Errors on simulating a wire-tapered helix antenna

I can simulate a typical helix antenna without any problem (in CST MWS)... but i want to have a tapered-wire helix* , so when i taper the helix and simulate it , i encounter with these errors :

in Integral eq. solver :

terminated abnormally
could not read mesh

in Time domain solver :

Terminated abnormally
.
.
.
Error in calculating solver matrix (parallelization enabled)

can any one help ?! what is wrong ?

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* it means that that the wire becomes gradually narrower.
picture of the structure :

Hi smhosseini , I'm working on archimedean spiral antenna, can u tell how to taper the wire of antenna