Page:Popular Science Monthly Volume 88.djvu/334

 TheStaticCoupledReceivingTuner

��By John L. Hogan, Jr.

��NEARLY all experimenters are fa- miliar with the action of the ordi- nary inductively coupled receiving tuner illustrated in Fig. i. \\'ith this arrangement of apparatus, if the ele- ments are well designed and manipu- lated, excellent results in tuning may be secured. The construction is not always easy, however, since the primary and secondary coils must usually be so built that one may slide within the other. It is difficult to devise ways to connect con- veniently to various taps on the movable coil without introducing losses in lead wires. Such losses in- variably result in weakened sig- nals, and prevent reception of signals from the greatest possi- ble distances.

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��Fig. 1.

��Magnetic coupling for receiv- ing tuner

��A somewhat different type of tuner, which is now coming into rather exten- sive use, usually gives sharp tuning and loud signals, yet is very easily assembled. The connections are shown in Fig. 2, and may be seen on examination to bear some resemblance to the inductively coupled layout of Fig. i. In both dia- grams the antenna and earth are shown by A and E, the primary circuit loading coil by Li, the primary by L2. the secon- dary by Lj, the secondary tuning con- denser by Ci, the blocking condenser by Cp, the detector by D and the telephone by T. In Fig. i the primary and sec- ondary coils are placed rather close to- gether, so that energy may be transfer- red electromagnetically by the action of the lines of magnetic force linking both coils. In Fig. i, the mutual inductance of the primary and secondary (and there- fore their coupling) is altered by moving the coils toward or away from one an- other; when near together the coupling is close and the selectivity poor, when

��far apart the coupling is loose and the selectivity or sharpness of tuning great- er. The gain in selectivity is often ac- companied by a reduction in signal strength.

In Fig. 2, the primary and secondary coils are set far apart, so that there is practically no magnetic coupling between them. A third condenser, Cj, which is preferably variable and of small mini- mum capacity (say of from 0.00005 to 0.001 microfarad range) is put in circuit as shown. This additional condenser gov- erns the coupling of the system ; when Cj has small values the coupling is loose and the tuning sharp, and when Cj is increased the opposite condition is ap- proached. The two coils need not be moved at all in order to secure any of the desired coupling effects ; therefore, either primary or secondary or both may be variometers and the end-switch losses thereby eliminated.

In tuning with the condenser-coupled circuit the ordinary procedure is fol- lowed. The coupling is made close and, with the secondary condenser discon- nected by opening switch S, the primary is adjusted until the desired station is heard with the greatest loudness. The switch 5" is then closed and the secondary system tuned by varying Lj and Ci. If interference is present, or if the incom- ing signals are very sharply tuned, the best results are se- cured by gradually loosening the coupling and at the same time adjusting L2 and Ci, to keep the

���: Fig. 2. Static coupling for receiving tuner

��signals at maximum strength. The reso- nant wave length of the coupling circuit L2, Cj, Lj. is generally much shorter than that which is being received. Test- ing this static-coupled received will be well-spent eft'ort.

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