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[SOLVED] Lumped port for matching antennas

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white4

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Good evening.

I am simulating in HFSS a short dipole, as well as a small loop antenna. The purpose is calculating the coupling between two short dipoles or two short loops. To match these antennas I use as excitation a lumped port with the same resistance and opposite reactance than the measured Zin, however, when I use this lumped port whith different impedance than 50Ohms the simulation returns S parammeters larger than 0dB... Which could be the problem?

Thank you very much.
 

It sounds like HFSS is still normalizing to 50 ohms, even though the characteristic impedance is no longer 50. When you first make your lumped ports, you have an option to check a box "normalize all modes" or something to that effect. Maybe try unchecking that and normalizing on your own? I would think that HFSS would normalize to whatever impedance you select, but I'm not sure.
 

I didn't check port renormalization. Furthermore, when I simulate that with a 50ohms port the simulation is fast and returns reliable results, but with the new impedance it is longer and strange S larger than 0dB. The port impedance is 5+j1000 Ohms to match the short dipole, 3m long at 10MHz. I attach the structure I someone wanna check that.
https://www.dropbox.com/s/64k8lzuldmxxzvf/shortDipole.hfss?dl=0
 

Another question... If I just wanna know the coupled power through S21 params, and i do not care the radiation pattern, can I set a small air box to make simulation faster?

Thank you.
 

Yeah, under Excitations --> Lumped Port1 --> Post Processing --> You can check Renormalize all modes and put in the input impedance you entered, (5+j*1000) Ohms. That should give you some S-parameters that make sense.

No I don't recommend shrinking the air box, the S parameters are calculated from the fields. If you make the fields and the radiation pattern less accurate then the S parameters will be less accurate as well.
 

If I renormalize to this impedance instead of seting it as lumped port impedance it doesn't converge... Perhaps HFSS cannot simulate in the right way this complex port impedances. I gonna try a serie RLC in the small loop to remove the reactance, can I use a RLC 3d boundary? Any suggestion to set a capacitor/inductor serie in a small loop in HFSS?

Thank you.
 

Huh, that's weird, wouldn't have expected any convergence issues. You should be able to use RLC boundaries to make your circuit, I've always just used 2D planes to make the boundaries. You can add one 2D rectangle between your lumped port and conductor to represent a series reactance and set that to the appropriate value.
 

Yeah, that's what I have done and it returns reliable results and good coupling. 3D RLC models work too, but simulation time is longer.
 

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