Popular Science Monthly 



155 



Receiving Various Wavelengths 



With the senders and receivers limited 

 in their activities to communication be- 

 tween a single pair of stations, it is usually 

 not necessary to provide for variation in 

 the tuned wavelengths of either. That is 



ANTENNA 



TUNING-COtL 



TELEPHONE 



MICROPHOME- 

 DETECTOR \ 



^ 



5= 



DRY-CELL -^^^^v _ GROUND 



~ flG.3 



A simple arrangement which has the detector 

 in series between the antenna and ground 



why the simple arrangement of Fig. 2 In 

 the December article could be used. When 

 it is desired to receive from a large number 

 of outside transmitters, all using different 

 wavelengths, it is necessary to provide 

 apparatus whereby the effective length of 

 the aerial at the receiver may be varied to 

 suit the incoming wavelength. A large 

 number of arrangements may be used for 

 this purpose. Some of them tune the 

 receiver very sharply, or in other words 

 make it respond energetically to a very 

 closely restricted range of wavelengths 

 for each adjustment. Other sets of connec- 

 tions are less critical in adjustment, but 

 easier to handle. 



The simplest variable tuning instrument 

 for use at the receiver is the so-called 

 "single-slide tuner." This is merely an 

 inductance coil with a sliding contact 

 whereby the number of turns in circuit may 

 be varied at will. It may be used in place 

 of the tuning coil shown in Fig. 3, and will 

 allow some latitude of adjustment, though 

 the tuning is very broad and unsatisfactory. 

 A better mode of connection for the single- 

 slide tuner is that of Fig. 4, in which the 

 detector is put in a side or by-pass circuit; 

 this gives sharper tuning and fairly strong 

 signals. 



A still better tuning arrangement fuses 

 the "double-slide tuner," which has two 

 variable contacts. In the catalogs of 

 manufacturers of radio apparatus there are 

 to be found a large number of diagrams 



showing different ways to connect the 

 double-slide tuner; but the best possible 

 results are to be secured from the arrange- 

 ment of Fig. 5. One end of the coil is 

 connected to ground, and one of the sliders 

 to the antenna. The larger the amount of 

 coil between the grounded end and this 

 first slider, the longer the effective length 

 of the aerial and the greater the wavelength 

 for which it is tuned. The grounded end is 

 also connected to one side of the blocking 

 condenser described in the November 

 article, and the other slider is connected 

 with one terminal of the cr>-stal-detector 

 also illustrated in November. The tele- 

 phone has one lead connected with ground, 

 and the other joins the open sides of the 

 detector and blocking-condenser. The 

 test-buzzer, which is not shown in Fig. 5, is 

 to be arranged as explained in the December 

 article, so that the cr>stal-detector can be 

 adjusted to its maximum sensitiveness 

 without waiting for signals from outside 

 stations. 



Operating the Variable Receiver 



In working the apparatus set up as in 

 Fig. 5, the first step is to make sure that the 

 detector is adjusted to a sensitive point, 

 and that the connections are all secure and 

 in good condition. Then the slider connect- 

 ed to the detector is set at a position about 

 half-way along the coil, and the antenna- 

 slider is moved back and forth slowly along 

 the length of the entire tuner. When a 

 station within range starts to send, his 



ANTENNA 



L0AD1N6-COIL 



?o 



CRYSTAL-DETECTOR 



y 



BL0CKIN6- 

 CONDtKSER 





GROUND 



FI6 ♦ 



TELEPHONE 



Connecting the detector around the loading- 

 coil produces a highly selective arrangement 



signals will be heard in the telephones; it 

 will be noted that the dots and dashes are 

 loudest with the antenna-slider at some 

 particular setting. Leaving the antenna- 

 slider at this point, the detector-slider is 

 moved back and forth until the position 

 giving the best signals is found. This is the 

 tuned or approximately tuned adjustment 

 of the receiver for the specific wavelength 

 being received. 



