Suppressing Ignition-Interference on Radio-Equipped Aircraft 300038
THE AUTHORS say in part that although it has been recognized for many years that electrical ignition systems on airplane powerplants are a prolific source of disturbances tending to prevent the successful reception of radio signals, the serious and intensive development of methods of suppressing such interference is a comparatively recent undertaking. This arises from the fact that only in recent years has special significance been attached to radio operations with limited collecting structures or antennas, over such distances that the signaling waves intercepted by these antennas are relatively weak. They then outline the systems in which interference is present and discuss how it can be suppressed.
Modern restrictions upon the receiving systems on both commercial and military airplanes have established a set of conditions which focus the designers' attention upon the complete suppression of electrical radiations from the ignition system, and these restrictions are both physical and electrical. Physically they include the elimination of the trailing wire and the use of fixed vertical or horizontal antennas positioned without regard to their proximity to the powerplant. The methods of interference suppression in vogue today are the results of wholly empirical and, in many cases, unscientific development, the authors assert.
After discussing the nature of the interference, the authors discourse on the suppression of ignition interference by means of shielding. A constructive tendency in recent shielding development, they say, is the extension of the idea that a shielding system which holds together under service conditions, if intelligently designed, may be just as successful in keeping water, oil, and dirt out of the ignition as it is in keeping noise out of the radio. The remainder of the paper is devoted to a discussion and illustrations of various shielding systems now in commercial or experimental use, which are designed for complete housing of all important current-carrying circuits on the airplane engine.
In the discussion* the belief is stated that no shielding at present has fully met the day-in-and-day-out grind of transport operations, and that it probably will develop for some time yet and will be evolved slowly. Tests of radio equipment made in a four-place cabin plane, having a J-5 nine-cylinder engine that was not shielded, are described. Doubt is expressed whether a worthwhile distinction will be made between the necessary shielding for long-wave and short-wave reception in the plane.
The remainder of the discussion is largely concerned with details regarding the merits of vertical versus horizontal antennas, sensitivity of receiving sets, the best types of mast and their manner of mounting, and the like.