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George:

You are very likely right (again, without either of us having sources to
cite) -- I thought of the wavelength issue right after I pushed the
"send" button.

For those who wonder what we are babbling about:  Even the smallest
mirror has dimensions a billion (thousand million) times greater than
the wavelength of light.   But radar reflectors are only a few times
larger than the size of a wave of radar energy.   Whenever relative
dimensions are so small, "reflection" is a lot less clean.   An analogy
we can all see with our own eyes is to watch waves hit a seawall (or,
even better, a large rock).  Yes, the waves bounce off the seawall (or
rock), but it's far from a clean "ray" ala light reflection.

Lu Abel

George Huxtable wrote:
> Lu Abel wrote, responding to Gary-
>
> | You seem to know a lot about radar reflectors, so I'm surprised you
> | didn't mention what many consider the most crucial aspect of radar
> | reflector construction:  that the angles between the plates must be an
> | accurate 90 degrees.  Since the radar signal bounces off one plate and
> | then another, only by having a very accurate 90 degree corner is it
> | guaranteed that the incoming radar pulse will be returned to the radar
> | that emitted it!   For this reason many experts advise against home-brew
> | radar reflectors (I can't tell you the number of times I've read letters
> | to the editor of various sailing publications with people bragging about
> | constructing a reflector out of bits of cardboard covered with aluminum
> | foil).
> |
> | Comments welcome.
>
> I suspect that Lu, and the "many experts", are misinformed about this
> matter.
>
> It's not a bit like light being reflected from a corner reflector, such as
> that of a "cat's eye" used for road lane markings. (do those exist in the
> US?). Well it is a BIT, but you have to take the differences into account.
> And the main difference is due to the very different wavelengths being used.
>
> In the case of a mirror reflecting light, the dimensions of any mirror are
> millions of times greater than the wavelength of the light, so the reflected
> light behaves in a geometrical way; just like a mirror, indeed.
>
> In the case of a radar reflector, with wavelengths of a few cm., and the
> dimensions of any mirror being only a few wavelengths, then "diffraction",
> which some of you may recall from schooldays, plays a major part. The result
> is that even the best reflector, unless is dimensions are absolutely
> immense, reflects its energy in a spread-out, diffuse, maner, with a
> beam-width of many degrees. It's for a similar reason that to get a narrow
> beamwidth, the swinging radar scanners on ships are so immensely wide.
>
> Because the beam from any such radar reflector is so spread-out in angle,
> that's why its efficiency in reflecting energy back to its sending point is
> so small; much of the reflected energy missing the target. And that's one
> reason why you get such an enormous increase in effective cross-section by
> increasing the size of the reflector, as Gary has pointed out.
>
> But the benefit, the only benefit, that you will get from the situation of
> reflectors being small(ish) measured in wavelengths, is that because the
> reflection is so diffuse, there's no call, at all, to get any high precision
> in the relative angling of the corner faces. I would go so far as to say
> that if it looks, by eye, to be about 90 degrees between the faces, that's
> good enough. And so, I suggest that the cardboard and foil solution, that Lu
> derides, might well be just as good as an expensive reflector.
>
> Let me say that my contrary view to Lu's is not based on any experience, or
> reading of authoritative texts, but just on pondering about the physical
> principles involved. So it could be wrong; but I doubt if it is. Neither
> physical optics nor radio propagation are "my subjects".
>
> If Lu can provide good authority for his claim that only a very accurate 90
> degree corner is good enough, then I will take it seriously.
>
> George
>
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>
>
>
> >
>

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