I need to run the feeder from my dipole (80m) up thro the eaves, thro'
the
loft-space to the shack at the other side (1st floor) of the house, approx
50'. No other routes feasible. Because of the route, I thought I'd have
less problems using coax, BUT from the remarks in the above recent post,
will someone kindly comment on viability of 450 ohm twin feeder over such
a
route?

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If necessary just squeeze heavy-duty 450-ohm ladder line wires close
together wherever they pass through a small hole. Line holes with a thick
plastic film or a pipe to obtain a higher breakdown voltage. Slowly twist
the cable every 2 feet between holes and keep it at least 1 or 2 inches away
from foreign materials, long metal conductors or otherwise.

If it was possible to do A-B comparisons at HF you would find little
difference - equivalent to a few pF change in a tuner capacitor setting and
1/2-turn change in the roller inductor. To be safe stick to 100 or 200 watts
unless you wish to test to destruction under worst case, but controlled
conditions (eg., very high SWR) just to see what happens. Probably nothing!

At HF a change in impedance Zo over a length of a few inches when passing
through one or two holes in timber or brickwork will have a negligible
effect on performance. The advantage of a balanced line all the way to the
transmitter may be that a tuner can be located in the shack. If an automatic
tuner is used then use easier-to-install coax and locate the tuner at the
antenna end.

Over a length of 50 or more feet, at 28 MHz, the lower loss in 450-ohm line
relative to 50-ohm RG-58 is worth thinking about. At 1.9 MHz forget it.

But it's only a matter of economics, time and labour, and the well-earned
satisfaction of having done a good job which works according to plan.
---
Reg, G4FGQ