  Scale separation between the flow and the magnetic field is a common feature
of natural dynamos. It has also been used in the Karlsruhe sodium experiment in
which the scale of the magnetic field is roughly 7 times larger than the scale
of the flow [R. Stieglitz and U. M\"uller, Phys. Fluids 13, 561 (2001)].
Recently, Fauve & P\'etr\'elis ["Peyresq lectures on nonlinear phenomena", ed.
J. Sepulchre, World Scientific, 1 (2003)] have shown that the power needed to
reach the dynamo threshold in a dynamo experiment increases with the scale
separation in the limit of large scale separation. With a more elaborate method
based on subharmonic solutions [F. Plunian and K.-H. R\"adler, Geophys.
Astrophys. Fluid Dynamics 96, 115 (2002)], we show, for the Roberts flow, the
existence of an optimal scale separation for which this power is minimum.
Previous results obtained by Tilgner [Phys. Lett. A 226, 75 (1997)] with a
completely different numerical method are also reconsidered here. Again, we
find an optimal scale separation in terms of minimum power for dynamo action.
In addition we find that this scale separation compares very well with the one
derived from the subharmonic solutions method.
