MainlySteam
New member
Re: On reflection
To answer the questions - No, it is not equivalent to a flat surface placed perpendicular to the radar beam. The Radar Cross Section of a target is a comparison of the strength of the reflected signal from a target to the reflected signal from a perfectly reflecting smooth sphere having a cross sectional area of 1 m2. A sphere reflects at an amount that is independant of frequency when the wavelength is small in comparison to the sphere.
However, other shapes do not reflect independantly of frequency - at 3 cm (X Band) a flat reflecting plate about 100 mm square perpendicular has a RCS about 1 m2, but at 10 cm (S Band) it is very much lower, maybe around 0.2 m2. At 3 cm a 1m x 1m plate has a very large RCS of 1000's of m2 and 100'sm2 at 10 cm. I think a flat plate reflects in proportion to its area squared divided by the wavelength squared but I stand to be corrected on that.
So getting back to the 10 m2 sphere that was mentioned in earlier posts it was said that was the area of the surface (so 0.9 m dia) in which case being a sphere its RCS would be roughly the same as its cross sectional area (around 0.6 m2). However, if the 10 m2 quoted was actually meant to be the RCD then one is talking about a very much larger sphere of course.
But, in fact, the requirement of 10m2 is nothing to do with spheres at all, nor the area of anything else either, because I have now checked the UK regulations (which are probably the same as IMO) to see what they say. They say that it is the the RCD that is to be 10 m2.
It can be seen from above that at X band even quite a small plate has a large RCD but for 10 m2 a 100mm square plate would not be big enough - needs to be about square root of 10 times bigger in area I think, if so would need to be about 200 mm square. The purpose of 'hedral shapes is to try and mimic a flat plate in multiple directions, but is not perfectly efficient at doing that, so a 'hedral will have to be considerably bigger than 200 mm square - I guess about the size of the ones one can buy.
Getting back to the RCD being dependant on wavelength, you can see that your analogy of an experiement using a light is incorrect as the wavelength of light is very much less than X Band and also there is nothing to suggest that light will even reflect in the same way (for example a black painted flat plate will reflect radar just as well as an unpainted one, but the same does not apply for light).
Also, at X Band it can be seen that even small areas have quite a large RCD, hence the possibility that Jimi's proposal, adapted to something more realistic than Xmas tinsel, may have some foundation. I suspect there could be a possibility that a rope done as a baggy wrinkle with aluminiumised plastic strips might perform better than some of the plastic cans people buy filled with crumpled up baking foil.
John
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To answer the questions - No, it is not equivalent to a flat surface placed perpendicular to the radar beam. The Radar Cross Section of a target is a comparison of the strength of the reflected signal from a target to the reflected signal from a perfectly reflecting smooth sphere having a cross sectional area of 1 m2. A sphere reflects at an amount that is independant of frequency when the wavelength is small in comparison to the sphere.
However, other shapes do not reflect independantly of frequency - at 3 cm (X Band) a flat reflecting plate about 100 mm square perpendicular has a RCS about 1 m2, but at 10 cm (S Band) it is very much lower, maybe around 0.2 m2. At 3 cm a 1m x 1m plate has a very large RCS of 1000's of m2 and 100'sm2 at 10 cm. I think a flat plate reflects in proportion to its area squared divided by the wavelength squared but I stand to be corrected on that.
So getting back to the 10 m2 sphere that was mentioned in earlier posts it was said that was the area of the surface (so 0.9 m dia) in which case being a sphere its RCS would be roughly the same as its cross sectional area (around 0.6 m2). However, if the 10 m2 quoted was actually meant to be the RCD then one is talking about a very much larger sphere of course.
But, in fact, the requirement of 10m2 is nothing to do with spheres at all, nor the area of anything else either, because I have now checked the UK regulations (which are probably the same as IMO) to see what they say. They say that it is the the RCD that is to be 10 m2.
It can be seen from above that at X band even quite a small plate has a large RCD but for 10 m2 a 100mm square plate would not be big enough - needs to be about square root of 10 times bigger in area I think, if so would need to be about 200 mm square. The purpose of 'hedral shapes is to try and mimic a flat plate in multiple directions, but is not perfectly efficient at doing that, so a 'hedral will have to be considerably bigger than 200 mm square - I guess about the size of the ones one can buy.
Getting back to the RCD being dependant on wavelength, you can see that your analogy of an experiement using a light is incorrect as the wavelength of light is very much less than X Band and also there is nothing to suggest that light will even reflect in the same way (for example a black painted flat plate will reflect radar just as well as an unpainted one, but the same does not apply for light).
Also, at X Band it can be seen that even small areas have quite a large RCD, hence the possibility that Jimi's proposal, adapted to something more realistic than Xmas tinsel, may have some foundation. I suspect there could be a possibility that a rope done as a baggy wrinkle with aluminiumised plastic strips might perform better than some of the plastic cans people buy filled with crumpled up baking foil.
John
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