Through joint simulation optimization of radiating elements and decoupling structures, the design achieves scanning range breakthroughs under low-profile constraints: 2D scanning in Ku-band (gain variation <3.3 dB in E/H planes) and stable scanning in Ka-band (gain variation <1.55 dB in E/H planes).
Figure 1 illustrates the preliminary design of the Ka-band element, where a coaxial probe excites the slot on the substrate-integrated waveguide (SIW) cavity, and the energy is radiated through a square patch. The ADS is located at the top layer, formed by a discontinuous square ring surrounding a square patch. By adjusting the patch dimensions and the thickness of the top dielectric substrate, a reflected wave with equal amplitude but opposite phase to the original coupled wave is generated, achieving coupling cancellation. Specifically, the square patch cancels the primary coupled wave, while the surrounding secondary annular patch suppresses cross-polarization coupling. The DGS is positioned at the bottom layer, where eight L-shaped slots are etched at the four corners of the ground plane to create band-stop characteristics, obstructing the propagation path of coupled currents and improving isolation.
To evaluate the role of the hybrid decoupling structure at the unit cell level, Fig. 2 presents a comparison of and gain performance for four Ka-band antenna unit configurations: (1) with both ADS and DGS, (2) without any decoupling structure, (3) with ADS only, and (4) with DGS only. To ensure a fair comparison, all results are obtained from simulations of thoroughly tuned and optimized models.
As shown in Fig. 2a, compared to the antenna unit without any decoupling structure, the proposed L-shaped DGS effectively blocks the coupling current paths within the ground plane and suppresses surface wave resonance, thereby significantly expanding the operating bandwidth. In contrast, the standalone ADS structure acts as an additional resonator, whose radiation field constructively superimposes with that of the antenna, contributing to gain enhancement. However, its individual application leads to a reduction in the lower-frequency bandwidth.Through co-optimization of ADS and DGS, the DGS effectively compensates for the low-frequency bandwidth loss caused by ADS, achieving stable impedance matching across the entire band and eliminating the mid-band gain depression. As visible in Fig. 2b, the unit without decoupling exhibits a noticeable gain drop in the 33.8-34.6 GHz range; this depression is effectively eliminated with DGS loading; while ADS further enhances the overall gain level. Their synergistic operation ultimately achieves a peak gain of 8.6 dBi.
The current distributions in Fig. 3a,b demonstrate that the hybrid decoupling structure significantly reduces edge currents on the ground plane and effectively suppresses surface wave resonance. This observation further confirms the structure's substantial contribution to bandwidth extension and gain stabilization at the unit cell level. The decoupling performance of this hybrid structure in antenna arrays and co-aperture designs will be discussed in detail in subsequent sections.
The core design concept of the Ku-band element lies in the structural reuse of a mirrored Ka-band array. As shown in Fig. 4a, the Ka-band elements are arranged in mirrored configuration, while Fig. 4b presents the derived Ku-band element after local modifications. Specifically, cross-shaped patches are added at the centers of the second and fourth layer surfaces in the Ka-band array to improve impedance matching and enhance radiation performance. The original 0.5-mm-thick F4BM217 substrate at the bottom layer is replaced with two 0.127-mm and one 0.25-mm F4BM217 substrates to facilitate crossed feeding lines for dual-polarization. Remarkably, the DGS of the Ka-band array naturally forms the central cross-shaped feeding slot for the Ku-band element. The ADS of the Ku-band element directly inherits the Ka-band array's ADS, achieving 100% reuse of the shared-aperture dual-band ADS.
Figure 5a displays the simulated surface current distribution on the slot layer of the model in Fig. 4b. The current distribution reveals strong edge currents at the upper and lower boundaries of the Ku-band element, similar to the Ka-band design, which would degrade the active VSWR. Consequently, the slot layer was optimized by interconnecting two originally isolated L-shaped short slots via a transverse connecting bridge, thereby forming a continuous-channel slot structure. This design modification transforms the structure into a continuous-channel slot configuration.Post-optimization simulations in Fig. 5c demonstrate significantly weakened edge currents compared with Fig. 5a, indicating reduced inter-element coupling. As evidenced in Fig. 5d, the optimized design achieves active VSWR below 2 across 14-18 GHz, showing substantial improvement over the initial configuration.
Under periodic boundary conditions, Fig. 6a shows that the dual-polarized Ku-band element maintains / below dB from 14-18 GHz with port isolation exceeding 37 dB. The radiation patterns at 16 GHz in Fig. 6b exhibit 4.4 dBi gain for both X- and Y-polarizations, with cross-polarization levels consistently below dB. Figure 7a further demonstrates that the out-of-band port isolation between the Ka-band and Ku-band units is below dB in the 14-18 GHz band and below dB in the 32-36 GHz band.
Figure 8 presents the final unit cell configuration of the proposed Ku/Ka-band SAPAA, comprising one dual-polarized Ku-band radiating element and four mirror-arranged single-polarized Ka-band radiating elements. The Ka-band patch elements are positioned at the four corners surrounding the dual-polarized Ku-band element. The entire structure utilizes a seven-layer stacked substrate configuration. The element spacing of the SAPAA is designed as 1.08, while the inter-element spacings for the Ku/Ka-band units are selected as 0.487 and 0.480, respectively ( = 36 GHz, = 34 GHz, = 16 GHz). This specific spacing configuration effectively suppresses grating lobes during wide-angle scanning while minimizing undesirable mutual coupling.
As illustrated in Fig. 8a-d, the main structure of single-polarized Ka-band elements occupies layers 1-4: The ADS resides on layer 1 using F4BTMS350 substrate (, ), while the radiating patch layer, H-shaped slot layer, and feeding layer are sequentially arranged on layers 2-4 using F4BME300 substrate (, ). The T-shaped feedlines are encapsulated within square substrate-integrated waveguide (SIW) cavities formed by metallized blind vias. Layers 5-7 utilize F4BME217 substrate (, ), with Fig. 8e showing the DGS for Ka-band elements on layer 5, effectively suppressing inter-element coupling.
The dual-polarized Ku-band element's main structure is shown in Fig. 8b,d-f. The Ku-band radiating patches occupy layers 2 and 4, while the Ka-band DGS on layer 5 is repurposed as a cross-shaped feeding slot for Ku-band operation. Figure 8f,g reveal that two T-shaped feedlines are primarily distributed on layer 6's top surface, where a segment of the Y-polarized feedline detours through metallized vias from lower layers to avoid the X-polarized feedline. Similar to the Ka-band feeding structure, the Ku-band dual-polarized feeding network is also encapsulated within square SIW cavities.