In article . 196,
"Ed_G" wrote:


Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each end
and sealed against water intrusion. Even if taped to the aluminum
mast.
This would provide a much better situation than a single coax feed.
Also if you are stuck with a single coax, then make SURE, that the
shield side of the coax is connected to the Ground Stud of the tuner.



Well! That was part of my original question in this thread! I had
intended to do just that, but some here seemed to steer me toward a
single coax, including SGC personnel. I do NOT think we have any
decent ground at all available anywhere near the roof location where
this antenna mast will be located, so I was unsure if the twin coax
feed would be OK without grounding the shields. Left floating, I
don't know what effect, if any, we will have for incidental radiation.
One thing for sure, it won't be any worse than using a single coax with
no balun at the feedpoint.


Ed

"Left Floating" so that there is less Capacitance to RF Ground, and more
distance between the vertical parallel Feedlines. It would even help
if you can use twin Coax Runs, to put them on opposite sides of the
Aluminum Mast, which would give you more separation with the same
Coupling Capacitance to the Mast. Capacitive coupling to RF Ground,
is the Killer here, and you MUST reduce that, as much as possible,
if your system is going to have any chance at reasonable operation.

--
Bruce in alaska
add path after fast to reply

"Ed_G" wrote in
. 192.196:


Bunk. The only reason the shields would not radiate would be if they
carried equal but opposite currents. That is most unlikely in this
case.



If both shields, ( ungrounded ) are tied together, and the two

At both ends?

This is the first mention of shields tied together, I certainly didn't
read that into Bruce's "with the shield left open on each end
".

center conductors are acting as a 'balanced' feedline, how can
current flow on the outsides of the shields, if the interior currents
of the two center conductors are always 180 out of phase?

The analysis in this case is different, but if I understand your
scenario, the outer surface of the two coaxes which are tied together at
both ends but connected to nothing else still carries the common mode
current. that exists on the two open wire conductors just prior to
entering the coax assembly. No, you cannot guarantee that those currents
are equal and opposite, ie that there is no common mode current, and the
common mode current will flow entirely on the outside surface of the
outer conductors of the coax assembly when connected as you now propose.

Bruce hasn't explained what his configuration is supposed to do, so we
are still guessing about that one.

There is no answer to this problem, because the problem is ill defined.
You have just added a new element in tying the shields together. Other
questions exist like what other connections exist between tx feed line,
ATU, ant feed line, mast, roofing / rain gutters, any other conductors.

Somethimes knowing how to describe a problem is knowing the answer to the
problem... or conversely, not knowning the answer is the result of not
knowing how to describe the problem.

Owen

If both shields, ( ungrounded ) are tied together, and the two

At both ends?

This is the first mention of shields tied together, I certainly didn't
read that into Bruce's "with the shield left open on each end
".


That's because I added that to the mix. Bruce's comment was a
suggestion for me. I have not done this yet, but my original post
under this thread was solliciting comments on using twin coax to feed a
balanced antenna, or using a single coax feed under the specific set of
circumstances I outlined.


About the common mode current.... please explain how this would be an
issue with the outer shields of two coaxes, shields tied together but
going nowhere ( no ground ), and the balanced antenna fed by the two
center conductors. I do not see common mode current being a factor, but
I'm willing to listen and learn.


Ed

"Left Floating" so that there is less Capacitance to RF Ground, and
more distance between the vertical parallel Feedlines. It would even
help if you can use twin Coax Runs, to put them on opposite sides of
the Aluminum Mast, which would give you more separation with the same
Coupling Capacitance to the Mast. Capacitive coupling to RF Ground,
is the Killer here, and you MUST reduce that, as much as possible,
if your system is going to have any chance at reasonable operation.



Please read my response and question just posted to Owen. With both
shields tied together, but not grounded, nor connected to the antenna
either, I do not understand how common mode current is an issue on the
shields.

We could use the mast as a physical separation as you suggested, (
the mast is not grounded, either, but again, what is the point, if the
two coax shields were "as one" anyway?


Ed
I

"Ed_G" wrote in
. 192.196:

That's because I added that to the mix. Bruce's comment was a

Ok, well here is a model to shape your thinking and moving the goal
posts.

At frequencies where skin effect is fully developed, and that is a
reasonable assumption for most practical coaxial cables at HF, the
current on the inside surfaace of the outer conductor is equal to but
opposite in direction to the current on the outside surface of the inner
conductor. This is TEM mode propagation.

At the end of the isolated outer conductor, this current must flow
somewhere, and it flows around the end onto the outside surface of the
outer conductor (effectively changing direction as it does so). So, at
that point, the current flowing on the outside of the outer conductor is
exactly equal to the current flowing on the outside of the inner
conductor.

