The reasons why the impedances don't make sense is because you are
measuring them through a length of coax rather than right at the
Go to VK1OD's transmission line calculator here:http://www.vk1od.net/tl/tllc.php
Set the "Mismatch" field to "Zin". Enter your measured values in the
window beside it in the form "30+j50". Set the length and type of the
coax and the measurement frequency.
After you hit CALCULATE look for the line that says "Zload" in the output.
That should be the actual impedance at the feedpoint. A quick check
with EZNEC says it should be about 20-j60 without the balun or the
Do make sure that the grounds of all the coax connectors are hooked
together. (They were mounted on a common aluminum plate on the
original.) It should be possible to ground this common connection to
the boom without adverse effect.
The design is, indeed, a bit puzzling. I'm not sure why they use a beta
match to go from 20 to 200 ohms, then step it down to 50. I think the
SWR bandwidth would be better (and adjustment easier) using a beta
match from 20 to 50 ohms and a 1 : 1 balun.