How can I improve the radiation pattern of a monopole antenna placed on finite ground

I am simulating a monocone antenna working on 2-6 GHz but the E plane radiation pattern seems tilted at 5GHz. I place the antenna on a skirt( Radius 37mm ) which I don't want to change.
How to decrease the elevation angle?
Many thanks.

Study conical antennas or use a longer vertical section

Thank you very much.