Roy Lewallen wrote:

Read again the fourth sentence of the posting you quoted.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:

If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.


What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the current become the same at both
ends?

73, Jim AC6XG

Okay. I read it. I'll try asking the question another way.

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

73, Jim AC6XG

Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground.

The only way I can see to model it and get these particular results
would be to have it as a device which increases in impedance at every
point along its length, and which has a current path to ground at each
of an infinite number of such points.

73, Jim AC6XG

Jim Kelley wrote:

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

Sorry to be obtuse, Roy. The point is only that antenna circuits
obviously present a problem to the assumption that such two terminal
black boxes will necessarily have equal currents at both terminals.

A solenoid should produce a field in the direction of its axis, should
it not?

73, Jim AC6XG

Jim Kelley wrote:
What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

The two-terminal black box must have the same zero dimensions
as the inductance. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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You also need to go back and read the fourth sentence of my posting.

Sheesh.

Roy Lewallen, W7EL

Cecil Moore wrote:
. . .
Are you saying I cannot coil up 1/2WL of coax inside a black box and
observe current flowing into both terminals for 1/2 cycle and current
flowing out of both terminals for 1/2 cycle? That would be quite a
revelation.

Jim Kelley wrote:

Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground.

The only way I can see to model it and get these particular results
would be to have it as a device which increases in impedance at every
point along its length, and which has a current path to ground at each
of an infinite number of such points.

For a 1/4WL vertical, that would be true even if the path to ground
is through displacement currents. But, what about free space?
--
73, Cecil http://www.qsl.net/w5dxp



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On 3-Nov-2003, Wes Stewart wrote:
....
Whatever we were doing required the use of a "magic" T. I, the ever
inquisitive student, asked; "Doc, how does a magic T work?"

Doc, former professor and the author of "Intermediate Mathematics of
Electromagnetics", replied,

"It's magic."

In the US Navy, we usually explained that such devices worked on the principle of
PFM. First word is Pure, Last word is Magic

-ken-

It would be even nicer if readers would read it.

An antenna could be regarded to have any number of terminals you'd like.

EZNEC does not make voltage measurements. Please go back and read the
lengthy recent thread regarding the meaning of voltage and voltage
measurement. I recall you were a major contributor. But it's really not
necessary to re-post it. Please don't.

EZNEC does show the voltages across sources and loads, both of which are
two-terminal lumped components.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Read again the fourth sentence of the posting you quoted.


It would be nice just to repeat it so 5000 readers don't have to
go searching for it.

In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground. To what is EZNEC referencing voltage measurements
on an antenna in free space?

Assume the box is non-conducting. There would be a current difference
between the two terminals unless you shrunk the length of antenna it
contains to a vanishingly small length. In the context of the antenna,
even a few feet of an antenna is a significant fraction of a wavelength.
Put the box around a few feet of a power transmission line carrying 60
Hz current, and you won't be able to measure any difference between the
current going in and coming out. In that situation, a few feet isn't a
significant part of a wavelength.

If you use a conducting box, you'll end up with current on the outside
of the box that gets into the problem. Its value depends on coupling to
the wire inside and to the antenna outside, so it gets stickier than I
want to deal with.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:

Read again the fourth sentence of the posting you quoted.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.


What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the current become the same at both
ends?

73, Jim AC6XG


Okay. I read it. I'll try asking the question another way.

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

73, Jim AC6XG

Cecil Moore wrote:

Jim Kelley wrote:

Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground.

The only way I can see to model it and get these particular results
would be to have it as a device which increases in impedance at every
point along its length, and which has a current path to ground at each
of an infinite number of such points.

For a 1/4WL vertical, that would be true even if the path to ground
is through displacement currents. But, what about free space?

Evidently there's some kind of an equivalence between electric current
and the electric smoke liberated by the antenna. ;-)

73, Jim AC6XG