Page:Popular Science Monthly Volume 88.djvu/171

 RADIO SECTION

Devoted to the Encouragement of Amateurs

and Experimenters in the Field of

Radio Communication

��Impedance of Oscillation Circuits in Wireless Telegraphy

By John Vincent

��IN last month's article it was shown that every antenna had a particular natural wave-length, or fundamental wavelength, which it would radiate if it were excited electrically and then left to oscillate. It was pointed out that this natural wavelength depended upon the capacity and inductance of the aerial, and that these in turn depended upon the total length of the antenna-to-ground system. It was also shown that if in- ductance were added in series with the antenna, so as to "load" the system elec- tically, the resonant w^ave- length would be increased. A simple rule for comput- ing arithmetically the reso- nant radiant wavelength in meters, when the capacity in microfarads and the in- ductance in millihenrys is known, was stated.

It should be noted espe- cially that the wavelength radiated depends upon the size of the capacity and inductance coils in the cir- cuit. The reason for this is that the length of radi- ated wave depends upon its frequency, or the number of times in one second the electromagnetic field ])ass- cs through a complete cycle of change in direction. This wave-frequency must

���Fig. 1

��Fig. 2

��Kig.

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��be the same as the frequency of the oscillating current in the antenna system, which produces it, and the oscillation frequency is determined by the amount of capacity and inductance in the anten- na circuit.

Considering ether-waves of the sort used in radio-telegraphy, which pass over the surface of the earth from the sender to receivers in any direction at a speed of 186,000 miles per second, the usual relation between velocity, wave- length and wave-frequency may be used. In these waves, as in any other traveling waves, the frequency is found by di- viding the velocity by the wavelength.

A wavelength of 2,000 meters has, therefore, a w^ave-frequency of 150,- 000 per second, since the velocity in meters per second (300,000,000) di- vided by the length (2,000 meters) gives this figure. Thus, to find the frequen- cy per second of any wave- length in radio, divide three hundred million by the wavelength in meters. Similarly, to find the wave- length in meters for any fre(|uency, divide the fre- quency per second into 3 300,000,000, which goes:

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