As 5G networks densify and massive MIMO scales, antenna engineers face persistent mutual coupling in compact arrays. Dr K. Mithra, an Arizona-based independent wireless researcher and inventor, explains to EFY’s Akanksha Sondhi Gaur how reengineering the inter-element gap, rather than increasing spacing.
Q. What inspired you to focus on mutual coupling reduction in microstrip patch arrays for the 3.5GHz 5G band?
A: The 3.5GHz band is globally allocated for 5G New Radio and is the most widely deployed sub-6GHz spectrum for commercial networks. It offers an optimal balance between coverage and bandwidth, making it particularly suitable for base stations and small-cell infrastructure. Designing at 3.5GHz ensures direct commercial applicability, practical antenna dimensions, compatibility with low-cost printed circuit board (PCB) substrates, and seamless integration into existing radio frequency (RF) systems. The primary objective was to create a solution that the industry could readily adopt, rather than one limited to simulation-based validation.
Q. What is mutual coupling, and why is it critical in compact 5G arrays?
A: Mutual coupling refers to the unwanted electromagnetic interaction between antenna elements when they are placed in proximity. When an antenna radiates, a portion of its energy propagates as surface waves through the substrate, exciting neighbouring elements. This interaction leads to variations in input impedance, distortion of the radiation pattern, increased envelope correlation coefficient (ECC), and higher signal correlation. In multiple-input multiple-output (MIMO) systems, elevated coupling directly degrades channel capacity, spectral efficiency, data throughput, and overall network reliability. Consequently, mutual coupling is not merely an antenna-level concern but a system-level limitation in dense 5G deployments.
Q. What is the principle behind the parallel coupled line resonator (PCR) technique?
A: The PCR functions as an electromagnetic barrier integrated within the antenna gap. It consists of multiple closely spaced microstrip lines placed between adjacent elements. At 3.5GHz, this structure behaves as a band-stop resonator, presenting high impedance. When surface waves attempt to propagate between antennas, they encounter this high-impedance region and are suppressed. The key concept is that the inter-element gap is not space;it can be engineered as a functional isolation structure. Treat the antenna gap as design space, not wasted space.
Q. How compact is the design compared to conventional spacing guidelines?
A: Conventional antenna arrays typically maintain an element spacing of approximately 0.5λ to minimise mutual coupling. In this design, however, the spacing is drastically reduced to just 0.07λ, corresponding to approximately 6 mm edge-to-edge at 3.5GHz. Despite this extreme compactness, the structure achieves up to a 26.2dB improvement in isolation. This result demonstrates that dense antenna packing and high isolation can coexist, which is essential for compact small-cell deployments and future massive MIMO systems.
Q. What measurable performance improvements were achieved?
A: At 3.5 GHz, the proposed design demonstrated significant performance enhancements. The isolation improved from –10dB to –36dB, corresponding to a maximum reduction in mutual coupling of 26.2dB and approximately a 400-fold reduction in coupled power. In addition to improved isolation, the antenna exhibited a gain increase of about 1.25dB and a radiation efficiency increase of nearly 7 per cent. Importantly, these improvements were achieved without disturbing the radiation pattern or beam-steering capability. Collectively, these measurable gains translate directly into improved MIMO channel capacity and higher spectral efficiency at the system level.
Q. Why was FR4 selected instead of high-end substrates?
A: FR4 was selected due to its low cost, widespread availability, and compatibility with standard PCB fabrication processes. For sub-6GHz applications, it provides adequate electrical performance while enabling scalable and industry-ready implementation. In contrast, high-end substrates increase material cost and manufacturing complexity, which can restrict large-scale adoption. The design philosophy emphasised practicality, manufacturability, and commercial readiness rather than dependence on expensive materials.
