Difference in Resonant Frequency Measured in S11 and Imped Plots

Hi all to the experts out there

I have simulated a half wavelength dipole using HFSS and have plotted the s11 and the impedance plots. However, I do not understand why the resonant frequency that I have measured from the two plots differ in value. My solution frequency is set to 30MHz. The resonant frequency from the s11 plot is around 28.1 MHz (point of minima) whereas the resonant frequency from the impedance plot is around 30.1 MHz (measured at point where Im impedance is zero). Please kindly enlighten a newbie like me.

Thank you very much for your time. (:

can you post your results? s-parameters are determined with a source impedance of 50 Ohm. So your resonant plot (s-par) shows a matched antenna to 50 Ohm.

Hi johnjoe

Thanks for your reply!
I have attached my HFSS project file. I have matched my impedance with around 71.4 ohms, but I still receive differing resonant frequency values from the s11 (29.8 MHz) and impedance plots (30.1 MHz). I think that the values are already very close to my solution frequency of 29.7748 MHz, however due to my shallow knowledge on antennas, it still is baffling to me as to why they are different. Please kindly enlighten me!

Thank you very much for your time! (:

In default, the minimum on the S11 plot indicates the best matching point with respect to 50 ohms. Note that S11=20*log10|(Z-50)/(Z+50)|, so S11 is a function of both the imaginary part and the real part of the antenna impedance. On the other hand, the resonance freq determined according to imag(Z)=0 has nothing to do with real(Z). Therefore it is reasonable to see some discrepancy between the resonance frequency found on the S11 plot and that determined based on imag(Z)=0.

Hi streamlet

Thank you very much for your explanation!
I've learnt something new.