John,
I would very much like to answer, but I don't get a clear picture of what sort of device you have, and what exactly you want to measure. Apparently the other contributors on this forum share my doubts. If you would give some more detail, probably you would get more answers!
Line isolator... Do you mean a power line filter against RFI? An isolation transformer? An RF transmission line choke? Coaxial or balanced? A circulator connected as one-way-device? Do you want to reject common-mode current on 17 meters, or differential mode current, or both?
Manfred
