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

If the goal of those guys is to sabotage amateur
radio communications, they have probably succeeded.
It's like them going to fight a fire with big holes
in the hoses.


It the goal was to sabotage our communications, they would have denied
permission to use their facilities totally, let alone, not have purchased
and fabricated the nice aluminum mast which they did for us. Admittedly,
they seem overly concerned with the visual impact of things, but beggars
can't be choosers! Tnx.

Ed

"Ed_G" wrote in message
. 192.196...

If the goal of those guys is to sabotage amateur
radio communications, they have probably succeeded.
It's like them going to fight a fire with big holes
in the hoses.


It the goal was to sabotage our communications, they would have denied
permission to use their facilities totally, let alone, not have purchased
and fabricated the nice aluminum mast which they did for us. Admittedly,
they seem overly concerned with the visual impact of things, but beggars
can't be choosers! Tnx.

Ed


Right off I would say they are the beggars. It is your group that is
supporting them.

Bruce in alaska wrote in news:fast-25B5D6.09195501032008
@netnews.worldnet.att.net:

....
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.

What does this do, what does it achieve?

Owen

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.

What does this do, what does it achieve?

Owen



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.

Ed K7AAT

"Ed_G" wrote in
.91:


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.

What does this do, what does it achieve?

Owen



My expertise is weak in this area, but just guessing.... using
twin
coax in the above configuration, if the shields were grounded, would

But that is not what was said. I read it to say "with the shield left
open on each end".

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.

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.

Let us just consider a simple example. Assumption is that skin effect on
the coax is fully effective, a reasonable assumption at HF.

Make a quarter wave vertical of a piece of RG213 supported on sky hooks.
Make no connection to the shield at either end, and connect the feed line
between a ground plane / counterpoise / whatever and the centre conductor
of the vertical piece of coax.

What current flows on the outside surface of the vertical coax?

The current on the outside surface of the vertical coax adjacent to the
bottom end of the isolated shield is the same as the current flowing on
the inner conductor adjacent to the same end of the shield.

Does the outer conductor 'shield' the vertical so that it will not
radiate?

No, the outside surface of the shield is the radiatior, it just has a
quarter wave o/c stub in series from the feedline to the radiating
element.



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


Owen

Ed_G wrote:
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 feedline 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.

Side-by-side runs of RG-11 (Z0=150 ohms) would reduce
the SWR on the coax and change the impedance seen by
the tuner from 0.3-j69 ohms to 1.9-j154 ohms which is
quite an improvement. You can increase that feedpoint
impedance even more to 2.5-j170 ohms by using side-by-
side runs of RG-133 (Z0=190 ohms) if you can find it.
--
73, Cecil http://www.w5dxp.com

Owen Duffy wrote:
Does the outer conductor 'shield' the vertical so that it will not
radiate?

No, the outside surface of the shield is the radiatior, it just has a
quarter wave o/c stub in series from the feedline to the radiating
element.

Owen, feedline radiation in this particular system might
increase the efficiency which would be a good thing.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ed_G wrote:
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 feedline 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.

Side-by-side runs of RG-11 (Z0=150 ohms) would reduce
the SWR on the coax and change the impedance seen by
the tuner from 0.3-j69 ohms to 1.9-j154 ohms which is
quite an improvement. You can increase that feedpoint
impedance even more to 2.5-j170 ohms by using side-by-
side runs of RG-133 (Z0=190 ohms) if you can find it.

I forgot to say that these figures are from EZNEC which
assumes lossless feedlines. Real-world impedances would
be much higher due to coax losses.
--
73, Cecil http://www.w5dxp.com

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
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?


Ed