I am doing a lot of PCB trace antenna design lately, and I am seeing
another occurence of something that I had seen before, where a given
antenna design that should be workable, simply won't tune in a given
design.

Previously all I had to address this was renting some time on a VNA and
hacking a physical prototype.

Today I am working with XFdtd simulating the entire PCB, plus all the
surrounding materials. That makes the tuning process one of time on a GPU
instead of rental lab time and X-acto knives.

It seems that there is some other structure on the PCB that is resonating
in some way that interferes with the antenna.   I've done some
investigation with stitching vias and definitely things are a lot more
stable with those in place, but it's not affecting the main problem that I
am seeing.

I am wondering if others have hit this, and if so what they did to isolate
and get rid of the problem?
--=20
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