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. 



AA/wyv 



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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