Hello.

I recently built a 500kHz oscillator with a high L and very low C. Part of
the capacitor is constituted by a plate left alone (without its counterpart
to ground) in free space, such that all surrounding conductive objects
constitute "the ground plate".

During my experiments, I put conductors and isolators near the plate; of course
I obtained variations in frequency proportional to the distance and shape of
conductors, but I noticed that a tiny change in frequency also happened when
I moved very close isolators, like PVC and PET pipes (long enough not to be
myself, the conductor to influence frequency).

I have not a ceramic or glass bar long enough to test it.

I am curiuos if the effect could be due to the change in dielectric constant
between air and the pipe material, or the pipe itself that, despite being
clean, is not a so good insulator, even at low voltage (about 10V) and
moderately low frequency.

Best regards,
Andrea Baldoni

I'm not quite sure what the issue is that you're trying to resolve, but
if it's the effect of different insulating materials affecting the
frequency when brought into proximity, then that should be no surprise
at all.

The experiment uses the capacitance of the air space around it to all
the conductive materials around it, and that capacitance must include
effects of any dielectric that substitutes for some of the air as well.
I would expect that the frequency will always go downward as any
insulator (dielectric constant always higher than that of air) is
brought into proximity, and the effect will be directional, being larger
if the material is between the sensitive node and a larger or closer
conductive surface.

Ed