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. 



