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What’s the practical minimum edge-to-edge spacing to achieve an ECC < 0.1 in a compact, two-element UWB MIMO antenna system?

I'm integrating a two-port UWB MIMO antenna (3.1–10.6 GHz) into a small IoT device. Mutual coupling increases as elements get closer, degrading MIMO performance. While I know isolation can be improved with decoupling structures, is there a fundamental rule of thumb for spacing to keep ECC low across the entire band? Does it relate to the wavelength at the highest frequency, or is the correlation more sensitive to the ground plane mode excitation? I need a starting point before I add complex decoupling.

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By Sumitra R Answered 1 year ago

In compact devices, relying on spacing alone is unrealistic. For UWB, the required isolation is frequency-dependent. A spacing of 0.5λ at the highest frequency (~14mm at 10.6 GHz) might give low ECC, but that's often too large. In practice, I've achieved ECC < 0.1 with edges as close as 0.1λ at the center frequency (~5mm at 6.85 GHz) by combining three strategies: 1) Orthogonal placement (e.g., one element at the top, one at the bottom of the PCB) to maximize pattern diversity, which reduces ECC even if S21 is moderate. 2) A strategically placed ground plane slit to block coupling currents. 3) Different antenna types (e.g., a monopole and a slot) for inherent pattern orthogonality. Optimize for pattern diversity first, then use decoupling to further suppress S21.

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