Radio broadcast .. (1922-30)

Shielded Receivers 519 id receiver. This may be accomplished by shielding" the receiver. SHIELDING IHE RECEIVER SETTING about to shield the apparatus, the entire interior of the receiver cabinet •uld be covered with copper foil which must connected to the ground post of the receiver, t care being taken to have all pieces in 'ect electrical contact. This can only be e by lapping and soldering the joints. Be•e the foil is put in place, the interior sur:es of the cabinet should be given a coat of Mac or varnish, allowing it to become very ckv." This will render it easy to smooth it the foil and hold it so. As each side is laid in, the foil should be further secured by \ inch brass brads driven through the foil into the wood. The nailing should be done through the lapped seams near the edges, after a row of brads has been placed lengthwise along the centre line of the sides. Cutting the foil in such shape that the enentire box may be lined with a single piece of metal will remove the necessity of soldering any joints. The shape of the metal is shown in Figure 2. Copper foil, however, is not readily obtainable in widths greater than 8 inches, and in case it cannot be obtained sheet copper known as "twelve pound" should be employed. The shielding of the cabinet should project sufficiently to enable connection to be made with the shielded rear surface of the panel. If the panel is set in a rabbet, the foil should project over the face of the rabbet and be secured by brass brads which should be sunken flush TOP REAR j RIGHT ! SIDE BOTTOM © FIG. 2 with the surface by means of a nail set, so that the shielding of the panel will make good contact with it when the panel occupies its position in the box. It is most important that the panel be shielded, as this serves the dual purpose of preventing the effect explained above, and also preventing "detuning" due to the presence of the hand or body near the receiver. No doubt all who use unshielded regenerative receivers have had this annoying experience. The shield for the rear of the panel should Pr/m. Anfenna Secondary facuum tube i Grid /eat I Var/omefer am/cant. / FIG. 3 be cut to size, and openings provided with f inch clearance around all projecting switch points and terminals, with the exception of the "ground" binding post, to which it should be soldered. After cutting the required openings in the foil, the rear of the panel should be coated with shellac or varnish and allowed to become very tacky. The copper foil is then warmed, laid in position on the rear of the panel, rubbed smooth, and dried under a slight pressure such as may be provided by placing a perfectly flat piece of wood — previously oiled to prevent its sticking — on the shielded side of the panel, and resting a weight upon it. Unless great temperature changes are experienced, the foil applied in this manner will retain its close contact with the panel. However, to make certain, thin strips or discs of bakelite may be placed under any nuts on the rear of the panel, projecting over the cut out space on to the foil, thus securing it. With panels of A inch in. thickness or more, "blind" holes may be drilled in the panel from the rear to within not less than iV inch from the face of the panel. These holes should be tapped for a suitable thread with a bottoming tap, or an ordinary tap with the point ground off, to accommodate round head machine screws, the heads of which will secure the shield. This is an operation which requires considerable skill in order not to have the panel bulge where the drill almost