Radio broadcast .. (1922-30)

View of radio transmitting and receiving apparatus in a modern Navy airplane. AIRCRAFT RADIO DEVELOPMENT THE FIRST telephone transmis- sion from plane to ground was accomplished in England during the summer of 1915 by Major Prince and Captain H. J. Round. A cw tube transmitter was used for this notable achievement. Since that time, radio has come to play an increasingly greater part in the development of commercial aviation. Ignition noise has prohibited the use of highly sensitive receivers until very re- cently. This necessitated the use of the long, trailing-wire antenna which was un- satisfactory on account of landing prob- lems, time involved in reeling the antenna in and out, frequency variation, losing the antenna, and other factors. Mechanical vibration and noise has made the choice of non-microphonic tubes a necessity. The problems of construction, instrument mounting, and dependability are also de- pendent upon the extreme amount of vibration encountered. The placement, ac- cessibility, and servicing of the component parts must be considered. The apparatus must be simple and fool-proof, because the average pilot and mechanic are not expert radio operators and only the larger com- mercial planes can accommodate a special radio operator. Frequency choice is im- portant, and depends largely upon the purpose of the equipment. Continuous communication with ground stations along standard airways must be considered. The apparatus must be able to withstand the unusual climatic, humidity, and tempera- ture changes encountered during any kind of flight. Tuning must be accomplished with heavy gloves at times. A locking device is frequently necessary to prevent "creeping of tuning," especially for beacon work. The apparatus must be extremely light, shock-proof, and compact. The pow_er supply should be such that the equipment can be operated during emer- gencies, such as a forced landing. Sufficient A Summary of the Factors Influenc- ing the Design of Aircraft Transmit- ters and Receivers. General Require- ments of Receivers for Aircraft Use. Sensitivity, Fidelity., and Band Width Characteristics of a Typical Receiver. By ELMER L. BROWN Engineer, Special Equipment Division, RCA-Victor Co., Inc. output of the receiver is necessary for oper- ating a beacon-indicating device. The trailing wire type of antenna, which has been used until very recently for both DISTANCE-MILES FROM TRANSMITTER Typical aerial field-strengh curve. the dirigible type of airship and the heavier-than-air type of craft, had several distinct advantages as well as disadvantages. Its use is confined largely to the lower frequencies, and it is compara- tively satisfactory around 300 kc. It is possible with this type of an- tenna to communicate over com- paratively long distances with a minimum of power. However, it is necessary to reel this antenna out when communication is desired and to reel it in when communica- tion is finished. The maintenance cost of this type is large, and the hazards encoun- tered when flying at low altitude make it undesirable. It is also impossible to use it when a forced landing must be made. The air resistance of this antenna increases the drag on the ship and materially re- duces its speed. It is impractical for mili- tary use, as a plane cannot be stunted. Effective shielding of ignition systems makes it possible to use receivers with a sensitivity as high as 5 microvolts per meter. Sensitivity greater than this is un- desirable, as with it one is enabled to get down to the noise level with a short ver- tical-rod antenna. The vertical antenna is justified by its elimination of physical hazards, burdens of operation and mainte- nance, and a substantial reduction in beacon errors. A streamlined duralumin vertical antenna 2 or 3 meters in height, triangular and guyed to the upper wing and fuselage, has much less wind resistance than a trailing wire, and is still short enough not to interfere with putting the plane in the hangar. This type is particu- larly satisfactory for high-frequency work, as it is rigid enough not to cause swinging of the signals. This eliminates the use of a kite for emergency landings as was neces- sary with the trailing-wire type. A modi- fied T or V type of antenna can be used successfully with larger ships and dirigi- bles. Almost any type of rigid structure can be used with dirigibles. 330 RADIO BROADCAST FOR APRIL