Keysight ADS - patch antenna input impedance (possible incorrect EM setup)
Here, a single patch was designed, accounting for effective dielectric constant, height of material, thickness, loss using LineCalc and Rogers' own tool. The source for the patch was a 100Ω terminated port. It was simulated in Momentum and was matched perfectly at the frequency of interest.
Now, I know that adding a source transmission line of any length and width matching the load to terminated load does not change the match. Without modifying any settings or models, I add transmission line of length to my antenna but it changes my resonance frequency and match significantly. Below are results for adding quarter wavelength 100Ω and full (effective) wavelength 100Ω lines. My resonant frequency changes significantly, up or down 100 MHz or more and match to the load changes too. First image is quarter wavelength, second is full wavelength, third is EM model of my antenna. I've also tried changing length of the line to a custom value and it affected results in the same was as described below.
I've looked through all textbooks I have and it doesn't appear length of terminated feed line should affect resonance but it does in my case. Am I missing something on transmission line fundamentals? Is my antenna not matched to air line? If neither of those, I'll need to call Keysight on Monday as there might be a problem with my simulation setup.
I would double check the patch input impedance, to make sure the good match is with 100 Ohm port (not the default 50 Ohm). Also, I would double check the port impedance for the model with line.
Resonance: There is certainly a little influence when extending the feed line, if the initial model had some residual coupling between EM port and path. But it shouldn't move you resonance too much. How much shift do you see?
Can you show your Momentum mesh? Coarse mesh that changes a lot when changing geometry details might explain your issue.
I placed a 100Ω line and terminated it with 100Ω sources on both sides and impedance changes significantly as I change the length. I checked the equations used for transmission line and my values are well within boundaries. This is perplexing, I'm going to contact Keysight next.