Leaving aside the effects of changing Zo by substitution of coax for
plain conductors:

If you use two coax lines in parallel with the shields isolated, it makes
very little difference, the current that would have flowed on the two
plain conductors now flows on the outer of the coax lines. The common
mode current is the sum of the currents in both coax shields, as it would
be for plain conductors.

If you join the shields together at each end, the sheilds together now
carry the common mode current. A different equivalent circuit, but almost
the same outcome.

Most of these 'shielded solutions' arise from a lack of understanding of
how the coaxial transmission line works in TEM mode.

For example, I saw an ham advise someone that station ground connections
were subject to noise pickup and the best improvement he could make was
to shield the ground lead. In his case, his shack was on the first floor
of the building, and his 7m vertical ground lead to the earth stakes etc
was a source of noise, so he used 7m of RG213 with the shield and inner
bonded to the earth stake and the shield left isolated at the top end.

Firstly, this is not a 'shield' at radio frequencies, but what he did
achieve was to insert a s/c stub in series with his station ground
conductor. The impedance of that series stub at 7.1MHz is 3056.20-
j1509.30 ohms... not a good outcome.

It might have 'fixed' his RF feeback problem, but it didn't improve the
station earth at all, it degraded it severely.

Owen

At frequencies where skin effect is fully developed, and that is a
reasonable assumption for most practical coaxial cables at HF, the
current on the inside surfaace of the outer conductor is equal to but
opposite in direction to the current on the outside surface of the
inner conductor. This is TEM mode propagation.

At the end of the isolated outer conductor, this current must flow
somewhere, and it flows around the end onto the outside surface of the
outer conductor (effectively changing direction as it does so). So, at
that point, the current flowing on the outside of the outer conductor
is exactly equal to the current flowing on the outside of the inner
conductor.

Leaving aside the effects of changing Zo by substitution of coax for
plain conductors:

If you use two coax lines in parallel with the shields isolated, it
makes very little difference, the current that would have flowed on
the two plain conductors now flows on the outer of the coax lines. The
common mode current is the sum of the currents in both coax shields,
as it would be for plain conductors.

If you join the shields together at each end, the sheilds together now
carry the common mode current. A different equivalent circuit, but
almost the same outcome.


The last paragraph above is where I lose you..... when the shields
are joined together. Yes, I understand inside the shield RF
current flowing around the end and to the outer side.... HOWEVER, the
OTHER center conductor is inducing RF current flowing in the opposite
direction. Since both these inside currents are 'shorted' at the ends
of the two shields, I fail to see how you can have any current flowing
on the outer shield since the two opposite currents should cancel....
????


Ed

"Ed_G" wrote in
. 192.196:

of the two shields, I fail to see how you can have any current flowing
on the outer shield since the two opposite currents should cancel....
????

I already said that as I understand your variable configuration, it is most
unlikely that there is zero common mode current, or close to it.
Irrespective of the magnitude of the common mode current, the mechanism is
that the coax doesn't 'shield' it.

Owen

I already said that as I understand your variable configuration, it is
most unlikely that there is zero common mode current, or close to it.
Irrespective of the magnitude of the common mode current, the
mechanism is that the coax doesn't 'shield' it.


No, I wouldn't say "shield" would be a proper term, either. But I
would suggest that a "cancellation" similar to radiation in a balanced
feedline, would be pertinent.


Ed

My expertise is weak in this area, but just guessing.... using twin
coax in the above configuration, if the shields were grounded, would
allow the feedling between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Perhaps others, here, will either expand on this, or correct my
misconception.

Using the center conductors of two pieces of coax, with shields bonded
together, does create a balanced transmission line. Its
characteristic impedance is twice that of the coax itself. The higher
impedance of this feedline will cause the voltages on the line to be
higher than on a single coax, and the currents lower (all else being
equal, of course) and thus reduce I^2*R losses. This can be
beneficial if this line is being used between an antenna and a
transmatch/tuner/coupler, where a relatively high SWR may be expected
on the transmission line.

The OP might want to consider a two-coax run of RG-6 cable, for a
total line impedance of 150 ohms. Satellite-dish RG-6 is available in
both black and white - the white coax might help reduce visibility if
it's cable-tied to the metal mast.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Ed_G" wrote in
. 192.196:



I already said that as I understand your variable configuration, it is
most unlikely that there is zero common mode current, or close to it.
Irrespective of the magnitude of the common mode current, the
mechanism is that the coax doesn't 'shield' it.


No, I wouldn't say "shield" would be a proper term, either. But I
would suggest that a "cancellation" similar to radiation in a balanced
feedline, would be pertinent.

Then I will leave you to your view that the system is balanced.

Owen