Page:Popular Science Monthly Volume 88.djvu/648

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��Popular Science Monthly

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��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

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