I am having trouble interpreting the S-parameters in Circuit Designer and HFSS. I am looking at the S21 between a tuned loop antenna (for f=2.4GHz) and two dipoles (Front Dipole and Back Dipole). I have attached the reported values for 15 different HFSS design variations. I am using circuit designer to tune the antenna (the schematic image is included below). The S-parameters computed in HFSS seem more realistic, but the HFSS design alone does not have any of the matching components for the loop antenna so I am concerned these numbers are not accurate for the situation I am trying to simulate. I would anticipate the circuit S21 to be more accurate (and higher) since the loop antenna is tuned for 2.4GHz, but it is much lower.
How should I be interpreting these S-parameter values in Circuit Designer vs HFSS? What is the "correct" value to report if I am looking at the S21 between the tuned loop antenna and each dipole?