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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Fatal interaction betweeu yacht and ferry.
From: Gary LaPook
Date: 2007 May 08, 12:36 -0700

```Gary writes:

I don't think most sailors realize how much size matters when it comes
to radar reflectors and may be tempted to "just get the slightly
smaller one." Since the echo varies with the fourth power of the size
a small change in size  will make a large increase in echo strength.
Changing the size by 19% changes the echo by a factor of two since
1.19 is the fourth root of two.

For an example take the standard Davis reflector which is 12.5 inches
in diameter. Adding only 2.4 inches to it to make it 14.9 inches
doubles the strength. Add another 2.8 inches up to 17.7 inches will
double it again to four times the original strength.

Reducing he size also has the same effect. Buy one only slightly
smaller, only two inches, down to 10.5 reduces the strength to one-
half. Get down to 6.25 inches and the strength is only one-sixteenth!
This is why those sexy streamlined mini reflectors don't work and
should be avoided.

gl

On May 8, 7:24 am, Lu Abel  wrote:
> 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).
> > |
>
> > 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 geo...@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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