  The spiral galaxy NGC 6946 was observed in total intensity and linear
polarization in five radio bands between 3cm and 21cm. At the inner edge of the
inner gas spiral arm the ordered magnetic field is only mildly compressed and
turns smoothly, to become aligned along the gas arm. Hence the field is not
shocked and is probably connected to the warm, diffuse gas. At larger radii,
two bright magnetic arms between the optical arms are visible in polarized
intensity. The field in the northern magnetic arm is almost totally aligned.
Faraday rotation measures (RM) in these arms are consistent with the
superposition of two low azimuthal dynamo modes. Three more magnetic arms are
discovered in the outer galaxy, located between HI arms. Due to strong Faraday
depolarization the galaxy is not transparent to polarized waves at 18cm and
20cm. The large-scale asymmetry in depolarization with respect to the major
axis may be another indication of large-scale helical fields. Three
depolarization rings of almost zero polarization seen at 20cm are probably
generated by differential Faraday rotation in HII complexes in NGC 6946 of
300-500 pc size. - In the gas/optical spiral arms, the total (mostly turbulent)
magnetic field is amplified to \simeq 20\muG. Its energy density is \simeq 10
times larger than that of the ionized gas and is similar to that of the
turbulent gas motions in the inner galaxy. The magnetic energy exceeds that of
the turbulent energy in the outer galaxy.
