On 11 Jul 2006 12:21:06 -0700, "
wrote:

I wanted to know what the current was in the center conductor of a
piece of RG-8 being used as a monopole. The answer to that question is
not trivially calculable, though it might be trivial in the sense of
being trivia... who cares? I did.

Dan

Hi Dan,

All the issues you focus on are quite particular and specific, and
perhaps too much so. I find them equally interesting and you ask
reasonable questions. Unfortunately, you question above suffers from
every problem imaginable for measuring, and any report you get without
considerable qualification is probably sheer fancy. It suffers from a
version of Heisenberg's problem of disturbing what you attempt to
measure, and invalidating everything in the process.

Another way of stating this problem, with more practicality, is that
you have to put a wire into the coax to add your meter to make the
measurement. This then brings that wire's own contribution, which, as
you've noted, can raise the stakes considerably.

I, too, have played with a variant of your model (I can push the mesh
finer and have worked with a 16 sided coax model). However, instead
of driving the line like a monopole, I simply plunked a source into
the wire skeleton of the "shield."

But to return to your own published model, I've played with the length
of wire 26 after discovering it emerged from both ends of the coax. I
don't put much credit to Cecil's invention of topics, so I am unaware
if this wire length meets some criteria (even if it did, I would still
suspect the detail would have been spurious). Be that as it may.
After truncating the wire 26 so that it did not come anywhere closer
to the mouth of the coax (either end) than 5 feet, "induced" currents
plummeted like a rock.

73's
Richard Clark, KB7QHC

Richard, You hit this nail on the head, for su

All the issues you focus on are quite particular and specific, and
perhaps too much so. I find them equally interesting and you ask
reasonable questions. Unfortunately, you question above suffers from
every problem imaginable for measuring, and any report you get without
considerable qualification is probably sheer fancy. It suffers from a
version of Heisenberg's problem of disturbing what you attempt to
measure, and invalidating everything in the process.

I don't have much trouble with the antennas I actually build. The
aluminum and wire I sling around tend to behave.

I do have some vested interest in overspecific modeling in the sense
that since I'm deploying all my antennas from the balcony of my
apartment (with its overhanging roof), and don't have much space to
build, adjustments and pruning are very hard to do. Some performance
gains can certainly be had by adjustment... but in my situation now
it's so much easier to push bits around in EZNEC.

This one, though, was just pure impractical theoretical curiosity.
That, and a little bit of being concerned that people seemed to "know"
the answer, that the answer was obvious, and, well, I didn't think it
was. The current in real experiment would be, as you pointed out,
pretty much impossible to measure without disturbing it.

But as I see it, from a practical standpoint, in a real system where
you're using the shield of a piece of coax as an antenna , the current
in the center conductor never matters.

The existence of the current matters, at least as a learning
experience. It matters in the sense that some seem to think that there
couldn't be a current, just because a piece of coax is "shielded". Or
because the center conductor is "floating". It's precisely the fact
that the center conductor is "floating" that makes the (probably) weak
coupling at the ends important.

Thanks for posting your results. What you describe certainly jives
with the fuzzy idea that I've got of what's doing the coupling ... or
at least where it's happening. Of course, the tube without center
conductor is certainly a waveguide well below cutoff, so we've got
solid footing saying a short wire inside a long conductive tube a small
fraction of a wavelength in diameter will have little current on it...
and the model also exhibits this.

I'm done with this one unless I find a leadless current meter (small,
battery powered with an A/D converter and optical fiber out?) and a
big piece of copper pipe laying around!

73,
Dan

Cecil Moore wrote:
wrote:
Cecil Moore wrote:
I have used that same circuit to switch a top hat
in and out. How much RF power did I lose by taking
that route?

That is NOTHING like the problem the ZL on eHam had.

Clutching at straws, are we? It's exactly like the ZL problem
except the relay is used to switch a top hat instead of a wire.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil,

In your alleged system, you carried RF through the coax and used the
coax as a swixcthing stub.

In the other fellows system, he had a high impedance feed and had to
get a relay conductor past the high impedance feed without affecting
the system.

They are TOTALLY different.

Even a card carrying mensa member can't be that out of touch with a
simple system like this.

73 Tom

wrote:
In your alleged system, you carried RF through the coax and used the
coax as a swixcthing stub.

You seem to be confused by my actual system in my yard
Vs the hypothetical piece of coax I introduced as a
simplified example for discussion purposes.

Forget the coax example. In my yard, I have run two wires
up a piece of 1/4WL aluminum tubing to a relay which
switched a top hat in and out just as the other fellow
wanted to switch a wire in and out. The only thing that
I would have to add to turn that system into switchable
half-squares, like the other fellow has, would be two
3/4WL runs of wire. I suspect the SGC-230 autotuner at
the base would match the new feedpoint impedance just fine.
I've never seen an impedance it couldn't match.

In the other fellows system, he had a high impedance feed and had to
get a relay conductor past the high impedance feed without affecting
the system.

The two physical configurations being discussed are *identical*,
being 1/4WL of tubing through which two DC wires are routed.

They are TOTALLY different.

Once again, you make an outrageous exclusive statement
completely divorced from reality. If any one thing about
the two antennas is not different, your statement is
completely false. One wonders what compels you to make
such strange assertions.
--
73, Cecil http://www.qsl.net/w5dxp