Sizing antenna feed trace?

[reagle]

Typically you would make a 50 ohm trace to feed an antenna, based on the distance to the ground plane and the trace width. That makes sense. But what about feed that's not over the ground plane? Many chip antennas specify no plane under, but still want a 50 Ohm feed. How does one size it?

[Kelbit]

As soon as you leave the ground plane you are no longer in a microstrip regime and the fields are no longer confined to the space between the trace and the ground plane, so your feedline will start radiating. As you are connecting to an antenna this is not necessarily undesired behaviour.

If it's for a chip antenna, usually the manufacturer will have a prebaked layout available in gerber or DXF format. Sometimes it's not available online but you can get it by emailing their applications engineering department.