Power Transfer between Two Antenna Arrays in the Near Field

We present numerical results with a focus on power transfer between two standard linear antenna arrays placed in the near field, where a much smaller active multi-antenna feeder (AMAF) space feeds a far larger passive array referred to as a reflective intelligent surface (RIS). The interest is in the regime of focal length to diameter ratio (F/D) less than unity. We address the question of center feed vs. end feed for array fed array antenna architectures and present the following novel findings and contributions: 1. In the regime of $F$/$D$ ratio less than one, the AMAF-RIS power transfer deviates from the classical inverse square law. Furthermore, the behavior of the power transmission coefficient is more sensitive to the $F$/$D$ ratio than to a particular RIS size. 2. For an end feed, non-eigenmodes provide better beam shapes than the eigenmodes, which are still inferior to beam shapes from the center feed eigenmodes. 3. The center feed provides more power gain than an end feed. This clearly illustrates that the center feed configuration should be used for array fed arrays, in contrast to the classical parabolic geometry.