Topband: A word from the moderator

Carl Clawson carlclawson at
Tue Apr 26 17:24:33 PDT 2011

> -----Original Message-----
> One thing to think about in this discussion - is where the 
> current "goes"
> as it diminishes as you work your way along a 1/4 wave wire 
> (let's not bring the inductor into it just now for this 
> discussion).  I have never really thought about this - but 
> one of our signal integrity engineers quickly said "return 
> current".  Thinking about that - I guess it is pretty obvious 
> that the current in the ground connection of a vertical near 
> the feedpoint is equal to the current going to the antenna element.
> Return current can also be thought of as radiation.  Probably 
> radiation resistance enters into this somewhere - but I am 
> not smart enough yet to understand how that works.  :-)
> Tree N6TR

I think "displacement current" is the phrase most apt. It made a brief
appearance earlier in this discussion. If current is changing along the
length of a conductor, charge has to be piling up somewhere. The derivative
of the time varying electric field resulting from this varying charge
density is the displacement current. The term is somewhat deprecated by
modern physicists -- it isn't really a current, after all -- but it was
crucial to Maxwell's formulation of the theory of radiation.

The classic textbook example of displacement current occurs between the
plates of a capacitor, resulting from charge accumulating on (or draining
from) the plates.

73 -- Carl WS7L

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