Simple Microstrip with MoM

I'm trying to learn FEKO after having learned and used Momentum (Keysight ADS) and AXIEM (NI-AWR Microwave Office) for the last 10 years. I'm looking for a lot of dialogues that I'm used to finding in the other software, and I'm not sure where certain things are located in FEKO. I've attached a project that was derived from one of the examples (Microstrip_Filter_FEM) though I also recreated it from scratch and obtain the same results. The result should be very close to a length of 50 ohm line of some length. The S21 result I get from the simulation is in the right ball-park, but S11 is way off.

The relevant project details are:

epsilon_r = 2.33

tand = 0

trace width = 4.6 mm

trace length = 18.4 mm

substrate height = 1.57 mm

perfect electric conductor

I suspect I have not specified something correctly and I think it might have to do with how the regions are defined (or lack thereof). It should be a thin copper strip with above dimensions (0 thickness), over infinite ground plane.

Thank you in advance.

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Torben Voigt

Hi @BrianFEKO,

I think the model is set up correctly. And I also think the results are expected. A microstrip of these dimension should give an impedance of 50.75 Ohm. I think that slight changes of impedance with frequency are expected for microstrip lines:

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Best regards,

Torben

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Altair Forum User

Thanks for the reply. I'm still new to understanding the underlying requirements and capabilities of the various solution methods...i.e. I'm still not clear on when 'regions' are required for a MoM solution. Do regions only apply when considering finite substrate or ground planes? For such a simple conductor and substrate definition, does FEKO just 'default' to a planar MoM solution?

I'm still a little surprised that there is that much difference in the return loss over frequency compared to my other results. My designs are usually very narrow band from a primary function point of view, so perhaps I'm just seeing things a little differently. Are you plotting the characteristic impedance or input impedance in your rectangular trace?

Altair Forum User

Also, can you point me in the right direction for how/where to control de-embedding/calibration of the ports?

Torben Voigt

Hi @BrianFEKO,

A region is created when faces form a fully closed volume. As soon as a region is present you can define the properties inside (e.g. set medium to some dielectric). The solver (MoM as default) will then use the Surface Equivalence Principle (SEP) to represent the region. I.e. the faces do behave as if there'a volume, without actually havin to mesh the volume. A surface mesh is sufficient. Note, one can also use Volume Equivalence Principle (VEP) instead of SEP. In that case the volume is discretized in tetrahedras. If not using MoM, with FEM geometry is also meshed with tetrahedras, with FDTD the whole problem is discretized with voxels (cuboids).

In your model, the substrate is modelled with panar Greens functions instead of a volume. That's a very efficient method for printed structures, because the substrate won't require a mesh.

In my graph you se the real part of the input impedance of VoltageSource1.

De-embedding unfortunately isn't supported directly in FEKO. It's under development but unclear when it will be available.

Altair Forum User

Thanks for the feedback...I learned a little more over the weekend so I'm getting there.

When you say de-embedding isn't directly supported, you're also including port calibration for microstrip ports and the like, correct? I just want to make sure I understand correctly.

Torben Voigt

Hi @BrianFEKO,

Any kind of port calibration would have to be done manually, e.g. with the help of lua scripting in POSTFEKO (e.g. adjusting S11 with some calibration model). Unfortunately I'm not an expert on this.