[UK-CONTEST] OT but Likely I'll get answer....

[F.G.Robertson]

Adrian, if you're talking single band (or harmonically related bands)
trim the feeder to be an electrical half wave length at the lowest
frequency (band) of interest.  The impedance at both ends will then be
the same, regardless of the feeder impedance, e.g. connect a 50R load at
one end and you'll see 50R at the other end.  I use hefty 75R CATV cable
in this way and it works well with low loss and no need for 50:75R
transformers.

I'd choose balun ratios that are in the right ball park to match the
open line, but I wouldn't think critical.  Using thick wire for your
open line will reduce I^2R losses, and use as wide a spacing/high
impedance that you can, consistent with the highest frequency in use.

73 Fraser G4BJM