Periodic arrays of Vivaldi antennas are considered. Planar arrays infinite along two coordinates are analyzed. A planar array is studied with the use of a well-known model in the form of the Floquet waveguide with variable parameters. Propagation of the fundamental mode of this structure is described in the single-mode approximation in which only forward and counter-propagating fundamental modes are taken into account. An equivalent circuit of the Floquet waveguide with variable parameters is proposed. The concept of the characteristic impedance of the wave of an array of slot transmission lines is introduced and used for calculation of the scattering matrix of the Floquet waveguide with variable parameters. Results of numerical calculations are presented.