Basically you just need to reduce the inductance
in the loading coil until the antenna resonates on
6m. The mobile antenna design chapters of the
ARRL Handbook or Antenna Book can help you
calculate the required inductance (though I don't
have either handy.) A dip meter or SWR Analyzer
will make the process easier.
First, unwind the existing coil (attempting to save
the wire if possible.) The coil will probably be attached
to the antenna at one end, ground at the other, with
a tap in the middle which goes to the feedline.
For starters, rewind the coil with half the number of
turns (a wild guess). Keep removing turns until the
antenna is resonant (lowest SWR) at 6m. Then
adjust the feedline tap point for best SWR. The
adjustments interact a bit, but the process isn't
difficult.
(I've also converted such antennas to a half-wave
on 2 meters, and they work quite well. I replaced the
coil with about 5 turns, tapped about 1 or 1 1/2 turns
up from the bottom. A 20pf trimmer cap across the
coil tuned it to frequency, and the tap adjusted for
best match. Works great, and doesn't require much
of a groundplane.)
- Dale WB6BYU