Iain,
It may be worth going back to basics and reading up on doublet antennas
as well as open wire feeders. The critical length is half of the top
plus the feeder length and this should be adjusted so that it isn't
resonant on any band you wish to use. This because it is easier to
match a medium impedance but reactive load rather that a high or low
impedance load. The impedance of the feeder is immaterial in this case
as are issues of balance so erect whatever you can and enjoy using it.
Based on your dimensions of 270ft top and 33ft feeder the critical
length will be 168ft which may give trouble on some bands so may I
suggest using about 148 to 150ft.
Hope your system works out OK
73
Peter
G3SMT
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