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Lumped Ports Confusion -- What are they supposed to be?
Posted Aug 3, 2014, 1:22 p.m. EDT Low-Frequency Electromagnetics Version 4.4 2 Replies
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I am thoroughly confused by ports in COMSOL. How are lumped ports actually solved in the simulation? And what, physically, are they supposed to represent when applied in the Magnetic Fields interface? I would like to use lumped ports to excite my structures because ports in COMSOL allow for the computation of impedances and S-parameters, but the ports never behave as I'd expect them to.
The documentation states that a lumped port "applies a uniform electric field between two metallic boundaries" and that "the excitation at the port can be expressed as a voltage or as a current, or via the connection to a circuit interface." The only equation I can find it discussing is Z=V/I.
I would expect a current-exciting port to drive the specified amount of current through the attached conductor, but this appears to NOT be the case. A lumped port exists in the example model inductor_3d. Changing the specified height/width of that port changes the amount of current that port excites in the attached inductor, even when the specified terminal current for the port is left constant at 1 A. This means the "terminal current" you specify for the port does not actually specify the current excited by that port. So what does "terminal current" actually specify?
On a similar note, what do width/height actually refer to for a user-defined lumped port? In inductor_3d again, for example, the lumped port is defined as the 4 exposed boundaries of a rectangular block. I'd expect the dimensions of this block to match the dimensions of the port in some way, but, again, that is not the case. So what, physically and mathematically, is going on with ports in COMSOL?
The documentation states that a lumped port "applies a uniform electric field between two metallic boundaries" and that "the excitation at the port can be expressed as a voltage or as a current, or via the connection to a circuit interface." The only equation I can find it discussing is Z=V/I.
I would expect a current-exciting port to drive the specified amount of current through the attached conductor, but this appears to NOT be the case. A lumped port exists in the example model inductor_3d. Changing the specified height/width of that port changes the amount of current that port excites in the attached inductor, even when the specified terminal current for the port is left constant at 1 A. This means the "terminal current" you specify for the port does not actually specify the current excited by that port. So what does "terminal current" actually specify?
On a similar note, what do width/height actually refer to for a user-defined lumped port? In inductor_3d again, for example, the lumped port is defined as the 4 exposed boundaries of a rectangular block. I'd expect the dimensions of this block to match the dimensions of the port in some way, but, again, that is not the case. So what, physically and mathematically, is going on with ports in COMSOL?
2 Replies Last Post Aug 5, 2014, 12:47 p.m. EDT
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Posted:
10 years ago
Hello,
Documentation on the Lumped Ports can be found in the AC/DC Module User Guide, in the chapter modeling with AC/DC Module, section Lumped Ports with Voltage Input (page 87 of the PDF).
As explained in the manual, the port represents a connection to a transmission line, with the two metallic boundaries constituting the conductors of the transmission line.
Physically, for a Current excitation, the port applies a Surface Current Density on the boundary, the magnitude and direction of which depend on the port geometry.
In the case of a Uniform port, the boundary must be rectangular, with two opposite sides touching metal boundaries. Comsol will automatically compute the width and height from the geometry of the model: the width of the port is the length of one of the "metallic" sides, while the height is the length of the one of the other two "non-metallic" sides. For a Current excitation, the surface current applied will be computed as the specified total current divided by the width of the port.
The User defined port setting works more or less like the Uniform setting, but it allows the user to specify manually the geometrical properties of the port (instead of automatically detecting them from the geometry). User defined ports are therefore more flexible and can be used even on non-rectangular boundaries (e.g. slightly curved boundaries, or groups of boundaries), but it is up to the user to enter values consistent with the geometry of the port. For example, if you specify a port width different from the actual geometrical width of the boundary, the total current excited by the port will not be equal to the specified total current.
In the inductor_3d model, the port is applied on a group of boundaries that extend between the two conductors. The width of the port is the length of the four edges in contact with each metallic domains (the edges perpendicular to the port direction), while the height is the length of the "free" edges (or, equivalently, the distance between the metallic domains).
--
Andrea Ferrario
Electromagnetics Group
COMSOL AB
Documentation on the Lumped Ports can be found in the AC/DC Module User Guide, in the chapter modeling with AC/DC Module, section Lumped Ports with Voltage Input (page 87 of the PDF).
As explained in the manual, the port represents a connection to a transmission line, with the two metallic boundaries constituting the conductors of the transmission line.
Physically, for a Current excitation, the port applies a Surface Current Density on the boundary, the magnitude and direction of which depend on the port geometry.
In the case of a Uniform port, the boundary must be rectangular, with two opposite sides touching metal boundaries. Comsol will automatically compute the width and height from the geometry of the model: the width of the port is the length of one of the "metallic" sides, while the height is the length of the one of the other two "non-metallic" sides. For a Current excitation, the surface current applied will be computed as the specified total current divided by the width of the port.
The User defined port setting works more or less like the Uniform setting, but it allows the user to specify manually the geometrical properties of the port (instead of automatically detecting them from the geometry). User defined ports are therefore more flexible and can be used even on non-rectangular boundaries (e.g. slightly curved boundaries, or groups of boundaries), but it is up to the user to enter values consistent with the geometry of the port. For example, if you specify a port width different from the actual geometrical width of the boundary, the total current excited by the port will not be equal to the specified total current.
In the inductor_3d model, the port is applied on a group of boundaries that extend between the two conductors. The width of the port is the length of the four edges in contact with each metallic domains (the edges perpendicular to the port direction), while the height is the length of the "free" edges (or, equivalently, the distance between the metallic domains).
--
Andrea Ferrario
Electromagnetics Group
COMSOL AB
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Posted:
10 years ago
Thank you, Andrea! That makes much more sense now. I hadn't found that section of the documentation yet, so I was trying to figure out ports from a couple example models and the lumped ports discussion linked to by the COMSOL help.
Now I understand that a port can be defined as a single boundary, where two of those boundary edges contact a conductor, and that the boundary drives a surface current out those edges into the conductor. The dimensions and parameters make sense in that context.
The examples I had seen created a block-shaped port out of four boundaries, so I had been assuming that creating a block was necessary and that the block itself would then behave as the port.
Now I understand that a port can be defined as a single boundary, where two of those boundary edges contact a conductor, and that the boundary drives a surface current out those edges into the conductor. The dimensions and parameters make sense in that context.
The examples I had seen created a block-shaped port out of four boundaries, so I had been assuming that creating a block was necessary and that the block itself would then behave as the port.