I used to use spray-on contact adhesive for flat spirals, letting it become
tacky before winding. For a trumpet coil, a winding jig that continuously
changes the orientation of the coil form so that the wire always follows
gravity would probably make winding much easier.
However, since this is intended as a scientific experiment, it may be
better to find a university or research laboratory that already has an
nScrypt 3Dn/Factory-in-a-Tool or DragonFly additive-electronics system.
Such a system could potentially fabricate the dielectric form and
conductive path directly from the CAD geometry, giving us much tighter
control over the taper, pitch, conductor placement, and reproducibility
between specimens.
Direct measurement of the completed secondary's inductance should suffice
for determining the capacitance needed for tuning. A conventional solenoid
equation using an effective radius near the midpoint of the conductor
length should provide a useful first estimate, although the trumpet
geometry will alter the mutual inductance between turns somewhat compared
with a cylindrical solenoid. Since we can measure the finished coil
directly, we do not need to depend on that approximation for the experiment.
I expect the primary-to-secondary coupling behavior to have some
similarities to working with a flat spiral, particularly toward the wide
end of the trumpet.
On Tue, Sep 1, 2026 at 11:40 AM Jim Lux <jim@xxxxxxxxxxxxx> wrote:
>
>
>
>
> I'm thinking that if you have some sort of form, and you wind the
> secondary with something sticky, and not under too much tension, it will
> stay in place.
>
> Or, one could "groove" the form (although making a spiral groove on a
> trumpet would make for an interesting CNC program).
>
> Conical secondaries have been built (Dick Aurandt's coil at Griffith
> Observatory) and there've been others who have built them on this list.
> Keeping the windings in place before coating/gluing has been the challenge.
>
> Predicting the inductance and coupling would be challenging analytically -
> but an FEM approach would probably work.
>
>
> On Tue, 1 Sep 2026 09:24:14 -0500, Ken <pupman.com@xxxxxxxxxxxxxxxxxxx>
> wrote:
>
> Note I've not committed to constructing anything for anyone. I'm curious
> about the requirements and based off that, maybe I can help, or not.
>
> The form for a trumpet shaped coil is probably not a big deal to
> fabricate. The challenge will be winding the coil so it doesn't collapse.
>
> I don't use NC machine tools, but you don't need to with a decent lathe,
> even for complex contours.
>
> There's still no hints as to whether this project needs a watchmaker's
> lathe or something they'd have at a heavy machine shop, or something in
> between.
>
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