Discussion Forum

Lucie,

you could start with AC/DC because even in the tissue the wavelength is probably more than 10 times the dimensions of your apparatus.
Of course you can model a capacitor in RF. The electrodes ARE the capacitor. You will need to couple the AC model to heat transfer.

Cheers
Edgar

Dear Edgar,
thank you very much for your answer. But if I model electrodes in RF, how can I set up which one is ground? It is important isn´t it? Excuse me, I am amateur..

You can model this in either RF or AC/DC. Have you looked at the example file here:

www.comsol.com/showroom/gallery/30/


In RF module, there is no such thing as a ground. Most likely you will probably need to surround your object with an air box. It really becomes a return path. Technically ground does not exist in its strictest sense at higher freqs. What you will have to do is setup port boundary conditions in order to get signal to your structure. This may or may not be easy to do but it depends on your structure. In AC/DC, I believe there is a way to declare a ground but I typically do not use that physics a lot so somebody else may offer better help.

I have modeled capacitors using the RF module so it is possible. I would not post what I have done because they are proprietary but if you have something generic enough that will help you do what you want to do, then post your file and ask questions. Sometimes just seeing a generic model will demonstrate enough that you can apply it to what you want to do.

If you want to model heating as well, you can use AC/DC or RF module along with the heating module or microwaving heating or bioheating (I am not familiar with this one). Microwave heating makes it easier because everything is in one physics.

Dear Dennis
You are very helpful, thank you for it!! Problem of microwave cancer therapy I have studied, but mine is somewhat different. This is one microwave anthenna, bud i need to model two metal electrodes and tissue between. This is my geometry with some solve..
My model surround with an air box (PML)

mysharegadget.com/254275124

I know that this is wrong solve.. Because in this I have two ports and bring the wave from both side of tissue (both electrodes have port with wave excitacion). It does not behave like a capacitor. I think, that I have to bring a wave only on one electrode (+) then the wave go through tissue to a second electrode (-). For a better idea I enclose some pictures.

In material I entered permittivity and electric conductivity for 27.12MHz, in biological tissue this values ale frequency-dependent. I this model I have open end of the electrode. When they were closed, there are reflections (maybe??) Maybe is my solution and my ideas totally wrong :-(. I will be pleased with any idea..
Thank you VERY MUCH for your willingness.
Lucie

I was not able to download your file at the site you posted it at. It would be easier if you just deleted the solution(s) from your file and then uploaded it here. If you do not know how to do this you can just go to the edit menu and select clear all solutions. You may have to clear all meshes as well. That should make your file smaller. This also assumes you are using ver4.2. Earlier versions required a more manual way of deleting solutions.

Overall my suggestion to you is to get the RF part working correctly before trying the heating part. So use emw before using heat analysis. This way you can check impedance parameters and s-parameters and see if those are correct before spending a lot of time debugging a heat problem.

Dennis,
thanks for your time! I thing, that I have some solution. I used electric current with frequency solver and than the bioheat. I thing that results are acceptable :-)..