Design of a Metasurface-based Bifunctional Folded Transmit/Reflect Array Antenna

We present a bifunctional folded reflect/transmit array antenna (FRA/FTA) utilizing a low-profile metasurface (MS) array. The design incorporates four linear phase variants of the transmit array (TA), facilitating cross-polar transmission at high frequencies ($30-40 \mathrm{GHz}$) and x-polarized wave propagation at lower frequencies ($\mathbf{1 0 - 2 2} \mathbf{~ G H z}$). Furthermore, a dual-band reflect array (RA) is implemented, operating as a folded reflect array (FRA) antenna in the lower frequency range ($11-22 \mathrm{GHz}$) and serving as a polarizer converter for the FTA at higher frequencies ($28-40 \mathrm{GHz}$). The proposed design achieves peak realized gains of 22.7 dBi and 28.1 dBi, with aperture efficiencies (AEs) of 42% and 31% at 16 GHz (FRA) and 35 GHz (FTA), respectively. It also provides 3-dB gain bandwidths (GBWs) of $\mathbf{4 2. 9 \%}$ and $\mathbf{2 8. 5 \%}$, maintaining frequency separation ratio of $1: 2.19$. The profile-to-diameter ratio (H/D) has been minimized to 0.27, with side lobe levels (SLLs) suppressed by 12 dB and 15 dB across the specified frequency bands. This design applies to satellite communication, synthetic aperture radar (SAR), radio astronomy, and related fields.