Recent polarimetric surveys of extragalactic radio sources ( ERS ) at frequencies \nu \lower 3.225 pt \hbox { $ \sim$ } \hbox to 0.0 pt { \raise 1.161 pt \hbox { $ > $ } } 1 GHz are reviewed .
By exploiting all the most relevant data on the polarized emission of ERS we study the frequency dependence of polarization properties of ERS between 1.4 and 86 GHz .
For flat–spectrum sources the median ( mean ) fractional polarization increases from 1.5 % ( 2–2.5 % ) at 1.4 GHz to 2.5-3 % ( 3–3.5 % ) at \nu > 10 GHz .
Steep–spectrum sources are typically more polarized , especially at high frequencies where Faraday depolarization is less relevant .
As a general result , we do not find that the fractional polarization of ERS depends on the total flux density at high radio frequencies , i.e \geq 20 GHz .
Moreover , in this frequency range , current data suggest a moderate increase of the fractional polarization of ERS with frequency .
A formalism to estimate ERS number counts in polarization and the contribution of unresolved polarized ERS to angular power spectra at Cosmic Microwave Background ( CMB ) frequencies is also developed and discussed .
As a first application , we present original predictions for the Planck satellite mission .
Our current results show that only a dozen polarized ERS will be detected by the Planck Low Frequency Instrument ( LFI ) , and a few tens by the High Frequency Instrument ( HFI ) .
As for CMB power spectra , ERS should not be a strong contaminant to the CMB E–mode polarization at frequencies \nu \lower 3.225 pt \hbox { $ \sim$ } \hbox to 0.0 pt { \raise 1.161 pt \hbox { $ > $ } } 70 GHz .
On the contrary , they can become a relevant constraint for the detection of the cosmological B–mode polarization if the tensor-to-scalar ratio is \lower 3.225 pt \hbox { $ \sim$ } \hbox to 0.0 pt { \raise 1.161 pt \hbox { $ < $ } } 0.01 .