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Tunable Primary coil
Just thought I'd give a quick update on the tunable primary I've been
building for my large magnifier.
It ended up being 29 inches in diameter. Magnifiers typically use a
cylinderical primary with a tightly coupled secondary inside, and this uses
this same design. Insulation between the two is usually very heavy poly
steeting.
All my primaries in the past have been roughly tuned by tapping the turns
for approximate tune, then half or quarter turns for final tune - sound
familiar? In about 1976 I came up with a design for a primary that allows
you to change the spacing between the turns to do precision tuning, even as
the coil is running. I built such a primary for a very large coil (primary
was 9 feet in diameter) and the tuning worked beautifully. This 29 inch
primary is the only other primary that I've built using this technique.
Because you have at least six vertical pipes which pvot in the center, and
the fact that each place where the wire connects to an upright has to have a
pivot as well amounts to drilling a lot of holes and making a lot of moving
parts.
I started out building an open frame out of 2 inch and 1.5 inch pvc pipe to
form a cage-like structure upon which to wind the primary. I found 12
verticals to be too many, operation was stiff and mechanical resistance of
the large coax wound on the form was excessive. I reduced the number of
uprights to six, and changed the pivot points on the 1.5 inch pvc from
threaded pipe fittings to a simple plastic pin arrangement and now it works
like it is supposed to.
A friend and I will be measuring the amount of inductance shift the tuning
arrangement provides later this Sunday. We also are abount to insulate the
secondary and hope to fire up this magnifier for the first time!
I'll post the results of the measurements and whether we set any smoke free
later.
Bert Pool
nikki-at-fastlane-dot-net