Page:Popular Science Monthly Volume 90.djvu/970

 954

��Popular Science Monthly

��Increasing the Efficiency of a Receiving Outfit

RH. RENDAHL'S U. S. patent No. • 1, 175, 472, recently issued, shows how a crystal detector may be coupled to a tuned receiver in such a way that the sharpness of tuning can be changed with- out affecting the resonant frequency. In older methods of coupling the detector to

���The antenna is connected with the ground through the load coil and transformer primsuy

the resonant circuits, any change in the transformer leading to the detector caused a change in the tuning also. Thus, when the detector was tightly coupled for weak signals with moderate selectivity, 600 meter waves might tune at 40 degrees setting of the secondary condenser; while if the coup- ling were loosened to tune more sharply the same wavelength might be tuned at only 30 degrees. This inconvenience is eliminated by the use of the arrangement shown in the figure.

The antenna is connected with the ground through the variometer load-coil V and the transformer primary K. The secondary Ki has in series with it the tuning condenser C, the loading inductance L, the tuning variometer V2 and the coupling variometer Vi. Across the coupling variometer is con- nected the primary Pi which has very tightly coupled to it the secondary P2. The detector D, telephones T and blocking condenser B are connected as shown. Coil P\ may be wound in ring form as a layer which lies directly between two other layers connected together to act as coil P2- When the coupling is as tight as in this case the primary winding acts practically as a simple ohmic resistance, for its inductance is al- most entirely neutralized by the opposing currents in the secondary.

The coupling of the detector to the secondary circuit is changed by means of variometer Vi. With this at its maximum value of inductance the greatest amount of energy is fed to the detector, while when Vi has minimum inductance the potential

��across the detector is least. Since the coupling variometer is shunted by the al- most purely resistive coil Pi, which is not of high resistance, its inductance is not effective in the tuning of the secondary cir- cuit.

Therefore it is possible to withdraw the energy for the detector at any desired rate without having to retune. The loudest signals are usually secured when the effect- ive resistance of the detector is adjusted, by means of Fi, to a value equal to the' effective ohmic resistance of the entire cir- cuit without detector. By the use of such an apparatus it is often possible to consid- ably increase the receiving efficiency of the wireless outfit.

��Connections for Vacuum Tubes to be Used in Wireless Telephony

IN THE illustration the microphone M is in an intermediate oscillary circuit, which is excited by a generating tube, V. The intermediate circuit is also coupled inductively with a three-electrode tube of large dimensions arranged for amplifying the oscillations occurring in the microphone circuit.

By adjusting the steady potential acting at the control electrode of the second tube it is possible to arrange that, uniform oscillations applied to it have no effect on the annode circuit, and then only the microphone variations are magnified. If this be done, the second tube may be made the same size as the first; or the micro-

���Diagram showing connections for a vacuum generating tube to produce oscillations

phone circuit may be arranged between the two coupled inductances belonging to the generating circuits of the tube V in which case the second tube will not be necessary.

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