From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical article on a new dipolar antenna for cars with steel roofs.
| Identifier | ExFiles\Box 63\2\ scan0073 | |
| Date | 27th April 1935 | |
| 530 86153, Car radio Steel Roofs Bring New Antenna Fig. 1—RCA Dipolar antenna under running board [Diagram labels: ADJUSTABLE 2FT TO 3 1/2 FT. ; DISTANCE FROM RUNNING BOARD ADJUSTABLE FROM 2 1/2" TO 4".] THE best location for the antenna of an automobile radio receiver is in the roof of the car, but the steel roofs on many 1935 cars make it impossible to locate the antenna there. Radio and car manufacturers therefore have been confronted with the problem of finding the next best location and the most suitable form of antenna for that location. RCA Manufacturing Company's solution of the problem is the Dipolar Antenna mounted under the running board (Fig. 1). The antenna is made of steel tubing, 4 ft. long, with the two legs spaced 5 in. apart. A strap is provided for connecting the lead-in at the center of the U. Two special brackets are provided to mount the antenna underneath the running board and to adjust its height so as to provide the proper ground clearance. The antenna should be placed under the running board which is furthest from the ignition system. There are rubber grommets in the bracket to insulate the antenna from the metal structure of the car, and the antenna itself is protected against corrosive influences by being first copper-plated, then given a coating of baked dull-gray crackle enamel, and finally a coating of a special wax. These protective coatings are said also to offer adequate protection against electrical leakage to the car body due to the presence of dirt, grease, ice, water, etc. A low-capacity transmission line is supplied with the antenna, and for maximum noise reduction this should be grounded to the frame of the car at a point near the antenna. The transmission line should be run as directly as possible to the receiver and kept away from ignition wires. It is admitted that this antenna does not give as powerful a pick-up as the roof antenna; on the other hand, ignition noise is said to be much less with it than with the roof antenna in many instances. As this antenna was specially designed for auto radios, some of its "reasons why" may be of interest to automobile engineers. The antenna is generally regarded as a conductor which for best pick-up must be strung high above the ground, and it will no doubt come as a surprise to many that with this particular running-board antenna the pick-up is best when it is in the lowest practical position, which is determined by considerations of road clearance. The reason can be readily explained by reference to Figs. 2 and 3. Radio signals induce a voltage in the car body, the latter acting as a flat-top antenna, and owing to the electrical capacity of the condenser formed by the automobile and the ground, a capacity current flows between automobile and ground. This is indicated in Fig. 2 by the symbol (sine curve) of an alternator at the left. If now a plate is placed between the car and the ground, and a radio receiver connected to the car and plate respectively, there will be a capacity current between ground and plate, but to complete the circuit to the car the current will have two paths, one being through the capacity between the car and the plate, and the other through the receiver. These two paths are in parallel, and the assembly acts as a capacity voltage divider which is similar in its action to the more familiar resistance voltage divider illustrated in Fig. 3. In the latter diagram there are two resistances connected in series between the car and ground, and the receiver is connected to the car and to the junction between the two resistances. It is well known that in this case the greater the resistance across which the receiver is shunted, relative to the other resistance, the higher the electro motive force applied to the receiver or, in other words, the greater its pick-up. Similarly in Fig. 2, the greater the impedance of the capacity across which the receiver is shunted, relative to the capacity in series therewith, the greater will be the pick-up. Now, the capacity impedance increases with the distance between the two conductors forming the capacity, hence by moving the plate B closer to ground the plate-ground impedance decreases while the plate-car impedance increases, hence the pick-up increases. In practice it is found that the best location is on a level with the lowest part of the chassis, which combines good April 27, 1935 Fig. 2—Circuit diagram with Dipolar antenna [Diagram labels: AUTO, RECEIVER, ANTENNA, EARTH] Automotive Industries | ||