post processing - How can I avoid circular banding artifacts in clear skies when de-noising?

I photograph in RAW, and convert using Darktable. Camera happens to be Nikon D40, 55-200mm lens. For a small number of photos, especially of clear blue skies, I cannot reduce noise without introducing artefacts. This remains true when I change de-noising method, although I cannot claim to have tried everything. Is there a solution to this? Example to illustrate my question follows:

1. 
   

   No denoise, no halo artefacts, but annoying noise:

   
   


2. 
   

   Very light denoise (profiled, wavelets, strength 0.097), halo artefacts:

   
   

Any thoughts on this appreciated. :-)