Stacked Rotman Lenses for Dual Band Beamforming

This article presents a thin planar stacked antenna array for simultaneous dual-band beamforming. The uniqueness of the design is the use of passive beamformers in a compact module with sharp filters to enable beamforming when transmitting at closely spaced frequency bands. It uses two identical Rotman lenses to feed two separate series-fed patch arrays operating at 15.3 GHz and 17.2 GHz. The lenses and arrays are vertically stacked, with simplified stripline-to-microstrip transitions, enabling efficient energy transfer between layers to reduce losses. This vertical stacking reduces the required planar area as compared to conventional multi-layer designs with airgaps. To improve performance, elliptic filters are used to achieve sharper transitions from passband to stopband, effectively reducing mutual coupling between the two bands. Low-pass filters are employed for the 15.3 GHz array inputs and band-pass filters are used for the 17.2 GHz array inputs. The entire module was fabricated using standard PCB techniques and successfully validated with measurements. Notably, sidelobe levels were below -10 dB with up to ±40∘ scanning from broadside.