How to Become a Wireless Operator
VIII. — Tuning with the Variable Con- denser in Primary and Secondary
��Bv T. M. Lewi
��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
��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-