Here's a tunable coil with an inductance between around 1.9 mH at minimum and 2.9 mH at maximum. The outer coil is around 200 turns of AWG16 wire around a 160mm diameter tube. The rotating inner coil is maybe 60 turns of AWG16 wire around a 110mm diameter 90mm length tube. The inner coil rotates on a 20mm solid rod with 10mm diameter holes through the ends for feeding the wires out. A small geared DC-motor rotates the rod.