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[HFSS] Realized Gain greater than Gain. How is it possible?

gcaricato

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Hi all,
I'm simulating a printed meandered dipole working at ETSI UHF Rfid band in HFSS 2019 R1. I noticed a strange thing: for some frequencies I got a Realized Gain greater than Gain (both evaluated in dB and for same Theta and Phi). I'll show you some plots to be more clear.

This is S(1,1) plot.
S11_plot.PNG


This is Gain and Realized Gain VS Frequency plot at Theta=Phi=90°.
Gain VS Freq.PNG


This is Gain and Realized Gain VS Theta at Freq=867MHz and Phi=90°.
867MHz.PNG


Can someone explain how this is possible? I'll give you more infos if needed.
 

vfone

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Realized gain contain the mismatch loss and losses that affect antenna efficiency, thus, because these losses are always present, is practically impossible to get realized gain higher that gain (which doesn't include these losses).
Try to set in the simulation both, realized gain and gain, for Theta=Phi=0°, and also both measured in dB.
 

gcaricato

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Hi vfone, thanks for your reply. Of course, I totally agree with what you say...That's exactly why I created this discussion. If you have looked at the plots, it is reported Realized Gain and Gain (in dB) at Theta=Phi=90° (I don't think it changes much at Theta=Phi=0°).
 

PlanarMetamaterials

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Hi all,
I'm simulating a printed meandered dipole working at ETSI UHF Rfid band in HFSS 2019 R1. I noticed a strange thing: for some frequencies I got a Realized Gain greater than Gain (both evaluated in dB and for same Theta and Phi).


I'd suspect it's just a numerical/simulation issue. What is the port impedance in this case? (Is it complex?) Also, what kind of frequency sweep are you doing? (discrete?)
 

gcaricato

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Hi PlanarMetamaterials, thanks for reply. I am using a Lumped Port with impedance 23.3+i*145, this to obtain a conjugate impedance matching (it is an RFID antenna and chip's impedance is 23.3-i*145). Regarding simulation, plots refer to a Fast frequency sweep (results are the same with a Discrete sweep). I also tried to make the mesh more dense but results don't change. I'm using a Region (air box) with Radiation Boundary and a distance slightly greater than lambda/4.
 

PlanarMetamaterials

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In that case, I would speculate that the error here is due to the use of the complex port impedance. I'm not sure what I could recommend; are you re-normalizing the port impedance in post-processing?
 

gcaricato

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No, I'm not renormalizing the port impedance in post processing. The approach I am using is the most popular for simulating RFID antennas (at least, that's what I learned from documetations and scientific papers). Am I doing something wrong? What would you recommend?
 

PlanarMetamaterials

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I don't think you're doing anything wrong, but I highly suspect that HFSS doesn't know what to do with complex port impedances when computing realized gain. I think the best approach in this case is to compute the realized gain manually (subtract the reflected power from the gain).
 

gcaricato

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Considering the impedance of the chip Z_chip as a parallel (R_chip//C_chip), I would like to try to simulate the project by inserting in parallel to the input terminals of the antenna a Lumped Port of value R_chip and a Lumped RLC element of value C_chip. Do you think this could be a good idea?
Unfortunately, the datasheet of the chip only reports the value of the complex impedance Z_chip. So how can I correctly estimate R_chip and C_chip?
 

PlanarMetamaterials

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Considering the impedance of the chip Z_chip as a parallel (R_chip//C_chip), I would like to try to simulate the project by inserting in parallel to the input terminals of the antenna a Lumped Port of value R_chip and a Lumped RLC element of value C_chip. Do you think this could be a good idea?
Unfortunately, the datasheet of the chip only reports the value of the complex impedance Z_chip. So how can I correctly estimate R_chip and C_chip?

That's not a bad idea, it might do the trick! I'm not sure if you need the conjugate impedance or not, but assuming you just have Z = R//C, then typically one determines these values using the derivative of Z (with respect to frequency) as well as Z.
 

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