Topband: "return" current - what is it?

[Tom W8JI]

> to make a confirming measurement.  When he did arrive at the chosen
> measurement point he would find nearly but never all of the injected 
> current
> had disappeared.  This would not have been a surprise to him but maybe to
> the rest of us. How could this have happened?

With an infinitely long conductor, the answer is displacement currents. 
There is no reflected wave, and no need for a reflected wave.

It's all part of Maxwell's equations.

If we look at any conductor at any frequency any change in current along 
length is always by charges going somewhere. The path can be through 
displacement currents with alternating current.

This is why current distribution is different in a small component of 
limited shunt capacitance to the surrounding environment than a large 
component when shunting capacitive reactance starts to be comparable to 
series impedance. That's why we should keep high reactance loading coils 
away from external conductors, and not put top hats right against coils, or 
use large metal end caps on coils.

This effect causes problems in physically large high impedance plate chokes, 
and is the main reason we should avoid excessive inductance and use minimum 
size chokes in RF applications.

http://www.w8ji.com/rf_plate_choke.htm

73 Tom