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Scale-down models for targets are used for radar cross-section measurements of huge-size objects. Is there any upper limit to the scaling factor on downscale modeling as a $100~\text{m}$ length of target becomes $2~\text{m}$ when downscaled by a factor of $50$? Will it satisfy the results?

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What are you building? Is it going to sink ships? 100 meters is too big for a plane. Why do you need to know the radar cross section? –  gecko Mar 10 '13 at 21:31
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I guess there can be some limitations. If you use a model that is 50 times smaller than the target, the wavelength for the model must be 50 times smaller than the wavelength for the target, so the frequency must be 50 times greater. The imaginary part of the electric permittivity is $\frac{4\pi \sigma}{\omega}$ (in CGS), where $\omega$ is the frequency, $\sigma$ is the conductivity. So, for example, if your target is made of copper, your model's material must have conductivity 50 times greater than that of copper, and there is no such material, as far as I know (I don't think using a supercoductive material would help). Similar (but probably less severe) problems can arise with the real part of electric permittivity, as it tends to be smaller for higher frequency.

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Well, kinda a tricky question. At a first glance I see no reasons to say "no", cause in most cases in ED equations the dimensions of the object are usually "scaled" by the wavelength (wavenumber). So if you change all (!) the dimensions in the same manner as the wavelength no changes in their proportion will happen. But to be strict one must investigate ED equations that govern the diffraction problem under investigation. And to my knowledge there are only few problems, that have analytical solution. All this covers only PEC bodies.:) In real life you have to take into account the interaction of electromagnetic radiation with the material of the object (cause usually electrodynamic parameters have quite vivid frequency dependence). In case of RCS calculation guess it would be crucial ;).

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