex with string so that they will not untwist further, and cut off the end you have untwisted, and no further adjustment will be found necessary, and any numher of locals may be tuned in without interference from each other. The Wave Trap. "THE second method is the use of a suitable wave trap which, if correctly constructed, will enable you. with either a valve or crystal set to tune in outside stations as well as other locals without reducing the volume of

the station wanted, while reducing the strength of the unwanted station to the vanishing point. In building such a wave trap a little care must be taken, and although we will not go into the technical reasons here, suffice to say that the proportions given here should be strictly adhered to. Wind on a bakelite former 55 turns of No. 24 D.C.C. wire, making a tap at the 27}th turn (be careful about the 274th turn), for it is this that makes the success of this wave trap assured. Connect each end of the coil to the terminals of a good low-loss variable condenser, .0025, which, of course, should be mounted on a small panel

of good quality bakelite or other insu- } lating matérial. : Two terminals should also be mounted on the panel, the lower one being connected to the moving plates of the variable condenser, the upper one being connected to the 273th turn tap of the coil. The upper terminal in use is connected to the aerial, which has been removed from the set, and the lower one is connected to the aerial terminal of the set. This trap may be used with any set and will, if correctly built, separate local station 10 ke, apart and distant from locals 15 k.e, apart. It should be stood, at least three feet from the set,, otherwise it would be found that there. would be coupling between the wiring of the set and the coil of the trap, which, being tuned to the interfering station, would act as a coupling coil feeding back to the set. When used either close to the set or with some types of A.C. set it is advisable to shield the coil itself, placing it in a can at least lin. larger than the coil each way, and although this may be a little more trouble it is well worth while. Other Methods. n indoor antenna may be used for receiving locals, or the aerial may be disconnected altogether, and in this case no interference will be had from one local station with another, and if the set is tuned carefully the volume will be quite as good as with an aerial, Especially is this the case with some A.C. sets which derive quite a large amount of pick-up from the power lines, which act as an aerial. However, with some of the latest types of all-electric sets which are well shielded it may be found necessary to connect a couple of feet of wire to the aerial terminal, or a good plan is to disconnect the aerial at the lead-in tube, or to switch the aerial to ground. Any or all of the methods described will give perfect selectivity and will enable any listener to tune out a local station and tune in either outside or other local stations without interference from each other, and at little trouble or cost to the listener. Fig. 1 shows the use of a small fixed condenser or midget variable condenser connected in series with the aerial, while Fig. 2 shows how to make a fixed variable coupling with twin flex. Fig. 3 shows the circuit diagram of the wave trap, and we know that any listener who cares to follow the instructions laid down here will have no further trouble with interference, and will be able to tune in any station he wishes. The main two things to be remembered are that the looser the coupling to the set the sharper the tuning, and the smaller the series condenser the looser the coupling to the aerial, and with a wave trap always remember that it does not impede but absorbs

the unwanted station while having little or no effect on the station required, and one of these wave traps has been used in Auckland a few hundred yards from the broadcasting station, and while all three of the Auckland stations! were operating was able . to tune in either of them or any outside station without inferference from any of them.