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(Abridged) We conducted new 6cm and 11cm observations of CTA 1 using theUrumqi 25-m and Effelsberg 100-m telescopes. Data at other wavelengths wereincluded to investigate the spectrum and polarisation properties. We obtainednew total intensity and polarisation maps at 6cm and 11cm with angularresolutions of 9.5 arcmin and 4.4 arcmin, respectively. We derived a spectralindex of alpha=-0.63+/-0.05 based on the integrated flux densities at 408 MHz,1420 MHz, 2639 MHz, and 4800 MHz. The spectral index map calculated from dataat the four frequencies shows a clear steepening of the spectrum from thestrong shell emission towards the north-western breakout region with weakdiffuse emission. The decrease of the spectral index is up to about 0.3. The RMmap derived from polarisation data at 6cm and 11cm shows a sharp transitionbetween positive RMs in the north-eastern and negative RMs in the south-westernpart of the SNR. We note a corresponding RM pattern of extragalactic sourcesand 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 isestimated to be about +45 rad/m2. A RM structure function of CTA 1 indicates avery regular magnetic field within the Faraday screen, which is larger thanabout 2.7 microG in case of 500 pc distance.

New 6cm and 11cm observations of the supernova remnant CTA 1