I have the possibility to run a 450hom ladder line from the rooftop
down to the shack in the basement of the house, through an unused
chimney made of a steel pipe of about 1 foot of diameter and about 20
feet long. If the multiband dipole antenna to be feeded is balanced,
the feed line should not radiate so it should not matter if it is
shielded.
A balanced antenna tuner in the shack should take care of any impedence
variation of the ladder line.

Advice on this implementation will be very much appreciated.

Many thanks,
Giovanni IW0BJR

Giovanni wrote:
I have the possibility to run a 450hom ladder line from the rooftop
down to the shack in the basement of the house, through an unused
chimney made of a steel pipe of about 1 foot of diameter and about 20
feet long.

The ladder-line needs to be centered in that 1 foot diameter
steel pipe. Otherwise, the fields are likely to become unbalanced
and losses occur. 6 inch spacing away from any conductive medium
is just about right.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote in message
...
Giovanni wrote:
I have the possibility to run a 450hom ladder line from the
rooftop
down to the shack in the basement of the house, through an unused
chimney made of a steel pipe of about 1 foot of diameter and about
20
feet long.

The ladder-line needs to be centered in that 1 foot diameter
steel pipe. Otherwise, the fields are likely to become unbalanced
and losses occur. 6 inch spacing away from any conductive medium
is just about right.
--
73, Cecil http://www.qsl.net/w5dxp

====================================

There is no great need to exactly centralise the feedline inside the
pipe. The amount of radiation from the balanced feedline feeding a
balanced antenna, inside the pipe or not, will be negligble.

Loss induced in the metal pipe will also be negligible. The
resistivity of steel is very low.

With a 1 foot diameter pipe the decrease in line impedance Zo will
also be negligible.

As a precaution, if the chimney is not vertical, try to keep the same
wire of the line away from the chimney walls for appreciable lengths.
Very slowly, about every few feet, twist the line about itself.

After construction, if the line is moved about and there is no
difference in the tuner setting then there's nothing to worry about.
---
Reg, G4FGQ