Through the use of multi-element antennas, wireless systems are accommodating increasingly higher data rates. At National Chiao Tung University (Hsinchu, Taiwan), a compact decoupling network for enhancing the port isolation between two closely spaced antennas has been investigated by Shin-Chang Chen, Yu-Shin Wang, and Shyh-Jong Chung. Using the proposed decoupling structure, two examples of printed antennas at 2.45 GHz were created. Isolation between the antennas went from 3 dB to more than 20 dB at the center frequency. Input return losses remained better than 10 dB.
The researchers connected two transmission lines to the input ports of two strongly coupled antennas. The length of these transmission lines allowed the transadmittance between ports to change from a complex value at the antenna inputs to a purely imaginary one. A shunt-reactive component was then attached between the transmissionline ends. Lastly, a simple lumped-element circuit was added to each port for input impedance matching. See "A Decoupling Technique for Increasing the Isolation Between Two Strongly Coupled Antennas," IEEE Transactions On Antennas And Propagation, December 2008, p. 3650.