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Q on defining lumped port value on HFSS RFID tag design

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Newbie level 3
Mar 6, 2009
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I am using HFSS to design a passive RFID Tag Antenna.
I'm using a microchip with impedance 16-j380 and, therefore,I have to design a RFID tag with 16+j380 to get the impedance matching with the microship. When I define a lumped port in my design, I have to enter the resistance and reactance value of the lumped port. My question is that Should I enter the impedance value of the microchip (16 and -380) or I should enter the value that I desired (16 and +380)?

I tried using both the values to obtain the simulation results. I get positive S11 value, which is impossible for an passive RFID tag, if I use 16 and -380. While using 16 and +380, I got negative S11 value. Both simulation results get the same impedance matching values.


I am having a similar problem with the one you described above.

Did you manage to find out the proper way to set the port impedance?

Best regards.

Glad to hear from you. Although it should not be happy facing the difficulties. But it proves that it may not be my own problem.

I did check all the settings with some of my university's research students and it seems that my setting have no problem. As what I need is to do the impedance matching, I just ignore the S11 value at this moment. I successfully match the impedance and I'm planning to fabricate my design in late December. I will try to do some measurement to prove whether the S11 obtained from HFSS is correct.

Another way I can think of is that try to simulate the same design using IE3D to see whether the same result would be obtained.

I would try to avoid complex impedances for the lumped port. Instead, define it as Z_port=Re(Z_chip) and then put an impedance sheet with the admittance (caution: the value is to be set in with respect on the width and length!) or a lumped RLC element (caution: then the admittance applied is frequency dependent). This way, the incident power should be correct 1W (if the meshing is sufficiently fine) and you can optimize the conjugate-matching from the antenna by just looking at the smith chart and trying to optimize onto Z_port.

Dear komplz
Could you please let me know what happen with your design? I mean in HFSS simulation you ignored the S11, and you focus on Z to get the matching condition and when you fabricate the design what happen.
Best Regards

use the negative value because it's the value of the ASIC that you should to define in the Design to cancel the positive imaginary part in the impedance of the antenna :smile:

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