620 



Popular Science Monthly 



that the 



with the 



That is 



millihenrys with 

 the antenna capac- 

 ity of o.ooi mfd. 

 times to the wave- 

 frequency of 60,000 

 cycles and there- 

 fore to the wave- 

 length of 5000 me- 

 ters. 



The principle of 

 tuning the antenna 

 circuit, then, is to 

 change its induct- 

 ance or capacity 

 or both in such a 

 way and by such amounts 

 resonant wavelength agrees 

 length of the incoming wave 

 to say, the free-oscillation frequency of 

 the circuit must be made practically the 

 same as the frequency of the forced 

 oscillations generated in the antenna by 

 the received electromagnetic waves. 

 These waves, of course, produce forced 

 oscillations of their own frequency; 

 hence it becomes necessary merely to 

 adjust the antenna so that it will natu- 

 rally radiate the wavelength which it is 

 desired to receive. 



If a secondary circuit is coupled to the 

 antenna, as in Fig. 2, the same general 

 conditions apply. In this diagram the 

 antenna A is connected to earth through 

 inductance coils Li and L2, as before. 

 The lower coil is used as the primary of 

 an inductive coupler, whose secondary 

 is the third coil L3. Across this seconda- 

 ry is connected a variable tuning con- 

 denser Ci, and in shunt to this the 

 crystal detector R and the stopping-con- 

 denser C2. This latter instrument has 

 connected to its terminals the telephone 

 receivers, T. In operation, the antenna 

 circuit must be tuned to' the frequency 

 of the incoming waves by varying the 

 inductance of Li or Lo, exactly as in the 

 example just considered. If the antenna 

 capacity is o.ooi mfd. and the incoming 

 wave has a length of 5000 meters, the 

 sum of the effective primary inductances 

 must be about 6.94 millihenrys. A dis- 

 tribution which would agree with good 

 practice would allow 0.05 millihenry for 

 the antenna itself, 5 millihenrys for the 

 loading-coil L; and the balance (1.89 

 millihenry) for the primary coil L2. It 

 would be entirely feasible to have the 



Fig. 3. Diagram of a two-circuit 

 tuner, the same as in Fig. 2, with 

 the addition of a potentiometer and 

 battery for adjusting the detector 

 to maximum efficiency 



entire inductance 

 of coils Li and L2 

 in a single primary 

 winding, but the 

 convenience of a 

 separate loading- 

 coil for long waves 

 makes it desirable 

 to divide the coils 

 as indicated. 



The secondary 

 coil L3 and the 

 tuning - condenser 

 Ci make up a 

 closed, oscillating 

 circuit of the kind discussed in the Janu- 

 ary article. In order to transfer the 

 most power from the primary or aerial 

 circuit to the secondary, so that the de- 

 tector may be operated by the strongest 

 impulses, it is necessary to adjust the 

 time period of the secondary oscillation 

 to agree with that of the primary. In 

 other words, the secondary must have 

 its inductance and capacity adjusted so 

 that it is tuned to the wave-currents 

 flowing in the primary. The resonant 

 frequency of the secondary must be 

 made the same as that of the primary, 

 and the same as the frequency of the in- 

 coming wave. If the secondary coil L3 

 has an inductance of 4 millihenrys, the 

 condenser must be set at 0.00173 mfd. to 

 give resonance for the assumed wave- 

 length of 5000 meters. When the ad- 

 justment is such that the effective values 

 of capacity and inductance are these, 

 and when the coupling between the coils 

 L2 and L3 is chosen so that the transfer 

 of power is at the rate which is best for 

 the detector in use, the loudest signals 

 will be heard in the telephones. 



The numerical values of inductance 

 and capacity given in these two exam- 

 ples, it must be noted, are the effective 

 values for the circuit considered. That 

 is to say, the assumed frequency of 

 oscillation will occur if the circuits be- 

 have as though these exact values of 

 coil and condenser were used. The 

 real measured values of capacity and in- 

 ductance may be somewhat different 

 (though not very much) from the quanti- 

 ties worked out by applying the simple 

 rules; this is because the coils in the 

 circuit react upon each other and par- 

 tially destroy the pure inductive effect 



