  Regular magnetic field structures can be derived from the Faraday rotation
measures (RM) of polarized background sources observable at 1.4 GHz with the
SKA. At lower frequencies ($<250$ MHz) polarimetry of radio sources with the
Low Frequency Array (LOFAR) will allow the investigation of extremely small RM,
to detect and map weak regular fields in halos and outer parts of spiral
galaxies, and in the interstellar and intergalactic medium. Very little is
known yet about the number density of polarized sources at low frequencies.
Observed distributions of polarized sources at 350 MHz and 1.4 GHz and
perspectives to detect weak magnetic fields with LOFAR are presented. Test
observations of polarized radio sources with the Westerbork Synthesis Radio
Telescope (WSRT) and the Giant Metrewave Radio Telescope (GMRT) are discussed.
