coupling space in strip line coupler

Hi
Do you know what should be the range of coupling space(min and max) between two strip lines
in a coupler to work well.
for example if the space is greater than .....the structure is not a coupler
thanks

It is not easy to give any rule of thumb on lower bound as it is related to coupling coefficient. However, for the maximum, I can tell the coupling becomes very small for spacings greater than overall height (i.e. the distance between two outer shields) of the striplines.

Hi

As a rule of thumb, i read that between two strips of same width (say D) the coupling become insignificant if the space between the two strips is more than 2D.
For example, when you make a filter base on open loops resonators made of microstrip, the space between the resonnators is less than approximatively 2 x resonnator width. If you could, simulate it.