Following are some useful links that might be helpful for you:
1. The functionality introduced in R2020a to create custom 3D antenna object using STL file - https://www.mathworks.com/help/antenna/ref/customantennastl.html
2. Antenna geometry created from scratch to use with PDE toolbox - https://www.mathworks.com/help/antenna/ug/rfid-antenna-design.html
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