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Port boundary condition: what's relation between "port input power" and "E field amplitude"?
Posted 14.04.2015, 08:28 GMT-4 RF & Microwave Engineering Version 4.4 6 Replies
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The port boundary condition is quite useful for exciting the model with an EM wave. However, the description of the port boundary condition's "Port Mode Settings - User Defined" in "RF Module: User's Guide" is quite unclear:
"The mode field can be entered with an arbitrary amplitude and is normalized internally."
Which means, no matter what value I use in the "Port Mode Settings - Input quantity (e.g. Magnetic field) - Magnetic mode field H0", COMSOL will only adopt the direction of the H0 value but normalize its amplitude with other parameters (am I right?). But what's the detail of the normalization?
Fortunately, I found a blog discussing it a little bit:
srdjancomsol.weebly.com/port-boundary-condition-2d.html
srdjancomsol.weebly.com/setting-excitation-in-3d.html
According to the author, for a 3D case, we will have
P=0.5*n*IEI^2*S*cos(θ)/η0
where P is the port input power, |E| is the amplitude of the electric field, S is the area of the port, η0=sqrt(μ0/ε0), and according to my guess, n may be the refractive index, and θ may be the port phase θin.
However, still, the relation between the Pin and E field amplitude is not clear. I realize a very simple 3D example to explore this relation: there is only one block, the top is a port, and 4 walls are perfect electric conductor and perfect magnetic conductor as we know the electric field will be perpendicular to two walls and the magnetic field will be perpendicular to other two walls. By changing the area of the port, I get different |E|, but the problem is, I can't obtain the relation P=0.5*n*IEI^2*S*cos(θ)/η0:
With Pin=1[W], θin=0[rad], H0=(0 1000 0),
width(=depth) [um] Port area [um^2] E [V/m]
0.1 0.01 8.0e6
0.2 0.04 2.0e6
0.3 0.09 1.1e6
0.4 0.16 8.5e5
0.5 0.25 6.7e5
0.6 0.36 5.4e5
0.7 0.49 4.4e5
Can anyone help me? Thank you!
"The mode field can be entered with an arbitrary amplitude and is normalized internally."
Which means, no matter what value I use in the "Port Mode Settings - Input quantity (e.g. Magnetic field) - Magnetic mode field H0", COMSOL will only adopt the direction of the H0 value but normalize its amplitude with other parameters (am I right?). But what's the detail of the normalization?
Fortunately, I found a blog discussing it a little bit:
srdjancomsol.weebly.com/port-boundary-condition-2d.html
srdjancomsol.weebly.com/setting-excitation-in-3d.html
According to the author, for a 3D case, we will have
P=0.5*n*IEI^2*S*cos(θ)/η0
where P is the port input power, |E| is the amplitude of the electric field, S is the area of the port, η0=sqrt(μ0/ε0), and according to my guess, n may be the refractive index, and θ may be the port phase θin.
However, still, the relation between the Pin and E field amplitude is not clear. I realize a very simple 3D example to explore this relation: there is only one block, the top is a port, and 4 walls are perfect electric conductor and perfect magnetic conductor as we know the electric field will be perpendicular to two walls and the magnetic field will be perpendicular to other two walls. By changing the area of the port, I get different |E|, but the problem is, I can't obtain the relation P=0.5*n*IEI^2*S*cos(θ)/η0:
With Pin=1[W], θin=0[rad], H0=(0 1000 0),
width(=depth) [um] Port area [um^2] E [V/m]
0.1 0.01 8.0e6
0.2 0.04 2.0e6
0.3 0.09 1.1e6
0.4 0.16 8.5e5
0.5 0.25 6.7e5
0.6 0.36 5.4e5
0.7 0.49 4.4e5
Can anyone help me? Thank you!
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6 Replies Last Post 16.06.2015, 16:46 GMT-4
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Posted:
9 years ago
Hello,
thank you for that good question, I'm also interested in this topic and can't find any answer, how the COMSOL Port-Node converts the Input Power in an electromagnetic Wave.
For interest, I'm studying the Microwave Cancer therapy File from COMSOL Examples and can't find a physical answer for that question above. With the formula mentioned in your Post, I get false values, that can't be right.
I hope somebody can bring a good answer for that.
Thank you very much for a answer.
Charles
thank you for that good question, I'm also interested in this topic and can't find any answer, how the COMSOL Port-Node converts the Input Power in an electromagnetic Wave.
For interest, I'm studying the Microwave Cancer therapy File from COMSOL Examples and can't find a physical answer for that question above. With the formula mentioned in your Post, I get false values, that can't be right.
