The microstrip-DGS has many advantages however there are several disadvantages and one of these is a more difficult structure to design. As a general statement, edge coupled features are more problematic than stub, or stepped impedance features and this is sometimes due to the environment variables that affect on the even / odd modes of the coupled features and / or circuit fabrication related issues.
Usually microstrip-DGS circuits are suspended in a housing so the ground plane of the housing below the microstrip-DGS is a fixed distance and is a ground. In other words, the open areas of the ground plane on the microstrip-DGS, should be a fixed distance to the chassis ground. Some times a microstrip-DGS circuit will be part of a power amplifier where the power amp has a large heat sink attached to the circuit and in areas of the DGS the heat sink will have a recessed pocket which acts as the fixed distance to ground.
If the distance to the chassis ground is large, such as 5X the thickness of the microstrip-DGS, then the fixed distance has little impact. As the distance is made to be closer, then more variation can be caused due to the chassis distance.
If the distance to the chassis ground is large, such as 5X the thickness of the microstrip-DGS, then the fixed distance has little impact. As the distance is made to be closer, then more variation can be caused due to the chassis distance.