I built the roll-up because it provides (perhaps) an option in a ziploc
that goes in the field bag with other stuff...
Yes, and they are great for that. I have a couple that I use the same way - a quick rope over a tree
branch and you can get by on low power where the rubber ducky was marginal at 5 watts.
To answer your earlier question on repeatability: there are a number of designs floating around, and
most are pretty close. One of the biggest variables is the exact characteristics of the twinlead, which
will vary from one manufacturer to the next, and in some cases from one end of a roll to the other.
As a result, you have to expect to do a bit of tweaking, though I've had some that came out close
enough without it.
One of the biggest problems, however, is that most designs really don't give good advice on how to
adjust the antenna when needed. The radiator (the length above the notch) is not subject to the
standard transmission line velocity factor, so that length probably doesn't need to change. If you
trim the top of the stub at the notch then you are simultaneously shortening the stub AND lengthening
the radiator. My preference is to adjust the length of the matching stub for resonance, and the
location of the coax feedpoint along the stub for SWR at resonance. With most designs the length
of the stub is not conveniently adjustable, since the twinlead is cut and shorted at that point.
So I'd recommend building it to spec and trying it out first. If the SWR at the minimum isn't as good
as you'd like, try moving the coax tap point up and down the stub a bit. If the frequency of minimum
SWR is not where you'd like it, try shortening the stub a bit (which may also require readjusting the
When I'm building my own rather than following an existing set of plans (like my first J-pole that
I built for 15m) the twinlead can get rather battered with all this experimentation. I've resorted to
using some straight pins to punch through the insulation for the coax tap point and the short across
the end of the stub. Then I cut the twinlead a bit long and cut the notch for the radiator at about the
right distance down from the top to form the radiator. (The radiator wires can optionally be shorted or
open at either end, or the unused wire can be removed entirely - it really doesn't matter.) Then I make
a guess for the stub length and coax tap point, stick the pins through to contact the wires inside the
insulation, and proceed with experimenting. If you have plenty of twinlead and want something more
tidy, build a second one using the dimensions you found from the first one.