Impact of using resistive elements for wideband isolation improvement

Improving the isolation between antenna elements in compact arrays has been a major focus of recent research. In this paper, we present ideas to improve the wideband isolation between closely spaced antennas. We do this by connecting lumped lossy (resistive) elements between the antenna feeds. A simple analytical expression is provided to compute the impact of resistive elements on efficiency to analyze the power lost in the resistive element. Three configurations of decoupling circuits are designed and fabricated for two closely spaced monopoles operating at 2.4 GHz. The decoupling circuit contains transmission lines of different lengths at the antenna inputs such that the mutual admittance between the antenna elements is: 1) resistive; 2) resistive and inductive; or 3) resistive and capacitive. Lumped elements are then connected between the transmission lines followed by matching circuit. This paper shows that with configurations 2) and 3), we can improve the wideband isolation compared with 1), as well as compared with using only lossless elements. The wideband isolation was improved by 17.6 dB across a 200-MHz band at 2.4 GHz, with a final isolation level of 20 dB over that band. Better than 30 dB isolation was achieved across a narrower band of 55 MHz. The proposed technique provides wideband isolation improvement for multiple-input multiple-output as well as narrowband performance with large isolation suitable for in-band full-duplex applications. The impact on efficiency is investigated to verify that the advantages from the improved wideband isolation outweigh the possible reduction in overall efficiency.