Radio broadcast .. (1922-30)

Shielded Receivers By THOMAS C. TIBBEY UNTIL the advent of the modern regenerative receiver, the term "shielding," as applied to radio apparatus, was quite unknown. The regenerative receiver, howver, being super-sensitive to all electrical influences, has been found to require protection against external electrical disturbances. Shielding accomplishes this. It is the process of surrounding the entire receiver, and sometimes even the individual circuits therein, by a metallic surface. This usually takes the form of a copper lining in the receiver cabinet and on the rear of the panel itself. This shielding absorbs any electrical influence which would normally find its way to the windings of the receiver, and induce in them an electric current, in the same manner that radio waves induce electric currents in a receiving antenna. In this instance the windings of the receiver virtually act as an antenna, absorbing energy from the ether. This energy may be radio telegraph or radio telephone signals, "induction" from electric light or power lines, or the summer pest of Radio — Static. SHIELDING IN RADIO COMPASS WORK THE Radio Compass, or as it is sometimes called, the Direction Finder, has been rendered accurate only by effective shielding of the apparatus. The heart of the Radio Compass is the "Compass Loop" which is a square coil of wire about two feet on a side and wound with six to twelve turns of insulated wire. These turns are spaced about one inch apart and held in this position by means of notched bakelite strips on the edges of the square frame of the coil. This coil is usually mounted above the operating room and arranged so as to be revolved by a shaft which passes through the roof into the room to be operated by means of a hand wheel. Wires leading from the terminals of the coil are likewise brought into the room and connected to the tuning apparatus. When the coil is revolved so that its edge is pointing toward a transmitting station, the signals are received with maximum intensity, decreasing as the coil is revolved, until the point is reached where the coil is broadside to the station. At this point no energy is induced in the coil, and no signals are heard. It follows, therefore, that when using the compass coil or "loop antenna" — a name probably more familiar to most of us — only those stations are heard which lie along the line in which the coil points. Under ordinary conditions this is not absolutely true, and here is where shielding plays an important rdle. As we have seen, the leads from the coil extend to the apparatus. These leads, together with the windings in the receiver and the wiring of the entire set, comprise an antenna, of no mean proportions. It may be readily understood, that when the coil is pointing so that no signals are being received from a certain station on the Compass Loop, they are being received on the wiring of the set. This wiring, acting as an antenna, has no directional properties and therefore seriously interferes with accurate determination of direction, or elimination of unwanted signals. The entire apparatus and leads, are therefore enclosed in a copper shield. This usually takes the form of a very fine mesh copper screen which surrounds the room, covering all windows and doors. This shielding effectually prevents the effect we have spoken of, and renders the Radio Compass an accurate and valuable agent for the guidance of ships at sea. AN IMPORTANT ADJUNCT IN BROADCAST RECEPTION F7IGURE i illustrates what is probably the L most highly developed receiver available to-day. This receiver is entirely shielded. The receiver cabinet is lined with copper, as well as the rear of the panel. Openings are provided in the panel shielding, in order that no part of the receiver wiring come in contact with the shield. A further refinement is effected by separating the primary, or antenna circuit, from the secondary, or detector circuit, except through such coupling as is controlled by a coupling coil. This type of shielding is valuable for preventing undesired reactions between the circuits of the receiver. A schematic wiring diagram of this receiver is shown in Figure 3.