376 Dr. Balth van der Pol on the Propagation of 



The moment M is defined as the finite product of the 

 infinitely increasing amplitude of the charges q (in electro- 

 static units) at the ends of the dipol into its length I in 

 centimetres, while I shrinks indefinitely. If, however, the 

 wave-amplitude is only considered at points at a great 

 distance from an oscillator of finite dimensions (great in 

 comparison with these dimensions), M can be defined in the 

 same way by the finite quantities q and I, so that also in 

 this case 



M = qjt. 



As it is usual to measure the current amplitude I at the 

 bottom of a transmitting antenna, we introduce the relation 

 between I and q . We have 



t dq 



1=-7T and 



at 



A/ y T 



q ° = 2ttc 0E - S ' U = 20tt oam P- 



When the antenna height is not small in comparison with 

 the wave-length emitted, the current distribution over the 

 antenna is not quasi-stationary. This fact can simply be 

 allowed for by introducing a form -factor a (Zenneck) giving 

 the ratio of the mean current amplitude over the sender 

 height into the maximum current, which is usually found 

 near the bottom of the transmitting aerial. 



The moment of a sender station can therefore be ex- 

 pressed as 



M _Vm. a i T 7, 



1Y1 — — ^ J-iamp. /fc l cm.' 



where h v is the actual height of the transmitting antenna. 



The amplitude of the electric force at the earth's surface 

 set up at an orientation 6 from such an oscillator is accord- 

 ing to (8) 



13 



&o e.s.u. - a a v /--^ ~ pi 207r "l^^lamp. e 



where all lengths are measured in centimetres. 

 Further we have 



•Eo volts/cm. = 300E e.s.u. 



If in this radiator field a tuned receiver antenna is placed 

 of height h 2) form-factor a 2 , and total equivalent resistance 



