I have tried the Slim Jim antenna on the 2m. I used a 1/4" copper tube,
antenna is very good. Then I made another one using 300 Ohm TV clear twin
lead, but SWR is very high and it seems to be not adjustable to the same 1:1
of the first antenna. Why? Does anyone know where is the problem? Could be
the clear cable plastic not good for the application?

The impedance of "300 ohm" twinlead is often not very well
controlled... it can vary quite a bit from one manufacturer / batch to
the next. It's quite common to find it necessary to adjust the
lengths of the radiator and matching section, and the position at
which the feedline is attached, in order to get a good match. The
distance from feedpoint attachment, to the bottom of the matching
section, seems to be quite critical (this appears as a shunt
inductance across the feedpoint, and small differences in length can
make a bit difference in the reactance seen at the feedpoint).

Once you figure out the exact segment lengths you need for a
particular lot of twinlead, you can usually reproduce this design
quite quickly... but only as long as you use that exact type of
twinlead. Switch to a different type and you'll have to do another
round of trimming-and-tweaking.

I have no reason to believe that the clear-dielectric twinlead is any
worse than brown- or black-dielectric twinlead, in terms of your
ability to achieve a good match. They just have slightly different
imepdances and velocity factors, and so need different trimmings.

It may also help to add a choke to the coax feedline, just below the
point at which it's attached to the twinlead. This will isolate the
outside of the feedline from the antenna, reduce RF currents on the
feedline, and make the antenna's effective impedance less sensitive to
the position and length of the feedline. Try a snap-on ferrite
two-part core.

A few years ago I made a J-pole for my bicycle, by taping wires to the
side of a fiberglass bicycle-flag mast... no twinlead was used. The
lengths of the various wires ended up being a *lot* different than
what I would have needed for a twinlead J-pole in free space... the
relatively high dielectric constant of the fiberglass had a very big
effect! Figuring out the correct length for the below-the-feedpoint
inductive shunt section was the trickiest... I used bare copper wire
for the two sides, wrapping a turn or two of bare stranded wire around
the pole to form the short at the bottom, and moving the short up and
down until I got a good match. It ended up being quite a bit shorter
(if I recall correctly) than a standard twinlead J-pole would have
required.

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