If you have multiple dielectric layers in the model, they must touch other dielectric layers or negative signal layers. This prevents “pockets” of air from developing. If you have a signal layer between two dielectric layers, the dielectric layers must touch. The signal layer may overlap the two dielectric layers; however, a signal layer may not touch a negative signal layer. You may create an air gap by defining a dielectric layer as air.
Note:
HFSS allows you to draw on all three primitive layer types: dielectric, signal, and negative signal. Objects on dielectric layers is ignored in Planar EM modeling, but they are displayed in Layout3D and they are exported.