  We use the new NLTE lightcurve and spectral synthesis code JEKYLL to evolve a
macroscopically mixed ejecta model of a type IIb Supernova (SN) originating
from a star with an initial mass of 12 solar masses through the photospheric
and nebular phase. We compare to SN 2011dh, and find that both the spectra and
the lightcurves are well reproduced. Our work further strengthens the evidence
that this SN originated from a star with an initial mass of about 12 solar
masses that had lost all but tiny (<0.1 solar masses) fraction of its hydrogen
envelope. We also investigate the effects of the macroscopic mixing by
comparing macroscopically and microscopically mixed models, and by varying the
clumping geometry. In the photospheric phase, we find strong effects on the
effective opacity in the macroscopically mixed regions, which affects the model
lightcurves. The diffusion peak is considerably narrower in the macroscopically
mixed case, and differs strongly if the radioactive material in the helium
envelope is allowed to expand more than in our standard model. The effect is
mainly geometrical, and is driven by the expansion of the clumps containing
radioactive material. These findings has implications for lightcurve modelling
of stripped-envelope SNe in general, and the effect would increase the
estimated ejecta masses. In the nebular phase, we find strong effects on the
collisional cooling rates in the macroscopically mixed regions, which affects
lines driven by collisional cooling, in particular the [Ca II] 7291, 7323 and
[O I] 6300, 6364 lines. As these lines are often used for mass determinations,
it highlights the importance of how the calcium- and oxygen-rich material is
mixed. As shown in this and earlier work, both NLTE and macroscopic mixing are
essential ingredients to accurately model the lightcurves and spectra of Type
IIb SNe throughout their evolution.
