Varactor-tuned 4 × 4 MIMO antenna with reconfigurable 2.38–5.1 GHz band and fixed 7.2–20 GHz wideband for sub-6 GHz, X-band, and Ku-band wireless systems

This study presents a compact, high-gain, frequency-reconfigurable $ 4\times 4 $ 4×4 Multi-Input Multi-Output (MIMO) antenna designed for sub-6 GHz, X-Band and Ku-Band communication systems. The proposed antenna operates across two distinct frequency ranges: a dynamically tunable lower band (2.38–5.1 GHz) and a fixed wide upper band (7.2–20 GHz). Frequency reconfigurability is achieved via varactor diodes with capacitance values adjustable between 0.39 and 2.25 pF. By tuning the reverse-bias voltage applied to varactors, the antenna demonstrates a dual-mode operation. At minimum bias voltage, the lower band exhibits a broad operational bandwidth spanning 2.38–5.1 GHz. Increasing the voltage introduces a tunable stop-band within this range, which can be dynamically shifted by approximately 320 MHz without degrading the performance of the upper wideband. This capability enables precise spectral suppression for interference avoidance while maintaining wideband functionality, effectively supporting both dual-band and wideband operations. The MIMO unit element consists of a tulip-shaped radiating patch with a defected ground plane. The ground of each element is interconnected through bent stubs, forming a common ground that ensures stable operation, maintains mutual coupling well below −15 dB, and contributes to an increase in overall antenna gain. With a maximum realized gain of 5.58 dBi at 0.39 pF, an envelope correlation coefficient (ECC) of less than 0.017, and a diversity gain exceeding 9.91 dB, the proposed antenna demonstrates excellent MIMO performance. Furthermore, a prototype was fabricated and tested, showing strong agreement between simulated and measured results, validating the antenna's suitability for sub-6 GHz 5G applications, including 5G New Radio (NR), Wi-Fi 6, and Internet of Things (IoT) devices, X-band (8–12 GHz) and Ku-band (12–18 GHz) applications, such as satellite communications and radar systems. This work addresses key challenges in MIMO antenna design, particularly the integration of both tunable and fixed bands. In achieving frequency reconfigurability in the tunable band, several practical issues were resolved, including the design of an effective bias circuit for the varactor diode, incorporation of DC block capacitors, mitigation of measurement mismatches caused by long DC bias cables, and overcoming difficulties in reliably triggering the varactor diode under varying test conditions.