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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

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.

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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