Page:Popular Science Monthly Volume 88.djvu/331

 Popular Science Monthly

��parts, one being of the wires themselves and another that of the earth's surface in the neighborhood of the antenna-base. All power losses in the antenna, includ- ing that due to the radiation of energy, represent additional parts of the effective resistance. All these component parts are added together to get the true total antenna resistance. For instance, in a large flat top aerial the wires might rep- resent an effective resistance of 0.3 ohm, the ground 0.4 ohm, losses ])y brush discharge 0.2 ohm, losses at the insulators 0.2 ohm, and the radiated power 0.8 ohm. Added together, the total resist- ance becomes 1.9 ohms; a closed circuit having the same capacity and inductance as the an- tenna, and including a resistance of 1.9 ohms in series, would permit the same current to flow as would the aerial when excited by the same frequency and voltage.

From the foregoing the fact appears that, for wavelengths long compared to the fundamental or natural wavelength, the electrical properties of an aerial sys- tem are in many ways equivalent to those of a circuit containing lumped in- ductance, capacity and resistance. An experiment with the arrangement of Fig. 3 will show this to be true. In the dia- gram A and G represent antenna and ground, which are connected to the "X" side of a double-throw double-pole switch. The "Y" terminals lead to a condenser Ci, inductance Li and resist- ance Ri, in series. Across the center points are connected the radio frequency alternator E, the inductance L2, and the ammeter /. Suppose the switch to be closed on the "X" side and the alterna- tor to be generating at 100,000 cycles per second frequency (which corre- sponds to a wavelength of 3,000 meters). Assuming the natural wavelength of the aerial to be considerably under 3,000 me- ters, if the inductance L2 be slowly in- creased the current reading of / will also increase, at first gradually and then

���Fig, 3

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rapidly, till it teaches a maximum value. If the inductance is still further in- creased, the current will grow smaller and smaller. ^ The largest current flows when the eft'ect of the inductance just neutralizes that of the capacity for the frequency used, or, in other words, when the antenna impedance is a mini- mum. The aerial system reactance is then zero, the impedance is equal simply to the effec- tive ohmic resist- ance, and the an- tenna is resonant or tuned to the al- ternator frequency. In this condition the current is de- termined only b y the total antenna resistance and the effective applied ^ oltage, irrespective of other factors.

If, now, the condenser Ci is made equal in value to the capacity of the an- tenna and the coil Li adjusted to equal the aerial inductance, the right hand cir- cuit will have a reactance equal to that of the antenna. If the switch is thrown to the "F'' position, with the alternator running at 100,000 cycles, and the in- ductance L2 is again gradually increased from zero, the current reading of / will fir.st increase and then decrease exactly as before. The point of maximum cur- rent will appear for the same value of L2 as when the antenna was connected; if the resistance Ri is set to a value equal to the total antenna resistance the greatest current in amperes will be ex- actly the same as with the switch in the "A^" position.

Thus it is evident that any antenna may be considered as an inductance, a capacity and a resistance in series, and that so far as current and voltage ef- fects are concerned the true aerial cir- cuit may be replaced by an artificial an- tenna consisting of equivalent condenser, coil and rheostat in series. This means that the considerations regarding the impedance of closed oscillation circuits and its arithmetic calculation, as given in the January article, may be applied almost without change to antenna cir- cuits. It is only necessary that the wave-

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