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I may not be understanding your question, but λ/10 usually refers to a grid resolution. The computational code is trying to approximate the derivatives in Maxwell's equations by computing the slope between adjacent points on the grid. If the resolution is to large, this approximation will not be accurate. The λ/10 quantity is essentially a rule-of-thumb for minimum grid resolution to obtain accurate results.
I think rrumpf misunderstood myem's question. I think myem's question is that "when a structure is less than lambda/10, people normally can use quasi-TEM methods to get reasonable results". I think rrumpf considers the lambda/10 is normally the maximum grid size for numerical method to get reasonable results. Both cases are ok.
Why is lambda/10, this is not an absolute limit. It just says the size of the structure is much smaller than a wavelength. Staitc or quasi-static methods normally assume the propagation constatnt is 0 or close to 0. This assumption means that the structure should be much smaller than wavelength. How much is much smaller? I think it is dependent upon the real situation. For some structures, lambda.10 is already small and quasi-staitc solvers can yield reasonable results. For some structures, quasi-static solvers may not be able to get good enough results even with hte size smaller than lambda/100.
For numerical methods, the maximum grid size should be lambda/10. If it is larger than lambda / 10, the chance of significant numerical error is very high. However, for many structures, lambda / 10 may not enough. For some structures, it may require the grid size smaller than lambda / 100 to meet the high accuracy requirement. It is dependent upon the structures and the requirements of different designs. Regards.
As to the size of the system, I guess this definition is a bit vague.
For example, transmission lines are well modeled with quasi-static 2D solvers no matter how long it is. In this case, its length is much larger than the 1/10 of the wave length. But its intersection geometry is much smaller than 1/10 of the wave length.
So, I guess the more strict definition is that the resonant frequency of the structure is much lower than 1/10 of the desired frequency.
Of course, the most strict definition in mathematics is to ignore a few items in maxwell equations.
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