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Re: Swallow roost

Subject: Re: Swallow roost
From: "freitojos" freitojos
Date: Wed Sep 19, 2012 1:52 am ((PDT))
Your technical points are never boring. To me it is very important to fully=
 understant what I am doing. Maybe you could said something (tchnical or no=
t) about my project to construct in fiberglass one 80cm diameter parabola?

--- In  "Avocet" <> wrote:
> > my parabola is 60cm in diameter and the focus is exactely in the
> > transition line between the inside and the outside of the parabola
> Jos=E9,
> Just to make a boring technical point, the diameter of most parabolic
> reflectors is the point where the slope is at 45 degrees. With short
> wave focussing, reflections from this point meet the point of focus at
> right angles to the source because this is also the distance of the
> focus from the centre of the dish. To complete the geometry, the
> diameter of the parabola at this point is four times the focal length.
> Continung the dish beyond this point gives a double reflection, and
> with radar and sound wavelengths, doesn't improve the reception in
> practice. It's also neat when the depth equals the focal length, and
> the radius is twice that, and the diameter four times.
> David
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce

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