stripline coupler - directivity

Hello,
directivity (coupling minus isolation in dB) of stripline couplers is related to equal phase velocities of the even and odd mode.
Normally, the odd mode travels faster than the even mode.
Can anybody imagine, why couplers with an even mode faster than the odd mode do not yield high directivity, if a comparable design with vodd > veven does.

Thanks.
-e

I try to guess:
Directivity is one port energy compared to the input port, So the energy is the same for both situations.

Hello tony_lth,

in theory, it is just the phase velocity difference that affects directivity.
Doesnt matter which of both modes is faster.

The problems with my design were based on our extraction method.

So thank you very much, we could already eliminate the error.

-e