A spiral antenna is a balanced antenna (like dipoles). In such cases, there normally is no ground plane. I do not understand how you have setup a ground plane for this antenna? Maybe you can post a photo ?
A spiral antenna has a bidirectional radiation pattern with opposite polarization in the top and bottom hemispheres. If you place a metal sheet below your spiral, then it will result in reflection of the backward waves. However, I have observed such sheets to cause a lot of impedance mismatch in the spiral.
Absorber loaded cavities are generally used to stop the backward radiation of the spiral antenna. However, this results in a drop in efficiency.