  In this paper we use the diffuse Galactic synchrotron emission to study the
properties of the Galactic magnetic field. We combined data from the
low-frequency WENSS survey with single-dish observations carried out by Brouw
and Spoelstra to (partially) fill in the central gap in the (u,v) plane that is
missing from the interferometer data. The small bandwidth of the WENSS data
meant that we could not determine rotation measures (RM) directly. Instead we
interpreted the polarization angle gradients that we derived as gradients in
RM, and to do this we found a new way to efficiently and reliably fit linear
gradients to periodic data. RM are available for the single-dish observations,
and we found that we have to rescale these single-dish RM to match the RM
gradients that we derive from the combined WENSS/single-dish dataset with the
single-dish only RM gradients. We tentatively show that the difference in
beamsize between the datasets can be responsible for this. We interpret the
scaled-up RM we find in terms of a simple toy model, and by combining our
results with those by Haverkorn et al. (2004) we reconstruct the full 3D
magnetic field vector along a number of lines-of-sight. For these
lines-of-sight we derive the properties of the magnetic field component
perpendicular to the Galactic plane.
