Hand Capacity

Radio Record, Volume III, Issue 27, 17 January 1930, Page 27

Hand Capacity

\How it May be Overcome ONE of the most annoying things, to anyone who has just built his first shortwave. receiver, is hand capacity. This occurs as the result of so many variable factors that it is impossible to lay down any Kard and fast rules for means of abolishing it. Nevertheless, it is generally correct to assume that it is caused by the aerial tuning and reaction control condensers, Of course, it is absolutely essential to see that the spindles of these two condens- | ers are earthed. But that alone will not always eliminate the trouble. The older type of condenser had the spindle . Insulated from the end-plates and, as spindle and dial had to be earthed, e end-plates were connected to the high-potential end of the circuit and #0, bringing the hands near the panel (i.e, one of the end-plates) hand capacity effects were clearly evident. In the reverse case, if the énd-plates were earthed, hand capacity was almost as bad owing to the spindle not being earthed this -time, So there was ho avoiding the trouble however the condenser was connected. The newer designs of condensers have the spindle and end-plates electrically connected, and the fixed plates are thus inside the two end-plates and effectively shielded. ,Hand capacity effects, therefore, will be minimised by just making sure~ that the spindle is earthed. In many cases a metal shield placed ' Immediately behind the panel will completely abolish these undesirable effects. Any non-magnetic metal can’ be used, such as aluminium, copper’ or even zinc. Care must be taken to see that the spindles and end‘plates of condensers make contact. with this shield and that the high potential plates do not. Here it might be noted that a good aluminium panel generally costs less than an ebonite one of the same size and adds to the neatness of the set by eliminating a certain amount of internal wiring, as well as reducing hand capacity effects. Still, theré are many who prefer an ebonite panel, and in such cases a metal shield behind the panel can be used. When operating a receiver with headphones, hand capacity effects will he noticed if H.F. currents are getting into the output leads. A simple. test for this is to tune in somé station and . then grasp the ’phone leads. If the tuning alters when this is done, it can be assumed that H.F. currents are m { the ’phone leads. These can be effectively blocked by using a filter cirecuit.. To each output terminal attach an ordinary H.F. choke and connect the free ends direct to the ’phones, Join two. .0005 mfd. condensers in series and take a lead to earth from the join. Connect the free terminal one to each side of the chokes nearest the ’phones. In certain cases the proper cure is ’ to put the whole receiver in a metal ‘ease, but even this will be of no: use if a poor earth connection is useé. Although a poor earth connection will not make so very much difference to actual signal strength in a short-wave receiver, it will make a great deal of difference to hand eapacity effects, and a really, good earth connection is essential to the smooth working of the receiver,