Topband: Keeping metal away from a loading coil . . .

[Dan Zimmerman N3OX]

For the coming season I want to be a little more frequency agile on
160... especially for the contests.

The antenna is a 60' base loaded wire vertical with a 2:1 SWR
bandwidth of only about 20kHz.  What I'd like to do is use a little
bit more loading coil than I need so the antenna+coil resonates below
160  and motorize a 2000pF Jennings vac variable that I have to tune
the resonance through 160.

This is mostly because I have a small ground system and a wire wrapped
on a fiberglass pole the antenna tuning changes a good bit when it
rains or snows.

The coil as it exists now is kind of stupidly big because I built it
to try to base load my 40 foot vertical as an experiment.  I'm only
using the bottom 30 or so turns now.

http://www.n3ox.net/projects/sixtyvert/topbandcoil_lg.jpg

I feel like I can make a very compact installation if I install the
capacitor (and maybe the motor)  inside the coil form tube and take
off some extra windings, but if I do this, how much space should I
leave between the end of the capacitor and the beginning of the coil
to avoid eddy current problems?  The coil form is 4" grey PVC
electrical conduit and the capacitor endcap is about 3" diameter.

I have plenty of room if I want it, but I'd like to cut off the tube
as short as possible and save the rest of the already-wound coil for
something else.

I suspect something like half a coil diameter is enough but do you
think that's overkill?

73,
Dan