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# [SOLVED]Same geometry but slightly different results

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#### waqsoh

##### Newbie level 5
i am doing HFSS simulations of simple microstrip. I designed two microstrips having same dimesnisons and everything, as shown in Fig 1. However when i see s-paramters for both microstrips, they are slightly different, as shown in fig 2. I dont understand why this happens. i tried denser mesh, but it still gives same result.

#### Attachments

• twomicorstrips.PNG
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• twomicorsparam.PNG
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#### vfone

Yes, is about meshing. The program choose a slightly different meshing for one of the structures (compared to the other one), and this gives slightly different results.

#### BigBoss

It might be dependent also on Boundary Conditions.One boundary may be closer to one Microstrip or else..
HFSS uses Boundaries in EM Simulations that should be taken into account.

#### vfone

Could be the Boundary also, but if use: Draw/Region/Padding Type/Absolute Offset/Value, then the boundary will be at the same distance (which is "value") from any part of the structure.

#### waqsoh

##### Newbie level 5
It might be dependent also on Boundary Conditions.One boundary may be closer to one Microstrip or else..
HFSS uses Boundaries in EM Simulations that should be taken into account.
I have checked, all the boundary conditions are same. There is a question relating to boundaries. I have a microstrip enclosed in a vaccum box, with radiating boundary condition.
If i include a second microstrip enclosed in a separate vaccum box, with radiating obundary condition. Will these two microstrips have effect on each other, in terms of s-parameters.
In other words, having two designs in same hfss project and then having two designs in separate projects. Do these both schemes differ in terms of s-parameters.

#### vfone

Do these both schemes differ in terms of s-parameters.
Run the simulation and see the results. Hard to guess something like this.

Did you try using a Region with absolute offset instead of a Box, for the boundary?

#### waqsoh

##### Newbie level 5
Run the simulation and see the results. Hard to guess something like this.

Did you try using a Region with absolute offset instead of a Box, for the boundary?
I used region with absolute offset as shown in fig 1. S-parameters are shown in fig 2. There is still difference but it is small compared to previous case and at 6 GHz, sparams are almost same.
For comparison fig 3 shows the case when the microstrips are in separate boxes and the distance of each side from the box is same as the absolute offset case. Fig 4 shows the s-params for this case. In this case the s-params dont converge to same value.

#### Attachments

• fig 1-design.PNG
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• fig 2-1-sparam.PNG
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• fig 3-design.PNG
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• fig 4-sparam.PNG
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#### BigBoss

Even the Microstrip Lines in separate Boxes Ports are Coupled in anyhow.Therefore you can not consider that "I separated and that's all" because Ports are still in interactions.This is RF propagation.Ideal curves are only and only obtained in case of infinite distance between the structures.