Onno,
Likely with a second floor shack, ONLY the tuned ground lead would help
your RFI.
That is because, as you state, your station in in the near field (close
to apex) of the antenna.
You are likely so far above Earth ground that it would be hard to not
have radiation off a long ground lead even if made of flat copper ribbon.
And, most places, the Earth soil is a poor, poor conductor of RF and NOT
suited for an RF ground.
Use of the tuned ground wire lead, insulated from other grounds, is the
first step as you took it.
The use of choke cores, is a valid and broadband solution to remaining
RF feeding back on cables.
You have done well for your station, and now have RF neutral chassis.
Good Work and enjoy good DX.
Stuart Rohre
K5KVH
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