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The Selective Double-Circuit Receiver
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ductance; with the vacuum tube detector, best results will ordinarily be had with relatively small condenser values in the secondary.
When a vacuum tube detector is used, it is feasible to utilize its radio-frequency amplifying power so as to neutralize a substantial portion of the receiving antenna resistance. This resistance-reducing property is one of the most valuable features of vacuum tube operation, and is obtained by means of the feed-back or regenerative action discovered by Armstrong. To use the tube detector for this purpose it is not necessary to rely upon the double tuned circuit receiver; regeneration applied directly to the aerial-to-ground circuit will give excellent results in selecting sharply between signals from moderately distant stations on slightly different wavelengths.
The feed-back circuit is based upon the discovery that the audion detector repeats into its plate circuit, in amplified form, the radiofrequency currents applied to its grid. In other words, the tube not only converts the arriving signal energy into an audible current form suitable for operation of the telephones, but at the same time transfers to the telephone circuit an enlarged copy of the radio signal impulses. If we place a condenser of moderate size across the telephone terminals so as to shunt these repeated radio-frequency currents across its winding (without disturbing the lower-frequency currents which are to act on the diaphragm), the amplified oscillations will pass freely through the entire plate circuit. By inserting a radio-frequency coil in this circuit, next to the plate connection, where the potential variations are greatest, we may set up a strong high-frequency magnetic field produced by the amplified oscillations. If, as indicated in Fig. 5, this coil is placed near to the antenna tuning coil it will act inductively upon the aerial circuit; by selecting the direction of current flow correctly, part of the plate-circuit oscillations may be caused to add their effects
FIG. 5
A so-called single circuit receiver employing a tickler or feed-back coil for regeneration
to those of the oscillations already flowing in the antenna, so as to produce a great increase in the total current strength. This is equivalent to reducing the antenna resistance, in its final effect, for, with no increase in the voltage which the arriving electromagnetic waves impress upon the aerial, there is produced a much greater flow of current of radio frequency.
Perhaps the most interesting feature of this regenerative effect is that it amplifies to a maximum only the signals to whose frequency the circuit is tuned: thus the behavior of the system is as though the aerial-to-ground resistance were greatly reduced for a single comparatively narrow band of wave frequencies. Sharpness of selection is therefore a prominent characteristic of the properly adjusted feed-back receiver, just as of any other low-resistance resonant circuit. The arrangement of Fig. 5, however, is subject to some disturbance from powerful near-by transmitters operating at wavelengths sometimes widely different from that being received. The cure for these transient interference effects is to use a combination of Figs. 4 and 5.