Change the surface property of the cuboid to perfect electric conductor
(PEC).
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In the model tree, select
Cuboid1
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In the 3D view, select the top face1 and front face2.
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From the right-click context menu of the selected faces, select
Properties.
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On the Modify Face dialog
(Properties tab), set the
Medium to Perfect electric
conductor.
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Click OK to change the face property and to close
the dialog.