Radio Broadcast (Nov 1923-Apr 1924)

Record Details:

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348 Radio Broadcast ging in phones at some preceding tube, it is only necessary to shunt the primary of the transformer for which the phones are going to be substituted, by a closed circuit jack (Fig. 3). Connect the inner prongs to the transformer, and the outside prongs to the original leads in the plate circuit. Make sure that the same lead runs (through the jack) to the same terminal on the transformer as it did before the change was made (assuming, of course, that it was connected rightly in the first place). All soldering paste, acid, and dirt, which will cause noises, should be carefully wiped away from the prongs. Fig. 1 shows our correspondent's circuit as he submitted it to the Grid, and Fig. 2 is the same circuit with the addition of the jacks. TRANSF. SEC. FIG. 3 Jacks may be shunted across any transformer where it may be desired to plug in phones How To Make A Wave-Trap / have had much interference from amateur and commercial stations. Would a wave-trap help me? Can I make one, or must it be purchased? What is the theory of its action? T. O. B., New York City. THE wave-trap, as its name suggests, is merely a device into which an undesired frequency or wave may be enticed, while the desired wave is passed on to the receiving circuit unimpaired or only slightly affected. Its most simple and common form is a coil of wire shunted by a variable condenser, which is placed in series with the "WW FIG. 4 One method of connecting parts of a "wave-trap" antenna lead to the receiving set, Fig. 4. This forms a resonant circuit, or one which will absorb energy from the wavelength to which it is tuned. Thus if it is tuned to six hundred meters, the wave on which our correspondent doubtless experiences the greatest part of his interference, energy on that wave will be "trapped" and dissipated in the trap circuit, while other waves will pass on to the set. The coil may be of the lattice-wound variety, about thirtyfive turns, or wound to the same number of turns with No. 26 wire on a 3-inch tube. The condenser should be at least a 43-plate variable, and if possible a still larger one (.0015 mfd.). The larger the condenser, the smaller will be the coil, or load on the set, with an additional bypass for the legitimate waves. It is a good idea to reduce the number of turns to the minimum (with full condenser capacity) for absorbing the highest of the undesired waves. This will lessen the reduction in strength of the broadcasting station. The tuning of the wave-trap is rather broad, and it affects signals anywhere from fifty to a hundred meters off its resonance point. In some cases, particularly in spark interference, the arrangement shown at Fig. 5, is more efficient than that just described, and will probably prove more efficacious in eliminating disturbances from amateur transmitters, though a correctly built and operated receiver should experience little QRM (interference) from such stations. The constants (the coil and condenser) remain the same as for the previous series arrangement, the trap merely being shunted across the antenna and ground rather than in series with the former. If the interference is confined to commercial traffic on six hundred meters, the 43-plate variable condenser is not necessary, and a 23-plate variable shunted by a .0005 grid condenser, without leak, may be substituted for it. SET m FIG. 5 This arrangement is more efficient in cutting out spark interference than that of Fig. 4 Bank Winding What is "bank winding"? I see it mentioned very often as an efficient form of inductance, but no one is able to describe it or instruct me as to the method of winding. W. B., Montreal, Canada. BANK winding, like the honeycomb and duolateral coils, is a form of inductance in which many turns of wire, or a high inductance, is possible, while the distributed capacity is kept at a minimum. Bank winding, however, can be achieved without special winding forms and apparatus such as is necessary with the variations of the honeycomb. Distributed capacity is the condenser action between the turns of a winding. It is naturally very high in ordinary multilayer coils in which each successive layer acts as a condenser plate. This capacity, which is virtually shunted across the coil, boosts up the wave and thus limits the inductance in tuning the lower wavelengths, or, more simply, there is an apparent loss due to the fact that the capacity offers a path to the higher frequency currents, which jump from layer to layer, the path of least resistance, rather than pass through the windings. Bank winding, to an extent, is a method of breaking up