Patch insert fed antenna S11 problems

Hello!
I have problems simulating a patch antenna, hopefully someone will help me. I'm working with HFSS!
I'm working on a patch antenna with microstrip feed. To match the antenna impedance I have to make an insert feed. I have calculated the insert length but to be sure I made an optimetrics analysis with different length. Length from 22 to 26 mm with 0,1mm step. The problem is: after I made a S11 parameter diagram a saw that the antenna resonate at different freq. and with different S11 for MINIMAL insert length difference (0,1mm).
Here are the pictures of my antenna and S11 diagram so you could understand what I'm talking about.
Does someone know what's the reason for such huge S11 difference for so small insert length difference? Is it a HFSS problem (I think it is)? If yes what am I doing wrong? Or what?

Thanks

look at the smithchart by doing that you change both the phase and impedance

pl

plasma said:

look at the smithchart by doing that you change both the phase and impedance

pl

Click to expand...

Are you saying this is correct? I made a sweep with 0.05mm of difference and S11 is still changing alot. How can I fabricate a patch antenna if such a small mistake can ruin the antenna?

I find your wave port lies on the radiating boundary , is that ok?I am not sure ,however.

Waveport is ok. The model should be ok.

The wave port takes precidence over the rad boundry so that should be OK.

Plot the surface current on the metalization and make sure it is not choppy, especially at the fed point. Increase mesh density if it is choppy (ie. red regions between two blues). That's a very thin substrate compared to your patch dimensions so it's not suprising the feed point impedance varies so drastically. If your substrate is thicker then you'll have a wider BW and easier match.