z parameters

In one model ,i set a lumped port on one side of a microstrip line which was connected to a monopole as a feed line.I set the resistence of the lumped port to be 50ohms and the reactance 0 ohms, after simulation i chected the matrix dada in solution data of "HFSS" option,i chose the Z parameters and i found the Z:lumpport1 value at 8.25GHz is 32.15ohms(magnitude) ,8.65GHz is 50.30ohms.In the simulation ,the sweep range i set is from 7.6 to 9 GHz,i observe the S11 plot and a resonance at 8.25GHz with a minimum s11 value of -8.5dB appears, then i have a doubt :why the resonance is not at 8.65GHz as its Z:lumpport1 value is more close to the preset resistence of the lumped port (50ohms) comparing to 8.25GHz ? S11=20log[(Z:lumpport1-Z0)/(Z:lumpport1+Z0)],is not it?

Any comment and help will be appreciated!Thank you !

Best regards!

resonance will appear where the input impedance of monopole matches the transmission which may not be necessarily 50 ohms that is why perhaps u got resonance at different place

Hi
The answer of HFSS is complete true. Because of resonances is related to minimum value of s11. your dimention of antenna and port imput impedance not correctly set. you cannot say that resonance frequency is relatived with impedance. I think.

Here antenna is a resonating strucure. Resonace depends on this only. S11 value will depend on the impedence at resonance frequency and microstrip line impedence.
If you make ur microstrip line impdence 30 then it will show better result or if you make 100 it will show worse result. But resonance frequency will not change.

Hi:

I think resonance frequency of a cirucit is changing with the excitation and load. If you use a different feed line to feed a structure, the resonant frequency will change. How much will it change? It depends upon how different the the source or load you put on it. Basically, a structure will have an intrinsic resonance and it is independent of the source and load. However, when you connect a source and a load to it and try to observe the the resonance, you are basically changing its resonance condition and the resonant frequency will change. A simple example is the following:

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L C
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R |
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Without connecting any source, load or monitoring circuit to the 2 terminal, you can find its resonant frequency easily and it will not change. Howver, when you put some monitoring circuit between the 2 terminals, it will change the resonant frequency of the structure because the monitoring circuit because part of the resonant structure.

For microwave applications, you put a different TLN of different Zc, you are basically connecting different source and load to the circuit. Its resonant frequency will change depending upon the Zc of the TLN line.

Regards.