Modulated oscillations appearing in the scan impedance of a finite phased array

For a linear array of horizontal dipoles aligned parallel to a ground plane, edge effects lead to oscillations of the scan impedance about the infinite-array solution. For a half-wavelength spacing and broadside scan, a slow modulation can appear on these oscillations. The modulation can be explained by the fact that the phase velocity of the waves diffracted by the edges of the array is slightly larger than the free-space velocity of light.