If you have a patch antenna with finite ground plane. How to setup the airbox correct?
I have used minimum \[\lambda\]/4 as a distance to my antenna structure.
But lets say you have an antenna with a very small ground-plane and you still need the \[\lambda\]/4 as minimum distance. Then there is a gap between the antenna and radiation box.
Should this be filled with a radiation plane?
"But lets say you have an antenna with a very small ground-plane and you still need the /4 as minimum distance. Then there is a gap between the antenna and radiation box. "
I don't see how you make a gap here. If you antenna is smaller, then your airbox/rad boundary) is smaller too -> less memory needed
you do not really need a space between the bottom of your ground plane and the bottom face of the radiation box -
on the edges of the ground plane, you should definitely put the lambda/4 distance to the side faces of your radiation boundary. Well, not up to lambda / 4. For your patch antenna there might be radiation to the sides of the ground plane. Kinda like a parachute shape.
You have several options and can use either radiation boundaries or a PML. Both have advantages.
The latest guidance from Ansoft is to use a PML and the set up differs slightly when the object touches the boundary vs. when it does not. Read the on-line help sections and if you need additional help call one of their application engineers.