SCREENING

Sun (Auckland), Volume II, Issue 349, 9 May 1928, Page 14

SCREENING

WHAT METAL TO USE This subject, which, is so necessary to the amateur who desires more than a single stage of high frequency amplification, offers many difficulties. The function of the shield is to isolate the effect of each stage and prevent parasitic oscillations being set up, the elimination of which usually means a marked loss in efficiency. The obvious remedy is to prevent the interstage coupling, which is the cause of such oscillations. The coupling between the stages is either electro-magnetic or due to capacity coupling, or both. Fieldless coils are largely made use of to prevent electro-magnetic coupling, but shielding is still necessary to prevent capacity coupling. Two metals are largely used in commercial sets, aluminium and copper. The American practice tends toward the use of aluminium, while in England and Germany copper is more popular. To prevent the capacity coupling any metal would be suitable, but the elimination of the electro-magnetic coupling has developed two schools of thought. The one who maintains that the shield shall be some metal of good conductivity, which will offer little resistance to the eddy current developed and whose fields develop a back effect, which balances out the original field due to the coil, and the other which has such eminent supports as Dr. Smith Rose, of the National Physical Laboratory, of England, and the'Federal Radio Corporation of America, which advocates the use of iron or an iron alloy. This use of iron is what one would expect for the elimination of magnetic fields, and the most successful screening accomplished by the writer was achieved with the aid of some wide galvanised down piping with lids made from benzine tin. The spacing of the shields with respect to thie enclosed coils is important, otherwise damping is introduced, which will seriously mar the efficiency of the receiver. With the advent of the screened grid valve the subject of screening becomes more important, and, without it, the set employing more than a single stage of high frequency amplification becomes unmanageable.