Hi,
I have plotted antenna input impedance on a smith chart vs frequency. Now, I wanted to add a VSWR = 2 circle on my plot in HFSS. Can anyone please let me know how I can do this in HFSS?
I know how to plot VSWR on a rectangular plot. Just can not figure out how this can be done on the smith chart in HFSS?
Thanks in advance!
There is a pdf file illustrate axis and uses of smith chart: smith_chart_basics
Hi Eanass Al-Shabkhoon , Thank you for your response!
This file is very useful in understanding the smith chart basics and how to use it manually. I just want to do it in HFSS. When tuning the antenna I wanted to see if the normalized antenna input impedance (Z11) is inside the VSWR circle over my band of interest. I have plotted the Z11 using the builtin smith chart feature of HFSS, but I don't know how to add a VSWR circle on my plot along with Z11 curve.
On a Smith chart, take centre as one point and r=2 circle cuts left side of horizontal line is another point. Keeping first point as origin and distance between two points is radius, draw a circle. This is a the vswr circle you required
In deed, impeadance is a complex number that can be plotted in polar coordinate as a magnitude and a phase that plot can be crossed with the VSWR and reflection coefficient there for be careful for the meaning of cross points and return to a reference, good luck.
I just got stuck at that problem. It is possible in Schematics, but not implemented in HFSS.So there is a workaround if you have a sweep. But the sweep should be very detailed.
Put there a formula plot:
rect(.13,(360 deg)*mag((Zo(port#)/Zo(port#))*((Freq-Fstart)/(Fstop-Fstart))))
Zo/Zo is important, or the plot will ignore the frequency sweep. Stupid, huh? The denominator just normalizes the quantity.
.13 is VSWR1.3
Actually, HFSS smith chart is almost the worst I ever seen. Only Excel is worse.
- It shows impedance, but accepts only reflection coeff Г as input, which is insane!
It shows Znorm="1" where the input value should be Г="0".
If not Z, you normally expect the Smith plot to show S parameters, where the angle is wound around the plot... but..
- It draws straight lines between impedance points, instead of drawing curved lines of linearly interpolated impedances between points.
And it can not plot VSWR circles, the most important parameter in passive RF and antenna design.
...who made that? And why does it stay in the same state for decades?
Hi Andrey Porokhnyuk ,
Thank you very much for your response and for sharing the formula. I really appreciate how nicely you have explained the details and the math involved. I have used the sweep formula as you had suggested and it worked perfectly for me.
I recently started working on this feature, and now I completely agree with you on your comments about this smith chart feature in HFSS, it's the worst.
I have been trying to display normalized input impedance of the antenna (Z(1,1)) and it's not working. In addition to this, I have also tried to plot VSWR indirectly (using both S11 and Reflection coefficient (Gemma)). But I'm it displays weird plots.
I wanted to observe how the normalized impedance of the antenna varies with different parameters. But, I'm not sure if the resulting plot is trustable/correct.
I 'm using the expression ((Z(1,1))/Zo(1)) but the results are way outside the VSWR circle (which is very strange given VSWR is less than 1 already). Been trying to plot this for over half an hour, but still no success.
It will be great if you could kindly comment on this!
Again thanks a lot for taking out the time to write a response!
1) Z11 is not an input impedance. But with a high load impedance of the free space, and low characteristic impedance of the feed, it converges to the input impedance. For Z-matrix explanation see Collin(Pozar, etc.) book.
2)You should feed the reflection coefficient to HFSS, which will plot the impedance/admittance/S Smith chart. Not very obvious, huh.
Gamma is a propagation constant (γ), not a reflection coeff. (Г). That's why you had no luck with it. See the manual. You should plot Г =(Zin-Zo(1))/(Zin+Zo(1))
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