Design and fabrication of waveguide-based graded refractive index Rotman lens with an improved insertion loss at Ka-band

The design and optimisation of a five-cascade layer Rotman Lens based on waveguide with a graded refractive index at K_a-band frequency are presented. The proposed lens incorporates 5 layers within the cavity, utilises a modified three focal Rotman lens design methodology to achieve perfect phasing for true time delay beam steering, and optimises each layer's parameters such as refractive index, vertex location, and contour angle to improve insertion loss between input ports and output array elements. This approach enhances energy concentration between input and output ports, reducing spillover to dummy ports. The graded layers are formed using periodic cubic metallic posts with varying heights within a parallel plate waveguide. The results show an average improvement in the insertion loss of 16% and a peak improvement of 48% across the bandwidth when compared to conventional Rotman lenses. The normalised average phase error across the array elements for all beam ports excitation is 4 × 10⁻³ and the peak degradation of the main beam during beam scanning from 0° to ±22° varies from 0.7 to 1 dB at different frequencies operating in the K_a-band (24–30 GHz) with potential applications for 5G communications. The simulation results demonstrate good agreement with the measurement results.