I have a SGC-230 tuner in the RV ... I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a
dipole fed with 450 ohm window line.

My SG-230 tuner manual says that is a no-no.
It says: "The coupler must be located *at the antenna*.
Never use a feed line or coaxial cable at the output
of the antenna coupler."

On the other hand, page 16 of the SGC-230 manual PDF specifically
shows a balanced dipole, being fed through "balanced line feeders,
300-600 ohms, up to 40 feet".

The ladder line is shown as being connected to the antenna "hot"
terminal, and to the case of the coupler.

The manual is quite clear about not using a coax feedline above the
antenna, or having a feedline out to a longwire antenna which has more
than a very small amount of capacitance to ground. The information
shown on page 16 suggests that you may be able to bend this
restriction if you're using a balanced feedline, although I suspect
that you'll still want to make sure that you've got low capacitance
between ground and the hot side of the line (keep the feedline well
away from metal, and use high-quality feedthrough insulators).

I don't know about using a balun at the base of the feedline... this
will (almost of necessity) require the use of a length of
low-impedance feedline of some sort, and this could be exactly the
sort of thing that prevents the SGC from matching the line properly.
Using a choke on the coax-and-control side might work better, although
this too seems to be something that SGC discourages.

I haven't been terribly happy with the SGC tuner I picked up
(admittedly for cheap, at a ham swap-meet). It's an early-model SGC
230, and its tuning algorithm seems very finicky. In particular, it
won't tune into any but the easiest loads when fed by my Kenwood
TS-2000. I suspect that the reason is that the TS-2000's high-SWR
foldback circuit is rather aggressive, and chops the power down to well
under 10 watts anytime the rig sees an SWR of more than 2:1 or so.
The fluctuating transmitter power seems to confuse the coupler's
matching algorithm... no big surprise there. Seems to work better
when fed from a simple TenTec Scout, which has a different high-SWR-
protection mechanism that doesn't cause the power to flop around as
much.

Anyhow... my impression is that the SGC tuners are best suited for
their original application - feeding a whip or longwire, whilst being
securely bolted and multiply-grounded to a Big Metal Vehicle Chassis.
They also seem to do OK feeding verticals over a big bed of radials.
I just don't think they're all that well suited to feeding balanced
antennas.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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