Barry,
the 28ft feedline is significant, it avoids the voltage peaks at the input to the ATU. It may have been a lucky find by the original designer, but I suspect not. If we know the VF of the flat data cable originally used we could check that out.
My 8.35m (~27.5ft) was calculated with my best guess of the VF of PTFE twisted pair being 0.7. There has been lots of work done on this by DL’s over the years, including spreadsheets and programs for calculating the lengths to use / avoid for multiband. I have a spreadsheet in German, but cant find a download link to it now. Today’s German test is attached!
Perhaps a native German speaker like Heinz HB9BCB will be able to add a bit more. But the feedline electrical length is important, even to the extent of accounting for the transmission line length in your balun!
73 Gavin
GM0GAV
