Kenneth Scharf wrote:
My shack is located off the garage toward the front of the house. I
can't run any feed line through the window or walls as these face the
front and sides of the house visible from the street. In order to meet
XYL approval we installed a 1.5" ID plastic electrical pipe in the wall
that gives access to the attic crawlway when we built the add on room.
Right now I have a single RG8/u coax cable running through the pipe in
the attic to my vertical. (The coax leaves the attic via a hole drilled
in the roof under-hang).

I'd like to put up a multi-band dipole feed with open wire line
(actually the 450 ohm web spaced twin lead type). I know that this type
of transmission line should be run in the clear if possible.
I can run it to the attic via the feed pipe, but it will be parallel
with the coax for the 10-15' length of the pipe. I can then run the
twin lead though the attic attached to rafters until I reach a turbine
attic vent and pass the cable out of the roof via a hole drilled in the
turbine base. The hole would be lined with plastic electrical fittings.

No, you need it to be clear some distance away from the feedline. You
cannot put it in the same conduit with coax. You could possibly put it
into a 1.5" plastic pipe all by itself as long as the pipe was clear
and didn't have any nails or anything within a few inches of the pipe.

Will this scheme work, or will the feed line be too negatively affected
by the surroundings to work with a tuner in the shack? We are not
talking about extreme QRO power, the largest amplifier I have in mind
would be about 350 watts out max, and usually I would run the IC-746
barefoot at 100 watts out.

It doesn't matter how much power you're running, or even if you are just
receiving. If the feedline becomes leaky, it becomes part of the antenna
and the antenna pattern is disturbed. If there is anything conductive near
the feedline, it will become leaky.

If you want to run a transmitter with a 450 ohm output, I would balun it
down to coax and use the existing coax.

If you absolutely have to run some kind of balanced transmission line
through a conduit, consider IBM Twinax.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."