Spiral shielded coil unstable

[F117]

Finally, I had some time to cast a spiral coil that I had wound some time ago.
28 turns inner diameter 20 cm, about 250uH. First layer = alu tape for airco tubing.
On top of that: spiral wound blank wire ( no idea why I did this, probably to enhance contact between different pieces of tape.
Casting material = "Blitshars", a 2 component resin. I always use this for all my coils without problems so far.
The result is a nice looking coil, but I just can't get stable 'readings' from it and it is far less sensitive than other similar coils without shielding.
What is wrong here: the alu tape or the spriral shield wire ?







Merry XMas from Belgium,
- Bernard

[Tepco]

Alu tape, positively. Use ONLY graphite shield for this. Blank wire is OK, just use smallest diameter available, single strand 0.2mm or so, to keep amount of additional metal at minimum.

[F117]

Alu tape, positively

That's what I also suspected.
Thanks for your quick response, Tepco !