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Slotline Simulation in Ansys HFSS

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Daniel_G

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Dear all,

as I am having some trouble with my antenna design, I started splitting it up in small problems:

My very first problem is about designing a slot line and determining it's characteristic impedance Z_0.

My simulation is quite easy:
I have defined a slot line on a low dielectric (epsilon_r=3.5) substrate (h=251µm). The slot line width is varied between 0.05mm and 0.8mm.
I put an air box around the structure at lambda/4 distance with radiation boundary (which is actually not needed here - I just put it to be sure not to have problems with any reflections).
At each end of the slot line I put a waveport with integration line defined from one conductor to the other and Z_0 calculated by Z_pv. The port is defined on a rectangle as it has to be (Width = ~ 7g and height > 4h or 4g). I tried PEC port caps as well as having the air box terminated at the ports.

The simulation (ports only, 3.1...10.6 GHz with f_c=6.85GHz) converges and gives out some results which are way smaller than what I expect due to closed form equations in [Janaswamy, Schaubert - 1986].
I calculaed the characteristic impedance using eq. 9 for W=0.1...0.8mm (since it is not valid for 0.05mm) in the above paper and plotted the result comparing it to the simulated results:
Slotline Impedance - calculated.pngslotline impedance - simulated.png

Does anyone have a hint for me? Is there a problem with the simulation or does the equation just not hold for my case for some reason?

Best regards,
Daniel

- - - Updated - - -

The port is defined on a rectangle as it has to be (Width = ~ 7g and height > 4h or 4g).
Exact values: width is 10*g and hight is 5*h.

The simulation (ports only, 3.1...10.6 GHz with f_c=6.85GHz)
Started with ports only but the reported graphs above have been calculated completely (not ports only).


Sorry for reporting imprecisely ;-)
 

Daniel_G

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I think they look fine. I attached the Field display as well as the mag(E) at the port.
Furthermore I attached the mag(E) field on the slot line.

Slotline - E-Field.pngPort Field.pngPort Field - E-Field.png
 

PlanarMetamaterials

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That does look fairly correct. Does the formula you used take into account the thickness of the traces? They look pretty thick with respect to the width of the gap -- this can have a large effect on the solution.
 

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