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).
Article discussing car radio antenna installation problems, focusing on optimizing diameter and placement to maximize signal pickup and minimize ignition interference.
| Identifier | ExFiles\Box 63\2\ scan0075 | |
| Date | 27th April 1935 | |
| 581 Installation Problems by P. M.{Mr Moon / Mr Moore} Heldt Engineering Editor, Automotive Industries pick-up with reasonable protection against mechanical injury. The next question that arose was with regard to the best diameter (area) of the antenna. The object, of course, is to assure the greatest energy input into the receiver for the particular location of the antenna. The problem is similar to that of the resistance of an outside line connected across the terminals of a battery of given internal resistance to assure the greatest possible expenditure of energy in the outside circuit. It is generally known among electrical men that this requires an outside resistance equal to the internal resistance of the battery. The electromotive force active in the outside circuit is then equal to one-half of the electromotive force of the battery. If a smaller outside resistance were used, the current would be increased, but at the same time the electromotive force across the outside line would be reduced and the product of current and line electromotive force would be less. Similarly, if the resistance of the outside circuit were increased, then the electromotive force active in it would be increased, but the current flowing would be reduced, and the product, that is the energy developed in the outside circuit, would be less. The electrical circuits of the car radio are represented diagrammatically in Fig. 4. There is one capacity between antenna and ground, another between antenna and car, the two being in series, and the receiver is connected between the junction of the two capacities and the car. The values of both capacities increase directly with the size of the antenna, hence the impedances vary inversely as the size of the antenna. If the antenna were made very small in diameter so that the two impedances were great, then practically all of the current flowing between car and ground would pass through the receiver and little through capacity C1. But since the current has to flow through the high impedance C2, the total amount of current will be small, and even though the receiver carries a large proportion of that current, it does not get very much. If, on the contrary, the impedances were made small by using a large antenna, then a comparatively large current would flow between car and ground, but most of it would pass through capacity C1 and only a small proportion through the receiver. Hence the conclusion seemed justified that a definite size of antenna would give the best pick-up and that either a smaller or a larger size would result in reduced pick-up. Experiment showed this to be the case, the pick-up varying with the antenna diameter as shown in Fig. 5. The particular form of the antenna and the instruction to connect the lead-in wire at the middle of same also are based on logical reasoning. A good deal of the interference experienced with car radio comes from the ignition system, which sends out electrical vibrations with a principal wave length of the order of seven meters. These emanations, of course, will induce corresponding high-frequency electromotive forces in the antenna, and the problem is to prevent these electromotive forces from affecting the receiver. This is accomplished by so designing the antenna that its natural frequency as an electrical oscillator is equal to that of the radiations from the ignition system. The electromotive force of each end of the antenna will pass through complete cycles at a very rapid rate, being soon positive and soon negative; and the opposite end will at all times be 180 deg. out of phase with it, being at its maximum negative value when the first is at its maximum positive value, and vice versa. This creates a node at the middle of the length of the antenna, a point at which there is no change in the electromotive force due to the inductive effect of the ignition currents, and it is at this point that the lead-in wire is connected. If the point of connection is moved toward one end or the other, an objectionable noise generally appears in the receiver. For minimum noise pick-up, the antenna generally must be mounted parallel with the running board. Fig. 4—Diagram illustrating principle of maximum pickup by receiver Fig. 3—Comparison of capacity- and resistance-type voltage dividers Fig. 5—Variation of pick-up with diameter of antenna tube Automotive Industries | ||