Topband: Re: where's the current in the coil?

[Tom Rauch]

> I'd like to support Barry's assertion that most of the
> current is only in the bottom few turns of coil. As

To do that we would have to have a coil that is either very large in terms
of wavelength, has considerable distributed capacitance to "ground", or that
violates basic laws of physics and electronic circuits.

To measure current we would have to actually measure current through the
coil, not watch a lamp light from exposure to the electric field in the
general area of a coil. Current is a "through" vector, voltage is an
"across" vector.

> one of the first users of the Minooka Special, I
> recall moving a florescent light up and down the
> length of the coil. Back in the AM daze we used to do
> that for grins. I noted then that the lamp would light
> just up from the bottom of the coil. I guess one could

A florescent lamp responds to the electric field Greg, not current.

In order to conclude anything about current distribution we must measure
current...not electric fields.

> map the current by measuring the distance from the
> coil where the lamp would begin to light then
> calculating the voltage at any given point along the
> coils length.

When we have a reactive system with phase errors between voltage and
current, such as occurs in a small antenna, maximum voltage can not be
directly multiplied with current to determine true power except at a point
where reactance has been cancelled and voltage and current are in phase.
This why we have VAR power or reactive power in systems.

When we have a two-terminal component, current into one terminal always
equals current out the other terminal. The sole exception is when that
component is physically so large that stray capacitance to the outside world
provides a third path.

Voltage from each end of a two-terminal component to "ground" can be and
usually is different.

An antenna must always have a given amount of ampere-feet to radiate a given
amount of power as EM energy.

This is why current is very high at the base of a short whip antenna, even
though voltage is also very high. It is also why feed impedance of a
current-fed single-conductor antenna always equals the radiation resistance
plus loss resistances normalized to the point of feed.

We can not possibly have high current in the first few turns only. There
really isn't anything complex, magic, or esoteric in any of this. It is
basic electronics.

73 Tom