"markvictor" wrote in
oups.com:

why then do you insulate it from ground?

Old habits are hard to die. You don't need to insulate the top of it,
just the feed point.....well, unless you run a wire in parallel with it
up 30' and attach the tuner ACROSS the backstay.

I used to take my Yaesu FT-900 and a Nye-Viking 3KW manual antenna tuner
to sea on Claire's Navie. That tuner will resonate almost any length
antenna to almost any impedance at high power. It's MUCH better made
than that little plastic Icom box with the tiny relays and crappy little
coils. This sucker is made for 40A of antenna current and has a coil to
take it.

I grounded the tuner through a jumper cable strap to the base of the
mainmast and fed the port side chainplate to the port shroud, which
wasn't grounded, shunt-feeding the mainmast and all the rigging attached
to it. As long as you didn't use it around 9.5 Mhz, where something
resonated enough to bite your hand holding the mike, even on 100 watts,
it got impressive signal reports on 20, 40 and 75 meter SSB from
offshore.

In the case of the little AT-130 autotuner (and its clones), you use an
insulated backstay because the autotuners have a limited range of
impedances they can actually resonate across the HF spectrum. It's just
easier on the non-technical amoung us.