how to icrease the directionality of wideband coupler?
directionality is about 20dB.
now a customer has a need for diretinality of 30dB,how to realize?
Directivity [dB] = Isolation [dB] – Coupling [dB]
So, to increase the directivity, in your simulation you have to increase Isolation or to decrease Coupling, keeping the other one constant.
Some theory about directional couplers:
http://www.minicircuits.com/appnote/coup7-2.pdf
http://www.bird-electronic.com/app_n...irectivity.pdf
Thats a big bandwidth for 2 sections. Try more sections. You might get better directivity with some lumped elements between sections. Really investigate the matches at your connector launches, and especially the internal 50 ohm load.
If you want to have good directivity, you must execute good matching for all loads in your design.
Also if you increase the intimate directivity of the directional coupler, the reflections due to connectors and launcher, and in general the loads will became dominant.
20 dB is a good value for common (industry) coax connectors.
30 db and more require precision connectors and very low VSWR loads.
In other worlds, directivity is not a performance dominated by the Coupler alone, but depend on the external match also.