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Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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RF Tool, low frequency range

Posted Apr 19, 2012, 10:51 a.m. EDT RF & Microwave Engineering, Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 4.2a 3 Replies

Matthias Kofler
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Hi everybody,

I'm using Comsol 4.2a and currently I'm taking a closer look onto the RF Tool which one is provided by Comsol. To get familiar with the Tool I spend some time onto the "Branch Line Coupler" out of the Model Gallery (Model ID: 11727).
Concerning the mentioned model I have two questions
- How can I run simulations in low frequency range (i.e. from 10kHz and less up to higher frequencies) ?
- The provided model is based on the use of Perfect Electric Conductors (loss-free). I would like to know how I can run
simulations using lossy conductors (e.g. copper conductors)

Perhaps, also other Comsol users are interested in this issues. If nobody has experience with the mentioned model, it would also helpful to get the questions answered in general.

It would be of great help to get an answer onto that questions.

With best regards,
Matt
3 Replies Last Post Apr 20, 2012, 2:51 p.m. EDT
Dennis Brown
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Posted: 1 decade ago Apr 19, 2012, 2:52 p.m. EDT
For your question on analyzing lossy conductors normally you would use an impedance boundary condition and select the boundaries you want. However, because the trace that you want in the branch coupler model is considered to be an "interior" trace, as of ver42a you cannot do this yet. From their docs wrt IBC's:

"The Impedance Boundary Condition

is used at boundaries where the field is known to penetrate only a short distance outside the boundary. This penetration is approximated by a boundary condition to avoid the need to include another domain in the model. Although the equation is identical to the one in the low-reflecting boundary condition, it has a different interpretation. The material properties are for the domain outside the boundary and not inside, as for low-reflecting boundaries."


It may be in a later version of the program but who knows when this will happen.

Analyzing really low freqs with the RF module can be tricky. If it was me, I would just use the ACDC module since it may be a little easier setup. Comsol supoprt will tell you that both ACDC and RF use virtually the same equations but there are some details to deal with before being able to use both cases. I don't think there are examples online here. For the branch coupler model, analyzing it at 10kHz seems to be pointless as the circuit is really meant to be at microwave freqs.
For your question on analyzing lossy conductors normally you would use an impedance boundary condition and select the boundaries you want. However, because the trace that you want in the branch coupler model is considered to be an "interior" trace, as of ver42a you cannot do this yet. From their docs wrt IBC's: "The Impedance Boundary Condition is used at boundaries where the field is known to penetrate only a short distance outside the boundary. This penetration is approximated by a boundary condition to avoid the need to include another domain in the model. Although the equation is identical to the one in the low-reflecting boundary condition, it has a different interpretation. The material properties are for the domain outside the boundary and not inside, as for low-reflecting boundaries." It may be in a later version of the program but who knows when this will happen. Analyzing really low freqs with the RF module can be tricky. If it was me, I would just use the ACDC module since it may be a little easier setup. Comsol supoprt will tell you that both ACDC and RF use virtually the same equations but there are some details to deal with before being able to use both cases. I don't think there are examples online here. For the branch coupler model, analyzing it at 10kHz seems to be pointless as the circuit is really meant to be at microwave freqs.
Matthias Kofler
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Posted: 1 decade ago Apr 20, 2012, 2:14 p.m. EDT
Hi Dennis.

thx for your answer. I need to think about this.

I'm using the Brach Line Coupler example, because I'm interested to extract the s-parameters.

Regards,
Matthias
Hi Dennis. thx for your answer. I need to think about this. I'm using the Brach Line Coupler example, because I'm interested to extract the s-parameters. Regards, Matthias
Dennis Brown
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Posted: 1 decade ago Apr 20, 2012, 2:51 p.m. EDT
I am not saying that you cannot use the RF module at low freqs. There is a node called "Gauge Fixing" (right click wave equation, electric) that may help with numerical stability. I myself have not used this a whole lot.

From the documentation:

"It is used for numerical stabilization when the frequency is low enough for the total electric
current density related term in the wave equation to become numerically insignificant."

You can definitely extract s-parameters using either module. It is just a matter of determining how best to approach the solution. Maybe you do both to verify that you are getting the same results. Getting all of this setup in comsol can be complicated for a new user.

I was only suggestion that at really low freqs, a branch coupler does not work as advertised. Of course, one could measure this same coupler with a real network analyzer at low freqs and determine this as well.
I am not saying that you cannot use the RF module at low freqs. There is a node called "Gauge Fixing" (right click wave equation, electric) that may help with numerical stability. I myself have not used this a whole lot. From the documentation: "It is used for numerical stabilization when the frequency is low enough for the total electric current density related term in the wave equation to become numerically insignificant." You can definitely extract s-parameters using either module. It is just a matter of determining how best to approach the solution. Maybe you do both to verify that you are getting the same results. Getting all of this setup in comsol can be complicated for a new user. I was only suggestion that at really low freqs, a branch coupler does not work as advertised. Of course, one could measure this same coupler with a real network analyzer at low freqs and determine this as well.

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