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    Re: Fatal interaction betweeu yacht and ferry.
    From: Lu Abel
    Date: 2007 May 08, 07:24 -0700

    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---.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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