  Precision measurements of active and reactive components of in-plane
microwave surface impedance were performed in single crystals of
optimally-doped Fe-based superconductor Ba(Fe1-xCox)2As2 (x = 0.074, Tc = 22.8
K). Measurements in a millimeter wavelength range (Ka band, 35-40 GHz) were
performed using whispering gallery mode excitations in the ultrahigh quality
factor quasioptical sapphire disk resonator with YBa2Cu2O7 superconducting (Tc
= 90 K) end plates. The temperature variation of the London penetration depth
is best described by a power-law function, delta {\lambda}(T) is proportional
to T with the exponent n, n = 2.8, in reasonable agreement with radio-frequency
measurements on crystals of the same batch. This power-law dependence is
characteristic of a nodeless superconducting gap in the extended s-wave pairing
scenario with a strong pair-breaking scattering. The quasiparticle conductivity
of the samples, {\sigma}1(T), gradually increases with the decrease of
temperature, showing no peak below or at Tc, in notable contrast with the
behavior found in the cuprates. The temperature-dependent quasiparticle
scattering rate was analyzed in a two-fluid model, assuming the validity of the
Drude description of conductivity and generalized expression for the scattering
rate. This analysis allows us to estimate the range of the values of a residual
surface resistance from 3 to 6 mOhm.
