HAND-CAPACITY TROUBLES

Sun (Auckland), Volume II, Issue 517, 21 November 1928, Page 14

HAND-CAPACITY TROUBLES

HINTS THAT WILL HELP Hand-capacity trouble, as its name implies, says “Ilopular Wireless,” is usually due to the fact that the operators’ hands, being really objects at earth potential, produce a slight increase in the capacity of earth when brought near to high potential parts of the receiver such as the “grid” side of the tuning condensers. When the operator’s hands are removed this slight additional capacity vanishes, so that tuning is naturally altered. Also bringing the hands near to a coil changes its inductance, very slightly, but sufficient with the capacity change to upset tuning appreciably. In short-wave receivers this “handcapacity” produces an even greater change in the tuning. As you are doubtless aware, a very small increase in the capacity of the tuning condenser produces a large change in tuning on these short waves. If handcapacity troubles are bad in a shortwave set, reception is quite hopeless and the receiver has to be re-designed in order to eliminate such trouble. DEPENDS ON DESIGN Now, experienced amateurs, knowing the root-cause of hand-capacity trouble, take precautions when building their receivers to prevent, movements of their hands from affecting the receiver. The surest remedy, in almost all cases, is quite simple and consists of ensuring that there is always an earthed conductor between the operator’s hands and the coils and condensers of the receiver. You will immediately think of receivers having all-metal panels, which from experience you know to be free from handcapacity troubles. The complete freedom here is due to the fact that there is a fixed smajl capacity between all components inside the cabinet and the metal panel. Since this is earthed, anything also at earth potential on the dial side of the panel cannot make any difference at all to these capacities of component to earth, so that “hand-capacity” no longer affects the tuning of the receiver. Now some circuits where all panel components are at earth potential lend themselves to the use of a metal panel quite readily, but it would be fatal to use anything but an ebonite panel where the panel components are not all at the same potential. We always try to juggle with the circuit so that all panel components shall be earth potential; but in some circuits, where no part of the tuning system is at earth pptential, this is impossible. In the old days the keen experimenter would have fitted the two condenser dials with long “extension” handles; but we more fortunate moderns can now purchase special dials which, besides giving a slowmotion adjustment, have a fairly large engraved metal dial which is insulated from both sides of the condenser. This dial can, therefore, be connected to earth, and we then have the equivalent of a metal panel just in front of the . condenser itself, with the added advantage of a slow-motion adjustment. If your receiver is at all prone to “hand-capacity” trouble and has an ebonite panel, fit these dials, when your troubles will in most cases disappear. Take care, however, to choose a reliable make having really good insulation between the shielding dial and the part which grips the condenser spindle. Short-wave receivers are very much more liable to “hand-capacity” effects than receivers tuning over the broadcast waveband. Unless a few simple precautions are taken in construction you will find that the tuning may change appreciably in spite of earthedscreen dials. Also, signals from distant stations very often weaken, and sometimes disappear altogether, when the operator moves a short distance away from the receiver. This is due to the capacity coupling via the headphones producing a very slight change in the capacity of the tuned circuits. This effect is only noticeable on short waves. ARRANGING THE LAY-OUT The necessary precautions are to see that the tuning coils are placed as far to the rear edge of the baseboard as possible. The coils are then farther away from the influence of the

operator’s body, and, for this reason it is advisable to use a wide baseboard when constructing a short-wave set. If the circuit does not permit a metal panel to be used for the reasons mentioned above it is a safe plan to mount the tuning and reaction condensers'well back on the baseboard, say 6in from the back of the panel, and link them to the dials with lengths of tin ebonite rod. Obviously the best precaution of all is to use a circuit which permits of a metal panel being employed, and then to construct a cabinet out of sheet aluminium or copper, when the receiver will be completely screened from outside interference. Take care, of course, that the metal does not come very near the coils anywhere. “Body-capacity” via the ’phones to earth can also be very troublesome in a short-wave receiver, and the only cure is to reduce to the lowest limits the back-coupling to the tuned circuits. Mere screening is useless in these cases, and may even make matters worse. The more - valves there are between the detector stage and the ’phone terminals, the less trouble there will be from body capacity. On some short-wave transmissions two L.F. stages following the detector will often provide sufficient volume to work on a loud speaker so that no trouble occurs, as the ’phones are not then worn. THE CHOKE CURE As many transmissions cannot be received at this strength we must still wear ’phones and overcome our difficulty by inserting H.F. chokes and earth-shunt condensers in the anode circuits in order to keep as much H.F. energy as possible out of the L.F. amplifier. This will often effect a cure, but in obstinate cases where, for example, a good earth connection does not exist* you will find that an H.F. choke in each ’phone lead will assist matters. In extreme cases binding the ’phone leads with 24 D.C.C. for a distance of two or three feet, and then connecting one end of this wire to earth, is very often effective. LEAGUE OF NATIONS WIRELESS “Headway,” the monthly journal o£ the British League of Nations’ Union, comments as follows on the importance of instantaneous communications if Geneva is to function effectively at critical moments: “Interesting possibilities in the development of the League wireless station at Geneva are suggested in the report of the committee of experts which has recently dealt with the subject. The primary reason for such a league station is the need for rapid communication with council members at a moment of crisis. The objection to the installation of a league station is, of course, expense. But it is pointed out that many nations maintain (at great cost ail sorts of pieces of emergency mechanism needed only for use in time of war, and i6> is therefore the more legitimate to incur a small collective expense in the interests of peace. But the experts do not think that such apparatus, if constructed, would be of value in emergencies only. They point out in the next passage in their report that the station could make a general distribution by circular wireless telegrams of press news. Government information, and important League documents; it could organise a letter telegram service between Geneva and the more distant member States, which at present receive their information by mail, and consequently are apt to be out of touch with the latest League developments; further, a considerable proportion of the telegrams now exchanged between the Secretary-General of the League and the different Governments, and between the Governments and their delegations, would be sent through the League wireless station. The proposal will come before the council shortly.