Bandwidth enhancement and size-reduction on a 4-port MIMO hilbert fractal antenna by using an open-ended stub-line

Abstract This paper presents the development and implementation of a 4-port MIMO antenna based on a combination of a Hilbert curve fractal and a reactive load implemented via an open-circuited stub. The integration of these techniques results in a highly compact array with a wide operating bandwidth, extending from 2.3 GHz to 5.8 GHz. This covers the 2.4 GHz ISM band and the 5G New Radio (NR) bands N41, N77, N78, and N79. The total array dimensions are 25 mm × 36 mm, equivalent to 0.18 λ 0 × 0.26 λ 0 at the lower cutoff frequency. The minimum separation between elements is 3 mm, approximately 0.02 λ 0 , which is a very small distance between radiators. However, by incorporating an electromagnetic barrier and giving galvanic continuity, also implemented with a Hilbert curve on the ground plane, an isolation ranging from 15 dB to over 20 dB is achieved across the entire bandwidth, with a peak gain of 1.8 dB. Furthermore, the use of the open-circuited stub stabilizes the antenna’s impedance behavior, causing the reactive component to approach 0 Ω, while the real part stabilizes within a threshold close to 50 Ohms, significantly increasing the bandwidth. The final result is a compact 4-port MIMO antenna featuring high isolation, an Envelope Correlation Coefficient ( ECC ) below 0.05 across the entire range, a Diversity Gain ( DG ) close to 10, and a stable Total Active Reflection Coefficient ( TARC ) below −10 dB throughout the operating bandwidth.