On the Constrains to Isolation Improvement in Multi-Antenna Systems

Different techniques are feasible to improve the antenna isolation between closely spaced antennas, especially for compact devices, which includes the use of lumped element networks between the feed points of the antennas. This paper systematically analyzes the achievable isolation between two closely spaced dipole antennas using a lumped element cancellation network. The improvement in antenna isolation is studied using a combination of resistive and reactive elements between the antennas and compared with using only reactive elements with phase shifting transmission lines. Design curves which propose optimal spacing between dipole antennas with lumped element networks for improved isolation are also provided. The results show better wideband isolation improvement than using only reactive elements together with phase shifting transmission lines, though at the cost of using a lossy element. The proposed method is validated with measurements between two closely spaced monopole antennas with 20 dB isolation achieved across 165 MHz bandwidth.