LESSON THIRTY. DEAD END LOSSES AND "NO DEAD END" SWITCHES.
Practically every radio circuit includes an adjustable inductance of some sort, usually consisting of a layer of wire wound over a tube and arranged so that the amount of wire in the circuit can be varied by means of a switch, a plug or a slider. These methods of variation are familiar in the ordinary tuning coil, loose coupler, loading coil, etc.
FIG. 60. Diagram representing the effect of Distributed Capacity.
If the plug, switch or sliding contact, depending upon the method of variation employed, is at E as in Fig. 60 so that only the portion of the coil A E B is in the circuit, then the portion E F together with A E may form an oscillator which, in order for the reader to obtain a better conception, may be likened to a sort of secondary winding, with A E considered as the primary. The oscillations of this part of the system may produce some very undesirable disturbances, especially so when the frequency of the currents in the circuit A E bear a certain relation to the natural frequency of the oscillator or E F. The losses due to the disturbance of these undesirable oscillations and also those resulting from eddy currents induced in the free portion or oscillator E F by the magnetic flux of A E are known as "dead-end effects."
These losses are very much more noticeable in receiving circuits than in transmitters on account of the very weak currents in the former and the importance of preserving all the energy when it is already very small.
Dead end losses take place principally in receiving transformers, loading coils and tuning coils. The losses are much more marked on short waves than long waves.
The presence of these highly objectionable losses, and they are large enough to not only seriously decrease the strength of signals but also to make selective tuning impossible, may be avoided by only using coils which are just the right size so that they can be entirely included in the circuit.
This is a very easy matter when only one wave length or at the most, two or three wave lengths are to be received, because it is then easily possible to quickly connect the coil of the proper size in the circuit. It is desirable however in most stations, and especially so in amateur stations that the apparatus be universal so as to be quickly and easily tunable to any wave length within its range.
Many amateurs build large loose couplers having a very wide wave length range under the impression that they have an ideal instrument. The truth of the matter is however that such an arrangement is decidedly inefficient especially on the shorter waves when only a portion of the windings are in circuit and there is a large dead end portion.
The better types of receiving transformers are now provided with "no dead end loss switches" which automatically break the windings up into a number of groups so that only that portion which is actually required to tune the circuit to a certain wave length is in circuit and the remainder of the coil is entirely disconnected.
These switches are located at certain definite points as previously determined by measurements of the coil with the aid of a wave meter.
FIG. 61. Diagram explaining how "end losses" are eliminated
The diagram in Fig. 61 illustrates the principle of such an arrangement. The points marked 1, 2 and 3 are the places in the coil where the switches are located so as to divide the winding up into separate parts. Suppose that it is necessary to move the slider or switch to such a point on the coil as represented by its position in the illustration marked E. The switch located at 1 would then automatically close as the slider or switch moved past 2 and 3 would however still remain open because that part of the winding which they connect would not be required. If it became necessary to include more of the winding in the circuit, 2 and 3 would automatically close as the slider or switch was moved along the coil and open again as it was moved back.
The automatic arrangement of the switches is easily accomplished in a number of different manners by means of levers, cams, trips, or some other mechanical means.