Cecil Moore wrote:
Dave wrote:

remember, 1/4 wave long (more or less) makes the open at one end look
like a short at the other end.


There is an open at both ends. Does that make it look like
a short at both ends?

If it looked like a short at both ends, it would look like an open at both ends.

Built a skeleton piece of giant coax in EZNEC, been playing with it as
a monopole over MININEC ground.

I'm not going to claim much since it's probably wildly inapplicable,
but I will claim that the answer to this question ain't trivial and
depends on the details of the ends.

I find a current in the center conductor, It's very much bigger when
the center conductor is slightly extended past the ends of the "shield"
than when it's slightly inside. The shield is eight wires arranged in
a regular octogon with the tops and bottoms tied together. It shows
more or less normal fat monopole behavior when used alone. (33 foot
element resonant around 6.4 MHz)

A fatter wire used for the center conductor has greater current than a
thinner one. A very thin wire exhibits almost no current.

The peak current in the center conductor is about 3% of what's flowing
in the shield for a 1 foot "diameter" (the circumscribed circle around
the octagon) shield and a 6 inch diameter inner conductor. The current
is very low at either end of the center conductor and peaks about 2/3
from the top.

This dumb model is full of holes of many kinds, I'm sure, but it does
seem to show nontrivial effects of changing things a little bit near
the ends of the coax (slight extension of center conductor outside
of/retraction of center conductor into the shield)

Dan

wrote:
The shield is eight wires arranged in
a regular octogon with the tops and bottoms tied together.

That doesn't seem to me to simulate a coax shield and
is more akin to parallel conductors. Doesn't skin effect
enter into the coax braid equation? It seems to me that
by floating the inside conductor and using the coax
braid as the radiator, we are forcing most of the current
into common mode on the outside of the shield. With zero
current at each end of the floating center conductor, why
would current flow on the inside of the braid?
--
73, Cecil http://www.qsl.net/w5dxp