Hello Sami Karkar
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Robert Koslover
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Posted:
7 years ago
Jun 27, 2017, 12:20 p.m. EDT
Updated:
7 years ago
Jun 27, 2017, 12:29 p.m. EDT
I have used lumped ports successfully in models with symmetry planes. Like so much in Comsol Multiphysics, the key is to understand the underlying physics. If your plane(s) of symmetry bisects the port (and it usually does), then you probably need to adjust the settings on it compared to a model of the full port. For example, suppose you were applying a voltage of 1v between a top and bottom edge of a lumped port, but then you cut the port in half via a horizontal cut, leaving only the upper half. Well, now you would have a port where you should apply only 0.5V, since the top and (missing) bottom would together have yielded 1V. Or consider setting an impedance. If you divide a port so that it is now like one half of two impedances in series, then the half port remaining should be assigned half the impedance (simple, right?). On the other hand, if you divide it in the other direction, so that the full port would be like two impedances in parallel, then the half port you have should be assigned twice the impedance of the full port. Interestingly, if you divide a full port into quarters (via two orthogonal symmetry planes), then the impedance doesn't change, because one of the divisions doubles it while the other reduces it by half. In summary, just follow the circuit rules! Just remember that resistances, inductances, and voltages (i.e., like batteries) add in series, while capacitances and currents add in parallel. That knowledge guides you in whether you need to be multiplying or dividing various port-related quantities by factors of 2. This can also be generalized to more complex symmetries; just think about and follow the equivalent circuits. I hope that helps.
I have used lumped ports successfully in models with symmetry planes. Like so much in Comsol Multiphysics, the key is to understand the underlying physics. If your plane(s) of symmetry bisects the port (and it usually does), then you probably need to adjust the settings on it compared to a model of the full port. For example, suppose you were applying a voltage of 1v between a top and bottom edge of a lumped port, but then you cut the port in half via a horizontal cut, leaving only the upper half. Well, now you would have a port where you should apply only 0.5V, since the top and (missing) bottom would together have yielded 1V. Or consider setting an impedance. If you divide a port so that it is now like one half of two impedances in series, then the half port remaining should be assigned half the impedance (simple, right?). On the other hand, if you divide it in the other direction, so that the full port would be like two impedances in parallel, then the half port you have should be assigned twice the impedance of the full port. Interestingly, if you divide a full port into quarters (via two orthogonal symmetry planes), then the impedance doesn't change, because one of the divisions doubles it while the other reduces it by half. In summary, just follow the circuit rules! Just remember that resistances, inductances, and voltages (i.e., like batteries) add in series, while capacitances and currents add in parallel. That knowledge guides you in whether you need to be multiplying or dividing various port-related quantities by factors of 2. This can also be generalized to more complex symmetries; just think about and follow the equivalent circuits. I hope that helps.
