Context :

Aims : We attempt to study spatial variations of the spectrum and rotation measures ( RMs ) of the large diameter , high-latitude supernova remnant ( SNR ) CTA 1 .

Methods : We conducted new \lambda 6 cm and \lambda 11 cm observations of CTA 1 using the Urumqi 25-m and Effelsberg 100-m telescopes .
Data at other wavelengths were included to investigate the spectrum and polarisation properties .

Results : We obtained new total intensity and polarisation maps at \lambda 6 cm and \lambda 11 cm with angular resolutions of 9 \aas@@fstack { \prime } 5 and 4 \aas@@fstack { \prime } 4 , respectively .
We derived a spectral index of \alpha = -0.63 \pm 0.05 ( S _ { \nu } \propto \nu ^ { \alpha } ) based on the integrated flux densities at 408 MHz , 1420 MHz , 2639 MHz , and 4800 MHz .
The spectral index map calculated from data at the four frequencies shows a clear steepening of the spectrum from the strong shell emission towards the north-western breakout region with weak diffuse emission .
The decrease of the spectral index is up to about \Delta \alpha = 0.3 .
The RM map derived from polarisation data at \lambda 6 cm and \lambda 11 cm shows a sharp transition between positive RMs in the north-eastern and negative RMs in the south-western part of the SNR .
We note a corresponding RM pattern of extragalactic sources and propose the existence of a large-diameter Faraday screen in front of CTA 1 , which covers the north-eastern part of the SNR .
The RM of the Faraday screen is estimated to be about +45 rad m ^ { -2 } .
A RM structure function of CTA 1 indicates a very regular magnetic field within the Faraday screen , which is larger than about 2.7 \mu G in case of 500 pc distance .

Conclusions : CTA 1 is a large-diameter shell-type SNR located out of the Galactic plane , which makes it an ideal object to study its properties without suffering confusion .
The rare breakout phenomenon known for CTA 1 is confirmed .
We identify a Faraday screen partly covering CTA 1 with a regular magnetic field in the opposite direction to the interstellar magnetic field .
The detection of Faraday screens in the Galactic plane is quite common , but is difficult at high latitudes where the polarisation angles of weak polarised background emission are rotated .
RMs from extragalactic sources are needed for this purpose , although the number density of extragalactic RMs is still small despite of significant observational progress .