Hey,
thank you for the quick reply, it does solve the problem.
Later I run into one more problem with my code:
TypeError: init(): incompatible constructor arguments.
I saw here that I should use meshtags:
pec_microstrip_facets_tags = meshtags(mesh, tdim-1, pec_microstrip_facets, np.full_like(pec_microstrip_facets, 1, dtype=np.int32))
ds = Measure('ds', domain=mesh, subdomain_data=pec_microstrip_facets_tags, subdomain_id=1)
So now it seems that the calculation works, thank you for the help 