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. 



=/ 



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. 



306 



