microstrip directional coupler

Hello,

when analyzing coupler circuits, like the rat-race-coupler, the isolated port is isolated due to destructive interference of two waves.

Is there a simple argument for the isolated port of a microstrip directional coupler, consisting of two coupled strips.

---------------------------(isolated) strip2
-------------------------------------- strip1


thanks

May be it is only possible to explain it by using L-C-matrices.
Then the coupling coefficent k is given by Lself + Lmutual and Cself + Cmutual....

simple arguments ?