wrote:
You still need to be able to couple to the ends, and a floating center
conductor is not the best way to couple energy in. However, there's no
fundamental physical reason why currents *won't* flow on the center
conductor in an open-ended piece of coax.

How about when there's two RF chokes in series?
--
73, Cecil http://www.qsl.net/w5dxp

I think it's just a matter of degree. The more RF chokes there are,
the less current will flow.

It would be a straightforward matter to add more wire to the model and
include loads for bypass caps and chokes.

In either case (wire inside or outside of the shield) good decoupling
where the wire lead exits is going to be important. The model so far
may suggest that being inside the shield is better than being
outside... but without that lead trailing away from the antenna some
distance, it's not time to conclude much about the relay+half square
problem.

The 40m coax monopole answer would seem to be "yes, there's current on
the center conductor, but it's small and coupled in through the ends"

If you're using the standard EZNEC, you're going to have to knock
another section off the top of the cage... if you've got EZNEC+ then
just add away...

I may try it when I get home... knock another section off and try a
control wire...

Might have to send off my money to Roy and go for EZNEC+... I know I
can get around segment limitations with other programs but I do like
EZNEC. This is probably the ninth time since I got the program a few
months ago that I've hit the segment limit ... i like meshing things...


Gives me a question about the (EZ)NEC limitations... should I be
watching out for fine 2D meshes? It seems to work OK in this case...
the base impedance of the meshed monopole and the current distribution
viewed on the segments all makes sense, and it seems to me that there's
not much reason to doubt that the currents are calculated correctly in
the mesh as long as it's not coarse with respect to a wavelength... any
caveats in this regard?



73,
Dan

I should have said "the more RF chokes there are, the less current will
flow in the center conductor of the case where the wire is inside the
shield" by the way.

If the RF chokes and wire are inside the shield, it will keep the
current off the wire inside the shield.

However, where the wire exits the shield, currents can still be induced
on it... this is what allows current to flow on the unchoked center
conductor, right? So why wouldn't it flow in the other direction.

So you need chokes some distance outside the shield to isolate the rest
of the wire from the antenna.

If you put the wire outside the shield and put chokes at the top and
bottom, some distance away from the antenna, then a very large current
will be induced on the wire between the chokes compared to the current
you get on the wire inside the shield. However, this doesn't
immediately translate into more current on the control wire trailing
away. It is just saying something about the wire between the top and
bottom chokes.

What you're trying to do is decouple the control wire going away from
the antenna from the antenna. Decoupling the control wire that runs up
inside or outside of the antenna isn't very important as long as
current doesn't flow on the control wire to the shack.

Whether or not inside or outside of the tower/tube helps depends on a
lot of factors and should be answered with some sort of calculation...

Seems that the various claims are testable...

I'll post anything I come up with.

Dan

wrote:
If the RF chokes and wire are inside the shield, it will keep the
current off the wire inside the shield.

I hope W8JI is reading this. He insists that it will still
function as a stub and such an approach is "useless".

However, where the wire exits the shield, currents can still be induced
on it... this is what allows current to flow on the unchoked center
conductor, right? So why wouldn't it flow in the other direction.

So you need chokes some distance outside the shield to isolate the rest
of the wire from the antenna.

Just outside the shield is a relay with an RF bypass cap
across its coil.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
wrote:
If the RF chokes and wire are inside the shield, it will keep the
current off the wire inside the shield.

I hope W8JI is reading this. He insists that it will still
function as a stub and such an approach is "useless".

However, where the wire exits the shield, currents can still be induced
on it... this is what allows current to flow on the unchoked center
conductor, right? So why wouldn't it flow in the other direction.

So you need chokes some distance outside the shield to isolate the rest
of the wire from the antenna.

Just outside the shield is a relay with an RF bypass cap
across its coil.
--
73, Cecil http://www.qsl.net/w5dxp

Turn in your mensa membership card Cecil.

You flunked basic grade-K problem solving logic.

:-)