Recently I built a helical resonator. It took me a good while to adjust it since I had to trim my coils much shorter than calculated. The calculation said 7.2 turns but it ended up being more like 6.5 turns. The coil was built on a custom 3D printed form (PETG material), so it's very close to ideal.
Could the difference in inductance be attributed to losses in the plastic form? If so, is it possible to estimate the decrease in self resonant frequency due to the form's dielectric constant, to allow for quicker tuning?