A simple analytical boundary condition to model electromagnetic properties of planar regular dense array of dipole particles is developed for normal plane-wave incidence. The impedance condition connects the averaged tangential electric and magnetic fields on the array plane which leads to an equivalent transmission-line model. The reflection and transmission coefficients are solved by using the transmission-line model. The results can be used in studies of periodic arrays of complex particles, including active arrays, and also in studies of multi-layer arrays.