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Page:Popular Science Monthly Volume 90.djvu/491

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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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