Q. How does this approach compare with DGS, EBG, and metamaterial techniques?
A: Each conventional isolation technique involves specific trade-offs. Defected ground structures (DGS) can provide good isolation but often increase back radiation and may degrade antenna gain. Electromagnetic band gap (EBG) structures effectively suppress surface waves but typically require a larger PCB footprint, limiting compactness. Metamaterial-based approaches can achieve strong isolation; however, they often require complex geometries and pose fabrication challenges. In contrast, the PCR approach is fully planar, fits entirely within a 6mm inter-element gap, uses standard PCB fabrication processes, and maintains overall antenna performance while achieving up to 26.2dB isolation. Its primary advantage lies in delivering high isolation without sacrificing compactness or manufacturability.
Q. How does reducing mutual coupling improve MIMO performance?
A: Reducing mutual coupling lowers the envelope correlation coefficient (ECC) and enhances spatial diversity in MIMO systems. When the correlation between antenna elements decreases, the system can better exploit independent signal paths, resulting in higher channel capacity, improved spectral efficiency, reduced interference, and enhanced signal quality. Even at an ultra-compact spacing of 0.07λ, effective spatial diversity is maintained, demonstrating that dense antenna integration does not inherently compromise MIMO performance.
Q. Can this technique scale to massive MIMO systems?
A: The proposed design has been experimentally validated for a 2×2 antenna array. Scaling the technique to larger configurations, such as 4×4, 8×8, or 16×16 arrays, will require detailed analysis of additional inter-element interactions, optimised placement of PCR structures, and potentially an increased number of resonator elements. Although further optimisation is necessary for large-scale arrays, the core concept of engineering the inter-element gap as a functional isolation structure remains inherently scalable and suitable for massive MIMO and future 6G architectures.
Q. What fabrication challenges were encountered?
A: Several fabrication-related factors influence performance, including variations in dielectric constant that affect resonance behaviour, frequency shifts due to substrate tolerances, and sensitivity to manufacturing accuracy. When flexible substrates are considered, mechanical bending can alter the effective electrical length and shift the operating frequency, introducing additional design complexity. Future enhancements may include tunable PCR structures using PIN or varactor diodes to enable dynamic frequency control, as well as dual-band and wideband configurations. The integration of active isolation mechanisms is also being explored to further enhance performance stability.
Q. Which industries can benefit from this innovation?
A: This innovation is particularly relevant to 5G base station manufacturers, small cell designers, RF module manufacturers, and compact antenna OEMs that require high performance within constrained physical dimensions. The approach is well-suited to dense deployments where both compactness and high isolation are critical. Beyond traditional telecom infrastructure, potential applications include IoT gateways, private 5G networks, defence communication systems, and, with suitable adaptation, automotive radar platforms. The core value proposition lies in delivering strong isolation within extremely compact antenna configurations.
Q. What is the current commercialisation status?
A: The work has been published in IEEE Antennas and Wireless Propagation Letters and has received nearly 190 global citations. At present, the technology remains in the research-prototype stage. Telecom certification has not yet been pursued, and patent filing is currently under consideration.
Q. How do you see compact antenna arrays evolving over the next five years?
A: Over the next five years, antenna technology is expected to advance toward massive MIMO systems incorporating hundreds of elements, along with chip-integrated antennas and millimetre-scale packaging solutions to reduce size and cost. AI-assisted RF optimisation is likely to play an increasingly important role in performance enhancement and adaptive tuning. As integration density increases, active and tunable isolation mechanisms will become increasingly important for managing mutual coupling and interference. In this evolving landscape, compact isolation structures engineered within the antenna gap will become essential design tools.
Q. What advice would you give to RF engineers working on 5G and beyond?
A: RF engineers should develop a strong foundation in antenna fundamentals while understanding their broader system-level implications. PCB space should be viewed as strategic design real estate that intelligent engineering can unlock to deliver significant performance improvements. Manufacturability must remain a priority, ensuring that designs are practical, scalable, and cost-effective. Simulations should always be validated through rigorous measurements to guarantee real-world reliability. Finally, solutions should be conceived with future scalability in mind, particularly as systems evolve toward higher frequencies and greater integration density. Innovation should ultimately be driven by measurable performance and practical implementation.
