Following up again to myself, and to the helpful comments from @Rene_Poschl and @pedro.
I have attached a copy of the footprint file that I am currently using. As I said, it appears to work, but it may not be ideal.C_0402_1005Metric_with_2.4GHz_antenna.kicad_mod (8.2 KB)
Here are some screen shots from the design and testing process.
First, I show how I started with a two-pad 0402 capacitor footprint. I added a duplicate pad numbered 1 and a polygon defining the antenna shape. I then created an irregular pad from the duplicate pad 1 and the polygon. The polygon touches the edge of the original pad 1 exactly but does not overlap it.
As guided by the post I found and linked above, I clicked on the irregular pad and unchecked the “F.Mask” layer in the Technical Layers section on the right, leaving only “F.Paste”. The technical layers for the other pad 1, where the capacitor will be soldered, were not changed.
When I imported this footprint into a layout, the copper layers looked fine by eye. But KiCAD does not recognize the two pad 1’s as connected, even though there is zero distance between them.
I was afraid to overlap the pads when I was making the footprint. Maybe that is permitted. I did not investigate. Instead I connected the two pad 1 anchors with a trace that you cannot see, but which satisfies KiCAD.
The 3D render looks a bit strange. KiCAD can’t seem to decide whether the antenna is above or below the solder mask.
However, when I uploaded the layout to our PCB manufacturer’s web site and looked at their stack-up image, it looks like the antenna is protected by the solder mask, as I want it to be:
Comments and suggestions for improvement are appreciated. Thanks!