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 



303 



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- 



