  We analyze the effect of polarized diffuse emission in the calibration of
wide-beam mm-wave polarimeters, when using the Crab Nebula as a reference
source for both polarized brightness and polarization angle. We show that, for
CMB polarization experiments aiming at detecting B-mode in a scenario with a
tensor to scalar ratio $r \sim 0.001$, wide (a few degrees in diameter),
precise ($\sigma_Q$ , $\sigma_U$ $\sim$ 20 $\mu$$K_{CMB}$ arcmin), high angular
resolution ($< \mathrm{FWHM}$) reference maps are needed to properly take into
account the effects of diffuse polarized emission and avoid significant bias in
the calibration.
