SIW Planar and Stacked Array Modular Antenna Structure Design for 38 GHz Millimeter Wave

Technical Summary: SIW Planar and Stacked Array Modular Antenna Structure Design for 38 GHz Millimeter Wave

Problem Statement
The increasing demand for bandwidth in next-generation wireless systems (5G/B5G/6G) necessitates a shift to millimeter-wave (mmWave) frequencies, specifically the 38 GHz band. However, higher frequencies incur significant propagation losses, requiring high-gain, high-directivity antennas to compensate. While antenna arrays are the standard solution for gain enhancement and beam control, their design at mmWave frequencies presents challenges regarding manufacturing complexity, cost, and the need for scalable, modular architectures that maintain performance consistency across different array sizes. Existing solutions often struggle to balance miniaturization, uniform power distribution, and the ability to easily scale from small to large arrays without redesigning core components.

Methodology
This study proposes a modular design methodology for 38 GHz linear array antennas using Substrate Integrated Waveguide (SIW) technology. The research focuses on two distinct architectures: a single-layer planar design and a modular stacked design.

• Radiating Element: The core radiating unit is an octagonal microstrip patch antenna fabricated on Rogers 880 substrate ($\epsilon_r = 2.2$, thickness = 0.254 mm). The design utilizes a specific feed line offset to optimize impedance matching.
• Power Distribution: A Y-shaped SIW power divider is employed to ensure uniform power distribution and phase consistency. The study designs and simulates 1×2, 1×4, and 1×8 power dividers, utilizing metalized vias to confine electromagnetic fields and suppress higher-order modes.
• Scalability: The authors validate the design by sequentially constructing 1×2, 1×4, and 1×8 linear arrays. The element spacing is set to approximately half-wavelength ($\lambda/2 \approx 4.85$ mm) at 38 GHz to minimize mutual coupling.
• Stacked Architecture: To decouple the radiating function from the feeding function, a slotted waveguide interlayer transition structure is introduced. This allows the SIW power divider (Layer 2) and the patch antenna array (Layer 1) to be fabricated on separate substrates and vertically stacked. This modular approach enables repeatable fabrication and flexible reconfiguration.
• Validation: The designs were simulated using HFSS for full-wave electromagnetic analysis. Prototypes were fabricated using standard double-sided PCB processes and measured in a millimeter-wave anechoic chamber using an Agilent PNA network analyzer (N5227A) covering 10 MHz to 67 GHz.

Key Contributions

1. Modular Scalability: The paper demonstrates a unified design strategy where the same radiating element and feeding topology are used to construct 1×2, 1×4, and 1×8 arrays without modifying element dimensions, ensuring design consistency and electromagnetic predictability across scales.
2. Dual-Architecture Approach: The study presents both a low-cost, single-layer planar solution and a high-performance, modular stacked solution. The stacked design specifically utilizes a functional separation of the power divider and radiating elements to facilitate modular expansion.
3. SIW Integration: The implementation of Y-shaped SIW power dividers and interlayer transitions provides a low-loss, high-efficiency feeding network suitable for mmWave applications, achieving wide bandwidths and excellent impedance matching.
4. Experimental Verification: Comprehensive experimental results confirm the simulation predictions, validating the feasibility of the modular stacked approach and the performance of the octagonal patch elements in linear array configurations.

Results

• Impedance Matching: Both planar and stacked arrays exhibited excellent impedance matching. The 1×8 planar array maintained $S_{11} < -10$ dB from 36.62 GHz to 38.23 GHz. The stacked 1×8 array showed low reflection from 35.53 GHz to 38.45 GHz.
• Gain Performance:
  • Planar Arrays: Measured realized gain increased from 9.63 dBi (1×2) to 15.27 dBi (1×8).
  • Stacked Arrays: The 1×8 stacked configuration achieved a peak measured gain of 14.45 dBi. The 1×2 stacked array reached 9.6 dBi with a radiation efficiency of up to 80%.
• Efficiency: The stacked 1×8 array demonstrated a radiation efficiency of approximately 72%, while the planar 1×8 array exceeded 70%.
• Radiation Patterns: All configurations displayed stable main-beam directions with no noticeable beam squint. As the array size increased, the main lobe narrowed, and directivity improved, with sidelobe levels remaining controlled.
• Comparison: Compared to existing literature, the proposed arrays offer competitive gain-to-aperture ratios and bandwidth stability while utilizing standard PCB processes, resulting in lower manufacturing costs and better mass-production consistency.

Significance
The paper claims that the proposed SIW modular antenna design successfully balances high gain, wide bandwidth, miniaturization, and standard fabrication processes. By separating the radiating and feeding layers in the stacked architecture, the design enables repeatable fabrication and scalable modular expansion, which is critical for next-generation mmWave communication and sensing systems. The study validates that a unified modular approach can achieve performance comparable to more complex or specialized structures (such as those with back cavities or non-standard materials) while maintaining the economic and manufacturing advantages of standard PCB technology. The results suggest that this architecture is a viable and efficient solution for the deployment of 38 GHz mmWave systems.