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**1. The Meaning of Antenna Decoupling Network**
In most wireless systems, antenna units are typically placed with as much spacing as possible to minimize mutual coupling. However, in compact devices like smartphones, the limited space forces the antennas to be closely packed, leading to strong electromagnetic coupling between them. Research shows that when the distance between two antennas is less than or equal to half the signal wavelength, their mutual coupling significantly affects signal reception. As the spacing decreases further, this effect becomes more severe, degrading the overall performance of the wireless system. Therefore, in such compact designs, it's crucial to address and reduce mutual coupling during the antenna design phase.
In engineering, isolation is commonly used to quantify the level of mutual coupling. For Wi-Fi band antennas, a typical requirement is an isolation of over 15 dB. Several methods exist to mitigate this issue, including adjusting antenna spacing or polarization, designing decoupling networks, using defective structures, or implementing current neutralization lines. These techniques can be analyzed and optimized using tools like HFSS. Among these, decoupling network technology is widely applied to reduce coupling between antennas. The design of the antenna unit and the decoupling network can often be done separately, which simplifies the overall design process. In this article, we'll walk through how to design an antenna decoupling network using HFSS simulation.
**2. Steps for Designing an Antenna Decoupling Network Using HFSS Simulation**
From a network analysis perspective, the goal of decoupling is to make the mutual impedance in the multi-port network matrix approach zero or to reduce the inverse transmission coefficient in the scattering matrix to near zero. A common structure of a decoupling network is shown below. The basic principle involves analyzing the network parameters step by step. In simple terms, the process can be broken down into three main steps:
First, due to good initial impedance matching between the antennas, there is a strong coupling effect. The role of the decoupling network (D) is to adjust the transmission admittance between the two ports from a complex number to a purely imaginary value.
Second, a parallel reactance is introduced to cancel out the previously adjusted imaginary admittance, effectively bringing its value to zero and achieving decoupling.
Third, the introduction of the decoupling network may cause impedance mismatch at the antenna port. To address this, a matching network is added to restore proper impedance matching.
Below is an example of a practical simulation using HFSS.
**1. Two closely spaced Wi-Fi band monopole antennas (2.4–2.5 GHz) show poor isolation, only around 3 dB at the center frequency.**
**2. After applying a lumped RLC boundary condition to simulate the decoupling network D, the isolation improves significantly, but the resonance point of the antenna shifts slightly.**
**3. Finally, a matching network is added, and after simulation, both the isolation and VSWR meet the required specifications.**
This step-by-step approach demonstrates how HFSS can be used to design and optimize decoupling networks, ensuring better performance in compact wireless systems.