Window-Shaped mmWave MIMO Antenna for 5G Devices Using Characteristics Mode Analysis

A compact four-element mmWave MIMO antenna designed using characteristics mode analysis (CMA) with wideband performance and high isolation is presented. The antenna is realized on a 0.254-mm Rogers RO5880 substrate with meandered conductive strips arranged in a window-shaped configuration and a partially truncated ground-plane notch. CMA is employed to observe the modal behavior of the structure, revealing that Mode 1 is the dominant radiating mode, while Modes 2 and 3 contribute to resonance formation and bandwidth enhancement. The single antenna element occupies a compact size of 12 × 14 mm² (0.1305 λ₀ × 0.1119 λ₀, where λ₀ is the wavelength at 28 GHz), whereas the four-port MIMO configuration measures 24 × 32 mm² (0.224 λ₀ × 0.229 λ₀). A decoupling structure is introduced to suppress mutual coupling, achieving interelement isolation better than 20 dB. The proposed MIMO antenna operates over a wide impedance bandwidth from 24.5 to 33.5 GHz and exhibits a peak realized gain of 6.2 dBi. Due to its symmetric layout, the design is inherently scalable to higher order MIMO systems. Experimental results obtained from a fabricated prototype show good agreement with simulations, validating key MIMO performance metrics, including ECC, MEG, CCL, and DG, confirming the suitability of the proposed antenna for mmWave RF applications.