With the rapid growth of wireless communication systems, there is a rising demand for multi-input multi-output (MIMO) antenna systems capable of adapting to various frequency bands and operating conditions. This paper presents an integrated design for MIMO antennas based on a varactor diode as a promising component for achieving frequency agility in the proposed system. A dual-polarized system is achieved by employing a combination of two antennas. One antenna is situated on the exterior surface of the side-edge frame, while the other is positioned on the substrate surface. The spatial configuration enables the creation of orthogonal polarization orientations, specifically vertical and horizontal polarizations. In each element, varactor diodes are positioned to provide reactive loading. By incorporating varactor diodes with a variable bias voltage (0.5-10 V) into the antenna design, the resonant frequency can be dynamically adjusted, allowing the antenna to operate across a wide range of frequencies (4.3 to 6.5 GHz) with more than 18 dB of mutual coupling in the working band. The presented reconfigurable antennas are printed on compact dimensions of 15 x 25 x 0.8 mm3 using a Rogers RT5880 material with a relative dielectric constant 2.2. Because of its flexible frequency range, extensive tuning range, small size, and planar structure, it is well-suited for various current and future wireless communication applications, including cognitive radio, software-defined radio, and next-generation wireless networks.
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