I thought about 75 ohm twin. I also thought about making my own open wire
line. But instead I use the heaviest grade of ladderline, 14g (US) stranded
copper plated steel. Sadly not sold in UK (as far as I know) but light enough
to carry home from Dayton. Apart from being incredibly strong and flexible,
the characteristic impedance is apparently a bit less than other grades,
somewhere between 350 and 400 ohms.
I run the TenTec auto ATU into about 15 feet of RG-8 coax to a balun hung
outside the shack, then around 80 feet of ladderline to the top of the pole.
The only downside is the change in SWR with rain/snow/humidity. But re-tuning
is a small price to pay for multi-band convenience without switches.
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