How to Become a Wireless Operator 



VIII. — Tuning with the Variable Con- 

 denser in Primary and Secondary 



Bv T. M. Lewi 



IS 



LAST month's article described in detail 

 the construction of a simple variable 

 condenser which could be built at 

 home and which is entirely suitable for 

 use in either the primary or secondary- 

 circuit of any normal radio receiving 

 apparatus. It is the purpose of the present 

 article to show how one or more of these 

 variable condensers may be used to the 

 greatest advantage, both for increasing 

 sharpness of tuning (and thereby lessening 

 interference difficulties) and for increasing 

 signal strength. The great advantage 

 which is secured immediately upon the use 

 of a variable condenser is in the flexibility 

 of circuit adjustment which can thus be 

 secured. Practically all the effects of mere 

 tuning to wavelength can be obtained from 

 variable inductances alone, using fixed con- 

 densers. However, to change the ratio of 

 inductance to capacity, which is one of the 

 important factors governing sharpness of 

 tuning, it is necessary that both the con- 

 densers and the coils be variable; if only 

 one element can be altered, it is not possible 

 to secure any desired ratio at any desired 

 wavelength. 



A second advantage gained by using 

 variable condensers is the mechanical 

 simplification of the inductance coils con- 

 nected with them. Since the condensers are 

 continuously adjustable, and can therefore 

 be set at any desired value of capacitv* with- 

 out the limitation of switchpoint steps, it 

 is unnecessary to take out many coil taps 

 in order to get close variation of inductance. 

 Any wavelength can be tuned to with ac- 

 curacy, with only a few steps or connections 

 on each inductance coil, because the con- 

 denser fills in the tuning range. 



Advantages of the Sharply Tuned 

 Secondary 



There has been some dispute as to 

 whether a secondary tuning condenser, 

 ' onnected in the usual way, shown in Fig. i 

 nd 2, aids in producing a receiver more 

 effective than the simple "broad tuned 

 secondary" circuit. Extended trials have 

 shown beyond a doubt that the sharply 

 tuned circuit of Fig. i and 2, when properly 



adjusted, will give greater selectivit>' for 

 the same strength signals than the circuit 

 without variable condenser in the secon- 

 dary. By building the secondaries specially 

 to suit each case, about the same maximum 

 signal strength may be had with both 

 types. With the secondary-^ variable con- 

 denser, however, the maximum signals are 

 secured with looser coupling between 

 primar\' and secondar>'. As a result, the 

 tuning is sharper and interference is much 

 reduced. The practical actions of such 

 tuners as Fig. i and 2 represent should be 

 studied in detail by every operator, and the 



TELtmOHtS 



FIG.l 



The receiver is set up by combining the 

 tuning coils with the variable condenser 



differences in operation dependent upon 

 closing and opening the condenser "switch" 

 in the secondary circuit should be particu- 

 larly noted. 



The receiver of Fig. i is set up by com- 

 bining the tuning coils described in the 

 February article with the variable con- 

 denser of last month. The resulting 

 arrangement is capable of excellent signal 

 intensity and tuning sharpness. Although 

 it is a directly or conductively connected 

 auto-transformer type of receiver, it has 

 variable coupling between primary and 

 secondar>', and is therefore not of)en to 

 much of the criticism applicable to the 

 ordinary "two-slide tuner" combinations. 

 For the same reason it requires more careful 

 handling in order to produce the best 

 results. 



In operating any of the tuners described, 

 it must be borne in mind that the instruc- 



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