Hand Capacity Troubles

Radio Record, Volume II, Issue 27, 18 January 1929, Page 29

Hand Capacity Troubles

Hints that will’ Help

QPERATORS are often troubled with hand capacity troubles, and in that respect the following from "Popular Wireless" should be of more than passing interest :-- Hand-capacity trouble, as its name implies, 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 arts 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 the 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 cages, 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 allmetal panels, which from experience you know to be free from hand-capacity troubles. The complete freedom here is due to the fact that there is a fixed small capacity between all compenents 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 at earth potential; but in some circuits, where no part of the tuning system is at earth potential, 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 slow-motion 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. HE 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 LF. stages following the detector will often provide sufficient volume to work on a loudspeaker 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 HUF. choke in each ’phone lead will assist matters. ‘