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HFSS and Planar EM Simulators >
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Including a Length of Uniform Cross-Section
Including a Length of Uniform Cross-Section
The geometry of a model with a port should include
a length of uniform cross-section at each port for optimal accuracy.
The length of the uniform cross-section should be
long enough to allow non-propagating modes to die out. Otherwise, the
conditions at the port will prevent the simulated solution from matching
the actual solution.
For example, if a non-propagating mode takes approximately
one-eighth wavelength to die out, then you should make the uniform cross-section
somewhat longer than one-eighth wavelength. Otherwise, you must include
the effects of that higher order mode in the simulation.
Note
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Reflected waves
attenuate as a function of  , assuming that the wave propagates in the z direction.
Therefore, the required length of the uniform cross-section depends
on the value of the mode’s attenuation constant, a.
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The reason you must make the port long enough
for non-propagating modes to die out is because Designer forces the
current pattern at each port to be some linear combination of the modes
you request. For example, if discontinuities in a structure are causing
higher order modes to be reflected back toward a port face, then the
actual current solution near the discontinuity is a linear combination
of all relevant modes. If the port length — that is, the length
of uniform cross-section leading to the port face — is not long
enough for the reflected waves to die out, then those modes must be
included at the port face.
In cases where port lengths are too short for
reflected modes to decay, a current solution involving only the dominant
mode will not be what you expected; the current solution will not be
for a structure being excited with only the dominant mode.
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