Overcoming Bandwidth and Complexity Challenges: Broadband High-Gain 3-Bit (U)LTCC-Based Transmitarrays Using Three-Layer Via-Less Subwavelength Unit Cells in the D-Band

Achieving broadband multi-bit transmitarrays (TAs) on LTCC (Low-Temperature Co-fired Ceramic) or ULTCC (Ultra-Low Temperature Co-fired Ceramic) substrates is challenging due to their high permittivity, which often requires complex unit cell (UC) geometries (e.g., seven-layer designs with vias). This work introduces a novel design of three-layer, viasless, subwavelength (U)LTCC-based UCs specifically optimized for linearly-polarized waves. These UCs enable 360° phase coverage with a nearly linear phase response, minimal phase errors, low insertion loss, and reduced cross-polarization over a wide frequency range. Implemented on LTCC (A6M, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\varepsilon r=5.72$</tex>) or ULTCC (ZSG8470, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\varepsilon_{r}=6.802$</tex>) substrates, the designs achieve an 18 % relative bandwidth in the D-band. To validate the approach, two broadside TAs, each comprising <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$20 \times 20$</tex> UCs, were developed using an in-house numerical tool and validated through full-wave simulations. The A6M-based TA achieves a peak gain of 23.5 dBi with 48.5 % aperture efficiency and an −1 dB gain bandwidth of 11.2 %, while the ZSG8470-based TA attains a peak gain of 21.9 dBi, with 38.1 % aperture efficiency and a 13 % gain bandwidth. Both TAs exhibit frequency-selective behavior, low side lobe levels, and high directivity, making them promising for cost-effective LTCC and ULTCC-based TAs for high-capacity wireless backhauls beyond <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 0 0 ~ G H z}$</tex>.