Due to the Non-TEM propagation of microstrip, the odd and even mode phase velocities will be different. This means both the coupling and isolation will degrad as the two phase velocities drift apart.
Thus the object of any compensation scheme is to equalise these velocities creating a "TEM" state.
Here are some ways of doing this:
Single section edge couplers tend to be narrow band due the different mode propagating through the coupled lines. There are many methods for equalising the modes. Additional bridge capacitors ( either lumped or interdigital), wriggle lines or interdigitated lines. however you will never get as good as a stripline.
