Hi Dear,
Now I see your problem. I don't have experience simulation structures with very low frequencies, but I will try to give my overview based on your problem.
You are simulating the structure in the CST MWS at 10 KHz range. I suppose you are using the transient analysis.
While using transient solver, CST needs at least lambda/8 (taking the middle of frequ. range) as surrounding space to compute farfield and give you good results. You could even reduce it, but it will not escape from the huge simulation time. In other words, lambda/8 @ 10 KHz means a distance of
3,75 Km from your source ! This would lead in a tremendous effort in the calculation domain of the transient solver. CST needs to discretize and mesh all this remaining space, quite impossible task.
Alternatively, you could use other solvers like CST EM Studio for simulate your coil with tetrahedral surface meshing, but limited to static analysis and output parameters. It will depends on what parameters you want to plot.
I hope it helps.