Consider a number of small sub-apertures that are mutually adjacent to one another, so that they form a [[filled aperture array]]. Suppose that they are in orbit, beaming [[microwave]]s at a spot on the ground. Now, suppose you hold constant the number of sub-apertures and the power emitted by each, but separate the sub-apertures (while keeping them mutually phased) so as to [[aperture synthesis|synthesize a larger aperture]]. The spot size on the ground is reduced in size proportionally to the diameter of the synthesized array (and hence the area is reduced proportionally to the diameter of the synthesized array squared), but the power density at the ground is unchanged.
