Compact Log-Periodic Single-Port Planar Antennas for D-Band Monolithic RIS Panels

The quality of wireless connections in the sixth-generation (6G) communication networks is tightly linked to the capabilities of smart antenna environments. They should enable effective routing of ultra-narrow radio beams, intrinsic to D-band channels, and make use of so-called reconfigurable intelligent surfaces (RISs). Practical RISs are seriously judged by their cost and operation efficiencies. In this letter, we report on the development of a robust single-port antenna that can be used as part of a D-band RIS panel. The antenna is presented by a grounded log-periodic spiral on thin quartz substrate. Two types of spirals are developed for either a static beamforming or dynamic beam steering in reflected light. The geometry- and load-controlled phase shifts consistently cover a 0<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> to 350<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> range upon reflection of 140 GHz waves with the angles of incidence within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula>75<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>. This is achieved by changing either first, the spiral scaling factor between 0.658 and 1 in half-wave cells or second the spiral load capacitance between 0 fF and 20 fF in quarter-wave cells. These values are feasible for gallium arsenide (GaAs)-on-quartz planar Schottky diodes. The simulation results are justified by prototyping. For a 1.3 k pixel proof-of-concept reflectarray with geometrically phase-configured cells, we measure the beamwidth, sidelobe, and cross-polarization levels of 4<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>13 dB, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>26 dB, respectively. All together suggests great potential of the developed antennas as basic elements for the physical layer devices of reflection-aided 6G wireless communications.