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.
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