  Magnetic fields in galaxies exist on various spatial scales. Large-scale
magnetic fields are thought to be generated by the $\alpha-\Omega$ dynamo.
Small-scale galactic magnetic fields (1 kpc and below) can be generated by
tangling the large-scale field or by the small-scale turbulent dynamo. The
analysis of field structures with the help of polarized radio continuum
emission is hampered by the effect of Faraday dispersion (due to fluctuations
in magnetic field and/or thermal electron density) that shifts signals from
large to small scales. At long observation wavelengths large-scale magnetic
fields may become invisible, as in the case of spectro-polarimetric data cube
of the spiral galaxy NGC~6946 observed with the Westerbork Radio Synthesis
Telescope in the wavelength range 17-23 cm. The application of RM Synthesis
alone does not overcome this problem. We propose to decompose the Faraday data
cube into data cubes at different spatial scales by a wavelet transform.
Signatures of the `magnetic arms' observed in NGC~6946 at shorter wavelengths
become visible. Our method allows us to search for large-scale field patterns
in data cubes at long wavelengths, as provided by new-generation radio
telescopes.
