Feko Giving Very Different Results Between Standard and Fine Mesh Settings

Hi,

I'm trying to design a quadrifilar helix antenna in Feko 7.0 for use in the 2.4GHz band. The antenna is shown below.

The response, specifically the impedance reflection varies quite considerably with different mesh settings. Shown below are the plots for fine and standard meshs. Using a custom mesh with the wire segment length equal to the wire diameter also gives a completely different result. I'm new to Feko and EM simulation in general but I feel the results should not vary this considerably between mesh settings, especially when using the automatic settings.

I would very grateful if anyone could give me a pointer as to what I'm doing wrong or whether this is normal?

Cheers,
Tom