Page:Popular Science Monthly Volume 88.djvu/330

 Antenna Circuits in Radio Telegraphy

��By John Vincent

��IN the two earlier articles of this se- ries, the simple relations between ca- pacity, inductance, wavelength and resonant frequency were explained. It was shown that in a closed circuit such as that of Fig. 1, the maximum current would flow when the impedance (or al- ternating current resistance) was made as small as possible. It was also shown that by adjusting the circuit capacity C and inductance L, they could be made to neutralize each other's effects for the particular frequency of the alternator E, and that when the circuit was in this resonant condition, the current flowing was dependent only upon the voltage generated at E and the resistance R.

The relations of induct- ance and capacity to frequen- cy and wavelength, and those of voltage and impedance to current, exist in "open"' an- tenna circuits such as that of Fig. 2, exactly as in closed circuits like Fig. 1. For most purposes the computations explained in the January ar- ticle will give good results for either open or closed circuits. The only error likely to cause trouble depends upon the fact that in the elevated part of an antenna circuit there are both capac- ity and inductance. In the closed cir- cuit (Fig. 1) practically all the capacity is lumped together at C and nearly all the inductance at L. In the antenna, however, for short waves the inductance L may be quite small and so the distrib- uted inductance of the antenna wires may play an important part in determin- ing the resonant frequency of the sys- tem. For most radio telegraphic pur- poses waves considerably longer than the natural wavelength of the aerial are used, and with these the antenna may be considered to be the equivalent of an inductance, a capacity and a resistance all connected in series.

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Fig. 1

��Fig. 2

��If one thinks of capacity as a prop- erty possessed by any pair of conductors separated by an insulator (which is a correct idea), it is easy to see that an antenna has capacity with respect to the earth. As the two plates of a condenser are separated by an insulator and have capacity with respect to each other, so, in the antenna system, the aerial wires and the earth's surface, (both of which are conductors) are separated by the in- tervening air. The capacity of the aerial system is a definite quantity depending upon the distribution of current in it, and like that of any other condenser may be computed or measured.

Inductance is a property of conduct- ors which makes itself known by the magnetic effects pro- duced upon these conductors when the currents through them vary. Since direct cur- rent is usually of uniform strength, in direct current circuits inductance is not often considered ; neverthe- less, the property is always present and ready to become prominent when the current varies. In radio antenna sys- tems, alternating current flows and therefore the inductance of the wires is important. One hundred feet of anten- na wire stretched out straight has about 0.07 millihenry inductance, which is equivalent to about tiventy turns of No. 24 wire wound in a coil of 4"' diameter. For a given length of wire a coil has much more inductance than a straight wire, because each portion of it can act magnetically on the turns beside it. Thus the inductance of an antenna wire can be represented by that of a small coil, just as its capacity may be represented by that of a condenser.

Antenna systems, like other conduct- ors, possess electrical resistance in addi- tion to their capacity and inductance. This resistance is made up of several

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