The Why of Wireless

Radio Record, Volume I, Issue 16, 4 November 1927, Page 15

The Why of Wireless

interesting Series Setting Out Scientific E acts Simply (By "Electron."’)

PART IX.-ATMOSPHERICS OR STATIC These disturbances are well known to most listeners even crystal users, They are due to little electric disturbances in the atmosphere like miniature lightning flashes. They may appear in many different forms; the familiar crash just like thunder, but without the rumbling after effects, then there is the rustling something like crushing a newspaper and also the sound like hail on an iron roof. ‘There are many modifications of these effects and. so far have caused much trouble as they cannot be tuned out. The reason for this is that the static hits the aerial a sudden blowjust like someone striking-a clock pen-dulum-and the current flows up an down the aerial at its natural frequency. Suppose two neighbouring aerials were tuned one to 420 metres and another to. 660 metres, it would be found that the atmospherics would affect each equaillywithont the slightest regard to their different wave-length. ‘This, therefore, is the teason why even selective sets" suffer from static. It seems that static is generally more troublesome as we approach the equator and becomes less as we approach the poies.. Also trouble from atmospherics is in nearly every case local, That is, suppose two sets, one in Auckland and one in Dunedin, were listening to 2YA, it is probable. that the first. would find great trouble from static while the latter would report perfect reception. If the first now tunes in to LYA, he will find that the static ceases. Yes, but that is because his set does not need to amplify so much with the near station, and the static is correspondingly re-. duced. The reason for static being so local is similar to the effect of using a lighted candle out of doors when a full moon is shining at night. Within a foot or so, the light of the candle will overpower that of the moon, while at a distance of severa] yards the effect will be negligible. Static can be reduced at times by means -f + super bterodyne set with

loop aerial. ‘hese sets, as will be explained. shortly, have a pronounced directional effect and if the aerial is pointed in the direction of the station whose signals are required, then any other signals-broadcast music or static -coming from any other direction are neglected and ignored. The ordinary

acrial, of course, picks up static no matter from what direction it comes, LOOP AERIALS. Reference was made in the last paragraph to the "loop aerial." This. aerial, instead of rising to 50 fret or so, and. then going horizontal to form. one plate. of a condenser, is wound into a coil or loop of fairly large diameter. If these wireless waves are made to pass through the coil, currents will be induced in it, as in the case of the seconlary- of a transformer. The position of the coil has a lot to do with the reception of the signals, however, because it is essential that the waves should pass through the coil. Iwo cases are shown in Fig. 1 with the coil in position "a," the waves from the transmitter X will obviously pass through. it, and induce currents in, as shown in Fig. Ila. In the case of Ib, however, the wave will hit all the sides of the coil at ‘the same time, and the effect will be as in Fig. Itb. No current will flow round the coil, and the station X will be ‘‘cut ont." ‘ It may be worth while for those whose sets permit of it to try to experiment with the position of the coils in their high frequency stages, if they find it impossible to cut out a broadcasting station to which they may be near. ‘hose who are within a mile or so of 2YA find great difficulty in cutting it out, because the waves are so strong. They can even get full loudspeaker strength without an aerial or earth, because the coils act like loop aerials. ‘Ihe relative positions of thie

three coils, however, must not be altered, since that arrangement (Fig. III} has been specially designed to prevent the currents in one coil from interfering with those in the other two. DIRECTION-FINDING. The rotation which a loop aerial must be given so as to enable it to re-

ceive a given transmitter has been given several most important uses. It will be seen ftom Fig. Ia that by means of it we are able to find the direction from which the waves come. This method gives us no cine as to the distance of the transmitter, but a simple addition will give us that information also. If: we find the direction of the station at a certain place, then move the. receiver through a given distance, and then find the new direction, we will. ve able to make a diagram similar to Mig. IV. Where the two directions. cross obviously gives us the position of the transmitter, This method’ is largely used in* navigation in places where fogs are frequent and where: traffic is dense, for example, in the English Channel. It is clearly much superjor to fog horns, etc:, because it gives the captain of the ship {the ship in this case being the transmitting station) definiie information as to his exact position, and he can then find all the rocks, etc., from his charts and his ccmpass. This method can also he used to trace and track down oscillators, whose persistance is inclined to fray the tempers of the neighbours and the P. and 7. inspectors. By means of a portable set on a motor-car the house can be located With ease. BEAM WIRELESS. The previous article gave indications for finding the direction from which the waves come, The next thing to do is to so construct the transmitter that we can have control over the direction im which the waves travel. ‘The reasons for re-} quiring the waves to travel in a beam

