Tom Donaly wrote:
The presence of anything at all near the coil should lower its resonant
frequency.

That's my experience. Conversely, the farther away from the
coil the ground plane is located, the higher the self-resonant
frequency. The location of the ground plane has a significant
effect on the self-resonant frequency of the coil.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com