directional coupler 20ghz

Hi,
I am curious about the design of broad directional coupler, especially the kind of instrument class directional couplers such that are used in VNA. No matter it is bridge based or transmision line based design, the extending coupling factor to a almost constant value from 100 kHz to 20 GHz is amazing to me.

I know Agilent/HP has some sepcial design based on bridge concept. One of the latest patent appending claims to extend the circuit to 40 GHz. Can someone out there give some input or share his/her collection regarding this topic (circuits, tips, pat. no. publications ...). So we can fins out how a home made stuff performs in this category?


Thanks for any contribution!


Regards


laipi