Gain-Equalized Multibeam Antenna Fed by a Compact Dual-Layer Rotman Lens at K_a-Band

This communication presents a gain-equalized $K_{a}$ -band Rotman lens-fed antenna by using phase gradient transmission lines (PGTLs). With PGTLs, the variation trend of the output progressive phase differences among the feeding ports is inverted. The aperture loss of the planar antenna array is then partially compensated by the efficiency of the lens when performing a wide-angle beam scanning. Hence, gain equalization is achieved among multiple beams with an acceptable gain degradation. To reduce the total size, the Rotman lens with PGTLs, the feeding networks, and the antenna array are stacked vertically. Meanwhile, the feeding network is implemented with substrate integrated coaxial lines, suppressing the parallel-plate mode inside the multilayer structure. In order to validate the proposed design, a compact Rotman lens-fed antenna operating at 24.75-27.5 GHz (FBW = 10.5%) is designed and fabricated. The results show that the gain drop of the Rotman lens-fed multibeam antenna is reduced from 4.4 to 1.5 dB at the design frequency of 26 GHz, covering a wide angular range between ±56°.