How to Become a Wireless Operator 



VII. — ^The Variable Condenser for Tuning the Secondary 

 By T. M. Lewis 



THE direct coupled receiving tuner 

 which was described in detail last 

 month will prove satisfactory for long 

 distance working, up to the limit fixed by 

 the sensitiveness of the detector and tele- 

 phones used in connection with it. The 

 loudness of signals, if the coils and switches 

 are properly built, should compare very 

 favorably with the best obtainable by using 

 an inductively coupled receiving trans- 

 former of equivalent design. To get the 

 strongest signals, in either the direct or 

 the inductively coupled outfit, the operator 

 must determine by trial the best settings of 

 primary and secondary inductance and 

 coupling, as previously pointed out. 



Sharpness of Tuning 



Although the simple tuners without 

 variable condenser in the secondary will 

 give loud signals, and in fact about as loud 

 as can be obtained by any arrangement of 

 tuning circuits when working with spark 

 stations, the best sharpness of tuning can- 

 not be secured. When two transmitters 

 are heard at about the same intensity and 

 on only slightly diflferent wavelengths, it is 

 cRffrcult to build up the signals of either at 

 will by the mere adjustment of the coupling 

 and primary and secondary inductances, 

 when the circuit without variable condenser 

 is used. For this reason the tuner, whether 

 inductively or conductively coupled, which 

 has the detector directly in series with the 

 blocking condenser (i. e., which has no 

 condenser directly across the secondary 

 terminals for tuning) is called "broad 

 tuned." However, if a variable condenser 

 is connected as shown in Fig. i and 2, the 

 selective powers of the circuit become very 

 much greater and it is called "sharp tuned." 



The details of adjustment which are 

 necessary in order to get the best results 

 from the sharply tuned receiver will be 

 taken up in full next month. The manipu- 

 lation of this receiver should be second 

 nature to all radio operators, since it is the 

 arrangement of apparatus used by the great 

 commercial companies. In the hands of an 

 unskilled operator better results are some- 

 times obtained with simpler circuit ar- 



rangements; the reason for this is simply 

 that the man does not know how to get 

 the most out of the sharply tuned lay-out. 

 The circuits of Fig. i and 2, when correctly 

 adjusted, provide the maximum selective- 

 ness which is to be had in the best commer- 

 cial receivers in general use. Securing the 

 correct adjustment, once the principles are 

 clearly understood, is merely a matter of 

 practice. It is essential for the student, 

 therefore, to familiarize himself with the 

 actions of such receivers under all condi- 

 tions likely to be encountered in practice. 



The Variable Condenser 

 Since Fig. i is exactly the same as Fig. 7 

 of last month's article, except for the addi- 

 tion of the variable condenser and a single 

 ANTENNA ^^^ switch for Cutting it out 

 of circuit, all of the instru- 

 ments described in the pre- 



FIG.l 



- " PHONtS \^_y 



Diagram for direct coupled receiver with 

 variable condenser and a single pole switch 



ceding articles of this series may be 

 utilized. The switch of Fig. i is preferably 

 a small single pole, single throw knife- 

 switch, since this type almost invariably 

 gives good contact, though any other form 

 will be satisfactory if kept in good condi- 

 tion. The variable condenser is the im- 

 portant new instrument, and must be of 

 good design if it is to be really useful. The 

 amateur who has sufficient funds at his dis- 

 posal will do well to buy one of the standard 

 variable condensers now on the market; 

 if he sticks to the ordinary semi-circular 

 rotary type, having a capacity of about 

 .001 microfarad and costing from $5.00 to 

 $25.00, he will be likely to get a good in- 

 strument for tuning. The cheapest ap- 

 paratus, as well as the various freak forms 



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