Design of Low-Profile Scanning Transmitarrays by Virtual Focus Synthesis

A novel approach for designing low-profile transmitarray antennas is presented, optimizing phase taper of the primary feed to emulate a spherical wavefront originating from a virtual focal point. The feed aperture is smaller than the transmitarray and placed at a distance <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$H=2 \lambda$</tex> from it. The performance of a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$24 \times 24$</tex>-elements TA with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$H / D=0.17$</tex> is analyzed. The method improves broadside directivity from 28.5 dBi to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{3 2 ~ d B i}$</tex> and reduces 1-bit quantization losses at 60° from 4.8 dB to 3 dB as compared to a similar configuration where no phase taper is applied on the aperture field. Finally, the effectiveness of the method is validated using an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$8 \times 8$</tex> array of open-ended waveguides as a focal array, demonstrating a 2.5 dB gain improvement and significant suppression of quantization lobes in a low-profile TA for beam steering at 40°.