We study a conspicuous ring-like structure with a radius of about 1.4° which was observed with the Westerbork Synthesis Radio Telescope ( WSRT ) at 5 frequencies around 350 MHz .
This ring is very prominent in Stokes Q and U , and less so in polarized intensity P .
No corresponding structure is visible in total intensity Stokes I , which indicates that the ring is created by Faraday rotation and depolarization processes .
The polarization angle changes from the center of the ring outwards to a radius \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \lower - 1.0 pt \hbox { $ > $ } } 1.7 \degr .
Thus , the structure in polarization angle is not ring-like but resembles a disk , and it is larger than the ring in P .
The rotation measure RM decreases almost continuously over the disk , from RM \approx 0 rad m ^ { -2 } at the edge , to -8 rad m ^ { -2 } in the center , while outside the ring the RM is slightly positive .
This radial variation of RM yields stringent constraints on the nature of the ring-like structure , because it rules out any spherically symmetrical magnetic field configuration , such as might be expected from supernova remnants or wind-blown bubbles .
We discuss several possible connections between the ring and known objects in the ISM , and conclude that the ring is a predominantly magnetic funnel-like structure .
This description can explain both the field reversal from outside to inside the ring , and the increase in magnetic field , probably combined with an electron density increase , towards the center of the ring .
The ring-structure in P is most likely caused by a lack of depolarization due to a very uniform RM 
distribution at that radius .
Beyond the ring , the RM gradient increases , depolarizing the polarized emission , so that the polarized intensity decreases .
In the southwestern corner of the field a pattern of narrow filaments of low polarization , aligned with Galactic longitude , is observed , indicative of beam depolarization due to abrupt changes in RM .
This explanation is supported by the observed RM .