Popular Science Monthly 



467 



tenna for the frequency of the closed 

 circuit, forced alternating currents of 

 the same frequency and largest am- 

 ])litude will be induced in the antenna 

 circuit. These large currents surging in 

 the aerial will produce electromagnetic 

 waves of the same frequency and cor- 

 responding length. Thus the discharge 

 of a condenser in a closed circuit may 

 be used to generate waves for radio 

 telegraphy ; for the best effect, the an- 

 tenna circuit must be adjusted so that 

 its natural frequency is the same as that 

 of the closed circuit, or, in other words, 

 both must be tuned to the wave fre- 

 quency. 



This principle may be applied to a 

 case corresponding to the circuits of the 

 average inductively - coupled amateur 

 transmitter. Since the present laws 

 limit amateurs to wavelengths below 200 

 meters, it is necessary to use such in- 

 ductance and capacity in the primary as 

 will give waves below this value. Prac- 

 tice has shown that it is net feasible to 

 use a condenser larger than 0.01 micro- 

 farad in size ; this, with an inductance 

 Lj equal to 0.0011 millihenry (includ- 

 ing lead wires), wall produce free alter- 

 nating currents of 1,500.000 per second 

 frequency, which corresponds to 200 me- 

 ters w'avelength. Since, for this size of 

 condenser, the total permissible induc- 

 tance is so small, it will often be better 

 to use smaller condensers and more in- 

 ductance ; for instance, 0.005 microfarad 

 capacity and 0.0022 millihenry induct- 

 ance or even 0.001 microfarad capacity 

 and 0.011 millihenry inductance (both 

 of which combinations tune to 200 

 meters) will give better results in 

 many stations. The average small an- 

 tenna, such as may be used for 200 

 meters sending, will ha\e a capacity of 

 about 0.0004 microfarad. The sum of 

 inductances in coils Lr and L2 will 

 therefore be 0.027 millihenry for 200 

 meters. The secondary may be made 

 identical with the primary, and the ])al- 

 ance of the inductance needed placed in 

 the load coil Li. 



The values quote<l are not absolutely 

 accurate, of course, for every station 

 will have small variations in length of 

 lead wires, closeness of coupling, regu- 

 larity of gap action, etc.. which may 

 modify slightly the amounts required. 



The best way to get true tuning-ad- 

 justment is to set the closed circuit at 

 the desired wavelength, by calculation 

 or wavemetcr, and then to alter the 

 coupling between Lj and L2 and the 

 amount of inductance in Li, until a hot- 

 wire ammeter in the antenna circuit 

 shows the greatest possible current to be 

 flowing. For good results, the coupling 

 must not be too tight. When very small 

 primary inductances are used in induc- 

 tively coupled transmitters, it is not 

 likely that the coupling will be tight 

 enough. 



The circuit of Fig. 4 is the equiva- 

 lent of Fig. 3, except that the closed 

 oscillation-circuit is directly coupled to 

 the antenna circuit. Part of the primary 

 coil is used as the secondary, as indi- 

 cated by the portion between the right- 

 hand clip and the earth, marked L2. 

 The computations given above apply to 

 this circuit as well as to that of Fig. 3, 

 but, with the direct coupling here shown, 

 it is sometimes possible to get satisfac- 

 tory operation with larger primary con- 

 densers than when the inductive coup- 

 ling is used. Since larger condensers 

 make it possible to use more transmit- 

 ting power for the same voltage and 

 spark frequency, the direct coupling 

 may be preferred in some senders. Con- 

 trary to the widely accepted idea, it is 

 possible to get just as sharp waves with 

 the direct as with the inductive coupling. 

 It is necessary to tune the circuits with 

 care, however, and to have the greater 

 part of the total antenna inductance in 

 the loading coil Li. 



The above stated principles of tuning 

 and adjusting various open and closed 

 circuits for maximum effect, with both 

 free and forced oscillations, include the 

 fundamental laws of radio telegraphy 

 and telephony. The simple rules which 

 have been given in the five articles of 

 this series may be applied to all types of 

 transmitting and receiving circuits, and 

 permit selection of apparatus which will 

 operate successfully in various circum- 

 stances. The computation of receiving- 

 circuit constants will be discussed next 

 month ; and after power in transmitters 

 is treated, designs will be given for coils, 

 condensers and other instruments which 

 may be combined according to these 

 rules. 



