  We discuss how the interaction between the electrons in a relativistic jet
and the Cosmic Microwave Background (CMB) affects the observable properties of
radio-loud AGN at early epochs. At high z the magnetic energy density in the
radio lobes of powerful radio-loud quasars can be exceeded by the energy
density of the CMB (because of its (1+z)^4 dependance). In this case,
relativistic electrons cool preferentially by scattering off CMB photons,
rather than by synchrotron. Thus, sources sharing the same intrinsic properties
have different extended radio and X-ray luminosities when located at different
z: more distant sources are less luminous in radio and more luminous in X-rays
than their closer counterparts. Instead, in compact regions where the local
magnetic field still exceeds the CMB in terms of energy density, synchrotron
radiation would be unaffected by the presence of the CMB. Such regions include
the compact inner jet and the so-called hot spots in the radio lobes. The
decrease in radio luminosity is larger in misaligned sources, whose radio flux
is dominated by the extended isotropic component. These sources can fail
detection in current flux limited radio surveys, and therefore they are
possibly under-represented in the associated samples. As the cooling time is
longer for lower energy electrons, the radio luminosity deficit due to the CMB
photons is less important at low radio frequencies. Therefore objects not
detected so far in current surveys at a few GHz could be picked up by low
frequency deep surveys, such as LOFAR and SKA. Until then, we can estimate the
number of high redshift radio-loud AGNs through the census of their aligned
proxies, i.e., blazars. Indeed, their observed radio emission arises in the
inner and strongly magnetized compact core of the relativistic jet, and not
affected by inverse Compton scattering off CMB photons.
