A scalable UWB-to-reconfigurable MIMO filtenna with single-varactor tuning and enhanced isolation for adaptive 5G and cognitive radio systems

This work presents a complete development framework that begins with a new fork-shaped ultra wideband (UWB) antenna. The antenna is designed, optimized, fabricated, and experimentally validated. The prototype achieves a very wide bandwidth extending from 2.4 to 8 GHz, with stable radiation behavior and high efficiency. Building on this design, a 4 × 4 UWB MIMO array is developed. The four elements are arranged orthogonally to enhance isolation and provide strong pattern diversity. Next, the UWB antenna is transformed into a frequency-reconfigurable filtering antenna (filtenna). A single varactor diode is embedded in a modified radiator to enable continuous tuning from 2.45 to 3.48 GHz. A stepped ground, inset feed, and RF-choke-based biasing network are added to achieve stable tuning and low-loss filtering. The fabricated prototype shows a clear frequency shift with excellent matching and good radiation efficiency. To extend the concept, 2 × 2 and 4 × 4 MIMO filtenna configurations are also developed. Each stage introduces further structural refinement, including inter-element decoupling lines, L-shaped ground extensions, π-shaped shared ground sections, and pairwise high-impedance biasing networks. These features significantly enhance isolation and suppress surface-wave coupling. The proposed MIMO designs provide outstanding diversity performance. An envelope correlation coefficient (ECC) of approximately 10−2, a diversity gain close to 10 dB, and a channel capacity loss of less than 0.1 bits/s/Hz are accomplished. Additionally, the antenna exhibits deep total active reflection coefficient (TARC) nulls near − 15 dB, along with mean effective gain (MEG) values that are well-balanced around − 3 dB. Taken as a whole, the results confirm that the developed UWB antenna, its reconfigurable filtenna derivative, and their 2 × 2 and 4 × 4 MIMO extensions form a compact, low-loss, and highly efficient solution for next-generation 5G and cognitive radio systems.