Radio broadcast .. (1922-30)

9 io Radio Broadcast A SHORT WAVE LOW LOSS SET BY SIMILARLY utilizing short wave low loss coils, a highly efficient short wave receiver, similar to that described in the August number of RADIO BROADCAST can be had. A receiver of this type is operating successfully in the R. B. Lab on wavelengths between 45 and 200 meters. It was used in intercommunication work with amateur stations for checking up on European reception during the International Radio Broadcast Tests. On the short wave set, the tickler should be so arranged that 180 degree variation is possible. It is, of course, impossible to secure satisfactory reception of short wave telephone broadcasts with the detector oscillating, and on the extremely high frequencies reversed feedback is generally necessary to stabilize the receiver. The tickler coupling must not merely be loosened but the coil must be turned around over ninety degrees. It is somewhat contrary to expectations, and therefore interesting to note, that the Roberts circuit employing low loss coils, responds more readily to adjustments of the neutralizing condenser, and little or no experimenting is required to stabilize the system. Operating and other instructions remain the same as those suggested for the standard receiver. For detailed information concerning the Roberts circuit and parts other than those just described, the reader is referred to any of the articles dealing with the set, or the "Knock-Out Book" published by RADIO BROADCAST. FIG. 4 The r. f. transformer and tickler coil. The primary shaft has been removed, and the coil mounted on the frame by a metal strip FIG. 3 Primary and secondary low-loss coils for the antenna coupler. These have been remounted from a standard three-coil unit ARTICLES on the wiring and operation of tickler-feedback regenerative receivers are almost invariably accompanied by rather indefinite instructions as to the proper connections to the feedback coil. The reader has doubtless run across a phrase (which the writer has often written) explaining that if the set failed to regenerate the connections to the tickler were to be reversed. Thus the wiring of the regenerative apparatus was a fifty-fifty chance which, in consideration of a permanent job, was decidedly inconvenient. There is really no reason why the exact manner of connecting the tickler coil should not be stated, for all doubt can be eliminated by a simple rule. Regeneration is secured by tickler feedback when energy is fed back from a coil in the plate circuit of a vacuum tube to a coil, generally the secondary of a vario-coupler, in the grid circuit, in such a manner that the returned impulse is in the same or assisting direction of the existing grid impulse. When the directions are reversed, that is, when the feedback impulse is in a direction opposing the grid impulse, the effect is naturally reversed, and instead of regeneration we have a^very noticeable weakening of the signal. To one familiar with the laws of induction, these considerations will throw light on the situation. Let us assume that the incoming signal places a momentary positive charge on the grid. This will, of course, cause an increase in the plate current, with an expansion of the magnetic field about the tickler. This