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    Re: Fatal interaction betweeu yacht and ferry.
    From: George Huxtable
    Date: 2007 May 8, 12:40 +0100

    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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