Thomas,

I'd suggest a remote tuner. You can have twinlead directly to the
tuner, and then your entire run of coax sees a nice flat 50 ohms.

A nice automatic tuner would be ideal; you don't have to worry about
controls. The price is not exactly right, since you've already got a
tuner...

If you've got a well stocked junk box (op amps, power transistors,
maybe a few multiturn pots) you could try my approach. I put a small
gear motor and sensing potentiometer on each shaft of my MFJ tuner,
stripped the spring detent balls out of the inductor switch, added a
servo circuit (very simple, it's an op-amp and a couple of transistors)
and can use the tuner more or less as if it were in the shack.

See http://www.n3ox.net/projects/servo for more information if you're
interested. Your MFJ-949 would lend itself nicely to this approach.
It can be done on the cheap, maybe.

A problem arises if you don't have 10-turn (or at least 360 degree)
pots... I've been thinking about this in that they're sort of the weak
point of my tuner project, as they're something like $15 per new. I
work in a physics lab and every few months an obsolete homebrew
instrument with a load of them hits the dumpster, so I'm set.

I think remote tuner of some description would be superior from a loss
standpoint to any feedline tricks you could do.

Now, the balanced, shielded line on the concrete is a good idea if
you're set on running twinlead all the way back to the shack. This is
quicker and easier than than building a remote control for your tuner,
and cheaper than buying an autotuner, so you might try it first and see
if you're happy with the results.

In my apartment, I originally ran about 20 feet of 75 ohm coax between
the antenna feedpoint and the tuner. I made some contacts, but saw a
DRAMATIC difference when I put the tuner at the antenna feedpoint
instead. Fewer RF feedback problems too.

Twinlead is better but twinlead isn't magic. Wide spaced open wire
line with an insulator every couple of feet only is super low loss even
with high SWR. 300 ohm twinlead, AFAIK, isn't.

73,
Dan
N3OX
www.n3ox.net