Topology optimization of decoupling feeding networks for antenna arrays

TL;DR

This work tackles mutual coupling in compact antenna arrays by introducing a density-based topology optimization framework that designs decoupling networks using impulse-response boundary conditions. Gradients are obtained with the adjoint-field method within a time-domain FDTD setting, and nonlinear filters enforce minimum feature size to yield manufacturable results. The approach is demonstrated on a two-element planar array, achieving substantial mutual-coupling reduction (exceeding 10 dB in simulations and measurements) while preserving port matching and radiation efficiency. The method is extendable to wideband and multi-antenna configurations and can address other interference problems where coupling plays a critical role.

Abstract

Near-field and radiation coupling between nearby radiating elements is unavoidable, and it is considered a limiting factor for applications in wireless communications and active sensing. This article proposes a density-based topology optimization approach to design decoupling networks for such systems. The decoupling networks are designed based on a multi-objective optimization problem with the radiating elements replaced by their time-domain impulse response for efficient computations and to enable the solution of the design problem using gradient-based optimization methods. We use the adjoint-field method to compute the gradients of the optimization objectives. Additionally, nonlinear filters are applied during the optimization procedure to impose minimum-size control on the optimized designs. We demonstrate the concept by designing the decoupling network for a two-element planar antenna array; the antenna is designed in a separate optimization problem. The optimized decoupling networks provide a signal path that destructively interferes with the coupling between the radiating elements while preserving their individual matching to the feeding ports. Compact decoupling networks capable of suppressing the mutual coupling by more than 10 dB between two closely separated planar antennas operating around 2.45 GHz are presented and validated experimentally.

Figures (13)

• Figure 1: Decoupling network design. The primary near-field coupling (Path I) between closely spaced antennas, Ant 1 and Ant 2, can be suppressed by optimizing a decoupling structure, in design domain $\Omega$, to create a secondary signal (Path II) that destructively interfere with the signal coupled through Path I.
• Figure 2: A two-port antenna system.
• Figure 3: Decoupling network with antennas at port 3 and port 4 are replaced by their impulse response as boundary conditions.
• Figure 4: Flowchart of the optimization algorithm.
• Figure 5: Geometrical parameters of the two-element planar antenna array with the domain for the decoupling network included. The system is built on a 4-layer FR-4 stackup with $\epsilon=4.5$ and 0.80 mm thickness. Side (left) and top (right) views of the substrate. The designs are placed on the top layer, and the second layer, separated by a distance $0.21$ mm, serves as a ground plane, excluding the area beneath the antenna. $L_0=21.04$, $L_1=10.52$, $d_0=4.21$, $d_2=13.05$, $d_y=9.68$, $d_z=8.21$, $w_0=0.42$ (unit: mm).