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