like the light from a motor-car headlight are several-(1) The elimination of interference in receiving sets in the neighbourhood listening to other stations, (2) Secrecy in war time. (3) Great reduction in the power used. Just imagine the size of the lamp requiréd in a motor-car to give the same illumina-

tion in front if it were entirely unshielded and placed on the hood As was explained in the article on fading, if wireless waves strike a flat conducting surface they will be reflected like light from a mirror, If, then, We surround the aerial by a mirror like that of a imotor-car headlight, we will reflect the waves in a beam. ‘There are however, two main difficulties in the way-(l) Lhe aerial must be smali com"pared with the reflector. (2) ‘Phe re-: flector must be large compared with the wave length. ‘hese seemed insuperable fat the beginning of the investigation--about 20 years ago-but with modern progress in the production of short | waves it has been found quite feasifile,: as the new Post Office beam stations in| LAustralia, Canada, ance India have proved. , In order to keep the aerial smali with: ‘regard to the reflector it was made with-: out a ‘‘top,’’ that is, the aerial consists" of a vertical wire rising straight up. The reflector consists of a flat. sheet of metal (or a form of: wire netting) surrounding the aerial, -as shown in fig, V. When the waves hit the reflector they are projected in the form of a beam directed towards the receiver. When these waves hit the receivers reflector they in turn cre reflected back on to the receivers. aerial when, 2s will be seen from figure VI, they will be highly concentrated, and a loud signal will be heard. , Other methods of directing the waves in a particular direction have been tried, | the only one worth mentioning now be--ing the inverted I, type. ‘This aerial will both transmit and receive, with a preference in the direction opposite to

that in which the flat top is pointing, but this preference is not noticeable unless the length of the top is at least ten times its height, so that with an aerial 20 feet high no appreciable effect would be obtained unless the top were £00 feet iong. So that to alt intents and purposes the I, aeriat of normat

dimensions will receive equally well from .alt directions, my _It has been stated earlier in this article that waves if they are strong enough will affect the most sensitive an¢ selective of sets, ‘The atmospheric be: ing untuned will break through the, best of sets, but we can if we take sufficient care cut out a near-by station fairly satisfactorily. If we go back to the condenser and coil and apply am: oscillating pressure of the correct frequency a oscillating current wilt flow | froin one plate of the condenser through the coil to the other plate and tack again. This current surging backwards and forwards through the coil produces an oscillating magnetic effect which cas be used to induce similar currents 1mneighbouring coils as has been explained. ‘ This effect will prevent any current from passing through the apparatus, as shown by the arrows of figure 7, because any electricity getting through the coil will be swept along by the surging current on to the condenser plate. If a higher frequency is applied, then it wilt not be able to pass through the coil, due to its choking effect, and will pass through the condenser with ease. A lower frequency will flow through the coil and leave the condenser alone. But the correct frequency will find great difficulty in passing through, and will pile itself up on the plates, thus producing a high pressure, whieh in the receiving set is used to operate the grid or the crystal.: We have then the peculiar state of affairs ‘that an arrangement-as shown will act more or less like an insulator to a particular frequency, and will prevent that frequency from flowing through. — . Were, therefore. is the secret of the wave trap. If we fit an arrangement like this in the aerial of our set and tune it to the frequency of the station which we wish to cut out, then all other signals will pass through to onr receiving set, but this one will be stepped (figure &). Since the wave trap 1s always most wselul at frequencies close to those which we wish to receive, it follows that its tuning must be exceedingly sharp, and tlis means that resistance must be as small as possible. ‘The wire should be very thick, din. diameter 18 quite small for this purpose-nothing will be ridiculous, and al! joints must be soldered. Because of the large wire the number of turns will be limited, and the condenser should therefore be large, and, of course, should be of ook quality. If these points were considered more there would be less complaints of weakened signals or incomplete ttrapping. One must take care also to sce that the coils of the receiving set do not act as toop aerials, and thus nullify the effects of the wave trap.- They. or perhaps better still the whiole set, should be shielded by a coating of copper or tin foil, which should be earthed so as to keep the waves out. Also the wave trap: shoukt be as near the set as posand the earth wire kept short or "else be shielded also.