  We report on the combined experimental and theoretical analysis of the
microwave-frequency electromagnetic response of BaFe2As2 single crystals with
different substitutions: K in the Ba site (hole doping), Co in the Fe site
(electron doping), and P in the As site (isovalent substitution). Measurements
using a coplanar resonator technique lead to the experimental determination of
the penetration depth and microwave conductivity as a function of temperature.
The whole set of data is analyzed within a self-consistent three-band
$s_{\pm}$-wave Eliashberg approach, able to account for all the main observed
features in the different properties. Besides the validation of the model
itself, the comparison between experiment and theory allows discussing the
possible role of the Fe-As planes in defining the superconducting properties of
these compounds, the relevance of coherence effects, and the presence of nodes
in the superconducting order parameter.
