The "Slim Jim" has no inherent advantage over a standard J-pole: they are the same antenna. All that
means in practice is that it doesn't really matter how you connect the ends of the twinlead on the
radiator - you can connect them together at the top, bottom, both or neither (as long as you don't
connect it to the top of the short side of the matching stub.) If you are short on twinlead you can
use regular wire for the radiator (the part of the twinlead above the notch) and just use twinlead for
the matching stub (the portion below the notch.)
When you put the J-pole inside PVC pipe it will need some readjustment because the dielectric constant
of the PVC around the matching section is different than air, which changes the characteristic impedance
somewhat. The tuning will also change if the twinlead flops around inside the pipe - the best solution
I've seen is to use some pipe insulation foam to keep the twinlead centered in the PVC.
How well it works will depend a lot on your local 6m repeater and/or the distance to the stations you
plan to work simplex. If there is no local activity on 6m FM, it won't appear to work well at all.
But there is nothing unusual about building a J-pole for bands other than 2m, even if dimensions
are more difficult. In most cases you just cut a half wave radiator, make the matching stub a bit
longer than a quarter wave, and experiment with the tap points for the feedpoint and a short across
the bottom of the matching stub to get good SWR. My first twinlead J-pole was for 15m, and the
theory hasn't changed since then.
I like the idea of the foam inside the pvc to keep it from moving for an outdoor antenna. I just wonder how well the antenna will work with 13' of twin wire running vertically in the house. I guess the best thing to do is set it up and try it out for a while and see. My tower is 130' away from the shack which would mean (preferably) lmr 400 coax at .90 cents a foot + lightning arrestor which all adds up to more money. the indoor slim jim will be the cheapest way out. I am sure the 2 meter will be fine in the attic and we will experiment with the 6 meter.