Hi Eric Chen,
Thank you for reaching out to the Ansys learning forum.
Please note that the polyline option can be used to define a single object by combining straight, spline and arc line segments. You can use "set edge type" from the shortcut menu while drawing. In your case, it looks like straight edge is selected.
More details can be found in the HFSS help document under section "Drawing a Polyline".
Best regards,
Praneeth.
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