Which antenna to use if close proximity to metal objects is unavoidable?
How to cope with IFA

So, if you place a large ground plan below an antenna you create an image of the antenna and equal distance from the ground plane but on the other side, as if the ground plane is not there. The only important part being any solution exists above the ground plane, and there is 0 below it.

Not getting caught up in the solution, try placing the ground plane plate one half wavelength below your IFA radiator. This should cause the radiation up to be doubled. You can test this, move it around, measure your RSSI with different distances, pick the one that you like the most.

I hope that is clear, let me know If i can do more.

Your solution

From a general perspective, you will be happy at about 2.4GHz, it has a lot developed at it, this reduces cost. An IFA antenna seems to get you a nice form factor, it is actually a type of patch antenna, and if the ground piece causes a problem just look into other patch. These are commonly used because of low cost and low form factor.

 Other options

If you need to radiate up from the disk(ie. The direction that passes through the disk) then the patch antenna is a very easy method. There are also chip antennas designed to have a ground plane. Both these will radiate very well.

If you need to radiate in the place of the disk, to its sides, this is where you cannot go wrong with a half wavelength dipole. Specifically the quarter wavelength monopole, using the ground disk as a conductor. This can be a very effective configuration.