I am simulating some lambda/2 series CPW resonators, and have been getting some disagreement between my results when simulating by Wave Port or Lumped Terminal. They both show resonance at roughly the expected frequency, but disagree on order of a few hundred MHz. I am inclined to believe the wave port results as they are close to ADS Momentum - Slot mode (differ by ~ 40 MHz), though was hoping to get some insight from anyone that has done this before. This is with the same capacitive coupling in all the simulations. Is it an aspect of the lumped terminal that is causing a frequency shift that I am not accounting for? I have passes set to 22 (most my machine can handle before running out of RAM), and (I think) I have set resolution of the mesh to a fine enough level to capture the geometries correctly.

As a bit of a side question, would anyone know off the top of their head a good source to look at for determining box modes in HFSS?