quellish said:
Trident said:
Well, if you were agreeing with what bipa said there, responding with what amounts to "No, you're wrong" when taken at face value seems like a patently unhelpful way of expressing your agreement.
I wasn't agreeing at all, I was pointing out that the explanation given applies to scale model testing, because the frequency is scaled. RCS itself is a measure of power the target reflects back at the radar - this is why it's typically expressed as dBsm. The dBsm is expressed as -20dBsm, etc. -20? Well, 20 less than what?
20 less than a (theoretical) perfectly conducting metal sphere at least several wavelengths in diameter. A sphere reflects in all directions equally for all viewing aspects. To calibrate a radar (or RCS test), a sphere is used for just these reasons. Once calibrated, you have the numbers for your ideal sphere - which removes frequency from the process (this is why the ideal sphere is x wavelengths in diameter).
The dBsm scale is relative to 1 square meter (ie zero dBsm). While the RCS of a sphere indeed becomes independent of frequency in the high-frequency regime, I don't see how this fact "removes frequency from the process". Actually it just removes it in the case of a sphere (or ellipsoid).
Maybe you were awkwardly referring to expressing RCS in "lambda squared" or "dB_lambda_squared" (lambda == wavelength), which is often used in theoretical literature to keep only the size/wavelength dependence and remove the dependence versus frequency alone. Indeed, the corollary of the "RCS scaling property" (as expressed in my previous post) is that RCS, when expressed in "lambda squared" instead of square meters, depends only on the size/wavelength ratio.
But in the real world, when you are facing an actual radar, only the square meter / dBsm scale counts, so the frequency dependence will re-appear: namely in the case of downscaled objects at downscaled wavelength, the RCS in square meters will also be downscaled (by the square of the scale, just like "lambda squared").
So I dare to insist: speaking in dBsm, the downscaled (bird sized) B-2 model at the upscaled frequency should have a much lower RCS than the actual B-2 at the actual frequency (and not the exact same RCS as you seemed to claim).
quellish said:
The idea that RCS is dependent on the "size/frequency ratio" is not accurate, because RCS itself has no frequency component. As stated above, frequency is removed on purpose. When testing a scaled model though, frequency is a factor because those several wavelengths for the idea sphere have to be scaled as well.
So RCS itself has no frequency, and is not affected by scale. Scaled model RCS testing does, because the whole point is to scale the results to a real world object.
I have
never said that RCS depends on "size /
frequency" ratio. Rather I have said that absolute RCS magnitude of an object (in square meters) depends on frequency
and on the "size /
wavelength" ratio (please re-read my previous posts carefully). In other words, that means it depends on frequency
and on the "size x frequency"
product.
Given that and all of the above, I really don't understand how you can conclude that:
quellish said:
RCS itself has no frequency, and is not affected by scale
because physics works in the exact opposite way... IMO you should clarify what you meant with this sentence.