I hope somebody can bring a good answer for that.
Thank you very much for a answer.
Charles
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Posted:
9 years ago
The relationship between the fields (E & H) in an RF port and the power at that port depend upon the mode and frequency of the electromagnetic wave. There can exist TE, TM, and TEM modes, and even combinations of them. Modes are further classified in terms of indices, which (very loosely-speaking) relate to how many wavelengths (or half wavelengths) wrap around and/or otherwise fit into/across the waveguide or transmission-line in question. The relationship of the overall power to the fields (the mode amplitudes) depends on both the type of mode, its specific indices, and the frequency relative to the cutoff frequency (if there is one) of the mode in question. Explicit equations for these relationships can be found in many (though not all) advanced undergraduate or introductory graduate-level electromagnetics textbooks typically used in courses taken by physicists and electrical engineers. (In general, you will likely find that EM books written for electrical engineers are easier to understand and contain more practical examples.)
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Posted:
9 years ago
Simple example: if a 2-d waveguide has thickness t, and the wavelength of light c / f = λ, then the light will travel down the waveguide at any of various angles Φ (0 = down the waveguide, π/2 = standing wave non-propagating) such that an integral number of half-wavelengths exist across the waveguide. The constraint is thus λ / sine Φ = n / 2 for some mode number n. The net electromagnetic field is thus the superposition of waves moving from the bottom to top boundary and waves moving from the top to bottom boundary, both otherwise moving down the guide (assuming no reflections). The component in the transverse guide direction is thus standing and has zero net power flux. You need to therefore look at E and H to determine the propagating power, not just E -- otherwise you'll overestimate power by counting the standing wave part.
In a fully enclosed perfectly resonant cavity there's no power propagating but the E field can be quite large. E and H are out of phase, though, yielding a zero E×H average.
In a fully enclosed perfectly resonant cavity there's no power propagating but the E field can be quite large. E and H are out of phase, though, yielding a zero E×H average.
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Posted:
9 years ago
Thank you very much for your answers.
But after searching hours for formulas in books, Papers and the internet I'm really frustrated.
I tried to understand and find the equations that are implemented in COMSOL, found under the "Equation View", but no chance, I even don't understand the equations there, e. g. the equation for the Port tangential electric mode field, the variable name is emw.tE0moder_1. Can anybody explain to me, why, only by calculating normal vector components and coordinates, I get an electric field in [V/m]?
Thank you very much!
Charles
But after searching hours for formulas in books, Papers and the internet I'm really frustrated.
I tried to understand and find the equations that are implemented in COMSOL, found under the "Equation View", but no chance, I even don't understand the equations there, e. g. the equation for the Port tangential electric mode field, the variable name is emw.tE0moder_1. Can anybody explain to me, why, only by calculating normal vector components and coordinates, I get an electric field in [V/m]?
Thank you very much!
Charles
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Posted:
9 years ago
Thank you for your interest in this question. And Robert, Daniel, thank you all for your answers.
Yet I haven't found an answer either, though my task was to calculate the enhancement in the plasmonic effect so that I was able to conduct a "control group" simulation to circumvent this question.
Still I am looking for an answer to this question. It will be of great help if anyone can post a simple 3D case example that can make a consensus between the theoretical calculation and the COMSOL result.
Yet I haven't found an answer either, though my task was to calculate the enhancement in the plasmonic effect so that I was able to conduct a "control group" simulation to circumvent this question.
Still I am looking for an answer to this question. It will be of great help if anyone can post a simple 3D case example that can make a consensus between the theoretical calculation and the COMSOL result.
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Posted:
9 years ago
Dear all,
Have you guys found the solutions for this problem?
I use my way to find the enhancement, but not sure if it is correct.
You can add a mode analysis for your input port (even if it is not numerical port, you can still use it). Then you plot the electric field tangential norm at the port. The color bar will tell you the E field at the input port. It might be uniform and the color is green, but that does not affect the color bar value.
I use that value for normalization and to find the enhancement. However, this method suffers from the standing wave pattern. I am not sure if the electric field is solely the input field or the result of standing wave.
Hope it helps.
Thanks,
Have you guys found the solutions for this problem?
I use my way to find the enhancement, but not sure if it is correct.
You can add a mode analysis for your input port (even if it is not numerical port, you can still use it). Then you plot the electric field tangential norm at the port. The color bar will tell you the E field at the input port. It might be uniform and the color is green, but that does not affect the color bar value.
I use that value for normalization and to find the enhancement. However, this method suffers from the standing wave pattern. I am not sure if the electric field is solely the input field or the result of standing wave.
Hope it helps.
Thanks,