Page:Popular Science Monthly Volume 88.djvu/172

144 Wavelength

requency

Meters

Cycles per second

300

1,000,000

600

500,000

1,000

300,000

2,000

1 50,000

3,000

100,000

These frequency values are not only the numbers of cycles per second in the radio waves, but also the frequencies of the oscillating currents which will set up such waves. Referring to Figure i, if E represents a radio frequency alter- nator which generates current of 100,000 cycles per second in the antenna-to- ground circuit A, I, B, E, G, the system will radiate waves corresponding to that frequency, or 3,000 meters in length. The stronger the 100,000 cycle current in the antenna, the more powerful will be the radiated waves. It is therefore desirable to do anything possible to in- crease this antenna current. Also, the higher the antenna the more powerful will be the radiation of waves for a given current. It is for this reason that great heights are sought in erecting sending antennas.

\\'hen a battery or direct current gen- erator applies a voltage or electrical pressure across the terminals of a circuit having resistance, an electric current flows through that circuit. The strength of the current is fixed by the amounts of voltage and resistance, and, measured in amperes, equals the number of volts pressure divided by the number of ohms resistance. This is simply Ohm's Law in its elementary form, and the fact is one of the first things learned in the study of electricity. But its extension to alternating current circuits is not so well understood, though it is very little more complicated. In fact, the same law in the same form applies to alternating cur- rents, if one uses instead of the simple ohmic resistance its alternating current eqviivalent, or impedance.

Impedance, or effective alternating- current resistance, is the property of cir- cuits which determines how much cur- rent will flow when a certain alternating voltage is applied. The current in am- peres is always equal to the applied electro-motive force in volts divided by the impedance in ohms. If, in Figure i,

Popular Science Monthly

the alternator E generates 100,000 cy- cles and 100 volts, and if the total im- pedance of the antenna-to-ground circuit is 5 ohms for this frequency, a radio- frequency current of 20 amperes will flow through the ammeter I, and waves of corresponding intensity will be radi- ated. If the impedance were 10 ohms, only 10 amperes would flow and the waves would be very much weaker. Evi- dently for powerful sending the antenna circuit impedance must be kept as small as possible, since then the current is largest.

How can the impedance be made small? Before this question can be an- swered it is necessary to find out what impedance really is, and whether it is always the same for any particular circuit.

Four quantities enter into the makeup of impedance, and these are the resist- ance, capacity and inductance of the cir- cuit, and the frequency of the current flowing in it. That portion which de- pends upon the capacity and inductance of the circuit is called the reactance, and changes as the frequency changes. This reactance is always added by a special rule to the simple resistance to make up the total impedance. The resistance it- self remains practically constant for rea- sonably small changes of frequency, but the reactance may vary greatly if the frequency is changed even slightly. The effort to increase antenna current by making impedance as small as possible must therefore be confined almost en- tirely to reducing the reactance portion, since the simple resistance of coils, wires and earth connection is always reduced to the smallest feasible amount to begin with.

The computation of reactance in al- ternating current circuits is not compli- cated, and may be considered in two parts. Referring to Fig. 2, a resistance R is seen in series with an alternator E and ammeter I. Since reactance de- pends upon the presence of inductance or capacity, and since the circuit of Fig. I has no inductance or capacity, there is zero reactance. The impedance is there- fore made up of the resistance R only, and the current I is found, in amperes, by dividing the resistance in ohms (which in this case equals the impedance in