Improve the isolation of 90-deg hybrid branch-line coupler

[winglj]

Hi, everyone. I have a question about how to improve the isolation of the 90-degree hybrid (branch-line) coupler.

I found that a microstrip line realization of such coupler provides about 20 dB isolation. Is it any way to increase the isolation to 30~40 dB?

Thanks.

 

[kspalla]

What about strip line design?