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Basic High Frequency S-Parameters
Posted Dec 6, 2015, 9:45 p.m. EST RF & Microwave Engineering Version 5.2 5 Replies
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This is my first time using COMSOL so this may be too basic of a question. I just downloaded COMSOL 5.2 and ran a few basic tests to make sure I was setting up the S-Parameters correctly. I chose the electromagnetic frequency domain and subsequent frequency analysis for my 3D model. First I made a 300nm x 300 nm block of air, with user defined e, u, and o of 1, 1, and 0 respectively. I set the PMC sides to be in the y direction, the PEC sides to be in the x direction, and ports to be in the z direction. I turned excitation on for port 1, the bottom, power to 1 Watt and an initial E-field in the x direction. I also set the same E-field with no excitation for port 2. After defining my frequency analysis range to be from 100 THz to 200 THz I ran my study. I ran an identical test with a box that was half n=1 half n=3. Both tests yielded the same result; S11 at 0dB throughout, S21 hovering around -260dB throughout.
This is not what I would expect. My question is, am I forgetting to set something rather basic? Is there something fundamentally wrong with my test?
This is not what I would expect. My question is, am I forgetting to set something rather basic? Is there something fundamentally wrong with my test?
5 Replies Last Post Dec 16, 2015, 10:21 a.m. EST
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Posted:
9 years ago
I suggest you upload your model, so that others here may take a closer look. Your description is confusing to me. Are you setting your E-vector perpendicular or parallel to your PEC walls? If the latter, then your mode is being set below the cutoff frequency.
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Posted:
9 years ago
Thank you for the response,
So I changed the size of the test model to 300mm by 300mm and adjusted the frequency range to 500MHz to 1000MHz. Set the electric field imposed by the ports in the x direction, perpendicular to the PEC sides. I switch the refractive index for one of the two layers between 1 and 3, keeping the other at one. If I'm not mistaken there should be a reduction in the S11 parameter of about 25% when I switch from 1 to 3 but In fact there is not change instead the frequency analysis explodes around 700MHz. I have uploaded my project, I'm sure I am making an obvious error this is my first time working with COMSOL.
So I changed the size of the test model to 300mm by 300mm and adjusted the frequency range to 500MHz to 1000MHz. Set the electric field imposed by the ports in the x direction, perpendicular to the PEC sides. I switch the refractive index for one of the two layers between 1 and 3, keeping the other at one. If I'm not mistaken there should be a reduction in the S11 parameter of about 25% when I switch from 1 to 3 but In fact there is not change instead the frequency analysis explodes around 700MHz. I have uploaded my project, I'm sure I am making an obvious error this is my first time working with COMSOL.
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Posted:
9 years ago
1. Your mesh is far too coarse.
2. In Settings, Port, you need to specify the propagation constant. For free space, use 2.0*pi*freq/c_const
2. In Settings, Port, you need to specify the propagation constant. For free space, use 2.0*pi*freq/c_const
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Posted:
9 years ago
1) I don't know what I should change to make the mesh less course. I set the physics-controlled mesh on electromagnet Wave to 3e-1m because that is the largest element in the model
2)I set the propagation constant to 2.0*pi*freq/c_const but should I set the freq variable in parameters? I decided to do that and set the freq and tried a variety of values from 500-700MHz always setting the frequency domain to [freq-50, freq+50].
After the second change when analyzing the S-parameters I found that the distance between S11 and S21 did decrease when I increased the refractive index but it I would have estimating it would decrease by 25%, it decreased by about 90+%. Also the S-parameters tend to bend together as frequency increases, I would have that they would remain parallel.
2)I set the propagation constant to 2.0*pi*freq/c_const but should I set the freq variable in parameters? I decided to do that and set the freq and tried a variety of values from 500-700MHz always setting the frequency domain to [freq-50, freq+50].
After the second change when analyzing the S-parameters I found that the distance between S11 and S21 did decrease when I increased the refractive index but it I would have estimating it would decrease by 25%, it decreased by about 90+%. Also the S-parameters tend to bend together as frequency increases, I would have that they would remain parallel.
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Posted:
9 years ago
OK, I've attached a revised version, which solves ok. Use a user-defined mesh.
"freq" is the internal name for the frequency variable. It is set by Comsol when you say you want to do a frequency sweep, so all you needed to do was refer to it.
Meanwhile, I encourage you to practice your skills using Comsol-supplied examples files.
"freq" is the internal name for the frequency variable. It is set by Comsol when you say you want to do a frequency sweep, so all you needed to do was refer to it.
Meanwhile, I encourage you to practice your skills using Comsol-supplied examples files.
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