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In the R. B. Lab
489
sated for by the circuit changes. The possibilities of such a receiver were first suggested to this laboratory by a reader, R. S. Ryan, and the resulting circuit is shown in Fig. 8.
Other than the substitution of the coil antenna for the secondary of the usual radiofrequency transformer, Ti, the changes of the circuit consist of a liberal distribution of bypass condensers and the two hundred-ohm potentiometer, which contribute controlled regeneration that is doubtless responsible for the effectiveness of the single-tube loop receiver.
FIG. 5
The circuit, connected to the last tube in the super-heterodyne designed by A. J. Haynes. If the extra battery 82 is not used (when Bi is above too volts) B may be connected to C
a ground connection, running to the minus side of the filament lighting battery.
A fixed crystal may be used with this receiver, though an adjustable detector, which can be operated on a comparatively high resistance spot, will permit greater regeneration. Try reversing connections to the crystal. A loop of standard dimensions will operate successfully with the receiver, though as usual signal response will vary directly with the size of the loop.
Though this laboratory has not been able to duplicate Mr. Ryan's reception record of 1000 miles, the results indicate that such a range, while perhaps not consistent, is quite possible. Local tations (within 25 miles) come in with enjoyable ear-phone volume, and when amplified with the Knock-out amplifier, described in the December number of RADIO BROADCAST, gives a signal of splendid volume and quality.
BUILDING YOUR OWN LAB
Ta corresponds to the original specificacations for this transformer, 63 turns being wound on a two and a half inch winding form, functioning as the secondary, followed by a layer or two of paper and the primary of thirty-six turns. Any convenient magnet wire, between No. 22 and No. 26 may be used. In the R. B. LAB. (Fig. 9) the Ballantine Varioformer has been found particularly effective in this one-tube loop receiver, the regeneration that is more or less objectionable when operated on an open antenna, adding considerably to the audibility and range of the loop set. When using the Varioformer, the condenser across the secondary T2 is, of course, eliminated.
The audio-frequency transformer T3 may be any reliable make — this laboratory recommending a medium high ratio — such as five to one — in which case the bypass condenser across the secondary is best dispensed with.
Under some conditions, it will be found advisable to use
O
UR suggestion this month is addressed in particular to the more serious fan — the experimenter — the embryo engineer, whose interest and inclinations prompt him to original and studied design. Our recommendation is a slide rule — preferably a Keuffel and Esser, ten inch polyphase rule. Such a rule, with leather case, retails at eight dollars, and is shown in Fig. 10.
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TO SET*
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FIG. 6
Suggested layout for the antenna coupler