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."

On 09/18/2011 09:42 PM, Scott Dorsey wrote:
Kenneth 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
I was told I could use parallel runs of coax with the shields connected
to each other (and nothing else). 93 ohm coax would be best if I can
find some!

A remote tuner is an option and there was an article in QST this month
on building one. But the differential cap (in a "T" network) and roller
coil driven by motors seems a bit "Rube Goldberg" to me. I'm also not
too sure how to build an end of limit switch for the coil.
(you don't want to leave that out!!!).

Kenneth Scharf wrote:
I was told I could use parallel runs of coax with the shields connected
to each other (and nothing else). 93 ohm coax would be best if I can
find some!

That gives you something like 150 ohms for RG-11, or 200 ohms for the
93 ohm stuff (which your Belden rep can sell you). Not 450 ohms.
Whether this is a problem or not depends on your antenna system.

A remote tuner is an option and there was an article in QST this month
on building one. But the differential cap (in a "T" network) and roller
coil driven by motors seems a bit "Rube Goldberg" to me. I'm also not
too sure how to build an end of limit switch for the coil.
(you don't want to leave that out!!!).

How does a remote tuner help you? You still have to run a 50 ohm
feedline and then transform to 450 ohms at the transmitter.

What's wrong with having a local tuner at the radio? I mean, besides
the feedline losses.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 09/19/2011 08:50 PM, Scott Dorsey wrote:
Kenneth wrote:
I was told I could use parallel runs of coax with the shields connected
to each other (and nothing else). 93 ohm coax would be best if I can
find some!

That gives you something like 150 ohms for RG-11, or 200 ohms for the
93 ohm stuff (which your Belden rep can sell you). Not 450 ohms.
Whether this is a problem or not depends on your antenna system.

A remote tuner is an option and there was an article in QST this month
on building one. But the differential cap (in a "T" network) and roller
coil driven by motors seems a bit "Rube Goldberg" to me. I'm also not
too sure how to build an end of limit switch for the coil.
(you don't want to leave that out!!!).

How does a remote tuner help you? You still have to run a 50 ohm
feedline and then transform to 450 ohms at the transmitter.

What's wrong with having a local tuner at the radio? I mean, besides
the feedline losses.
--scott


Nothing wrong with a tuner at the rig. The issue is running 450 ohm
twin lead (nothing magic about that impedance, just what is available in
a "open wire" balanced transmission line with low lose that will fit
though a 1.5" plastic pipe) from the shack to the attic next to coax
cable. I can't run the feed line "in the clear" which makes for more
loss. Putting the tuner in the attic allows running coax to the tuner
and the balance line to the antenna, the run from that point on is
'cleaner'.