A three-element retro-directive antenna array

In this paper, a novel approach is developed for achieving a three-element retro-directive antenna array. The array is designed by properly making use of 90° hybrids. For a 90° hybrid, it can be shown as the two signals with relative difference of ΔΦ are connected to its input and isolated ports, the phase difference of the output signals from these two ports become -ΔΦ, if the direct port and coupled port are terminated with the same reflection coefficient Γ. In other words, a 90° hybrid becomes a phase conjugation circuit. The formulas and measurement results of the phase conjugation circuit and a three-element retro-directive antenna array are presented with the measurement arrangement.