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Re: Swallow roost

Subject: Re: Swallow roost
From: "Avocet" madl74
Date: Tue Sep 18, 2012 4:46 pm ((PDT))
> my parabola is 60cm in diameter and the focus is exactely in the
> transition line between the inside and the outside of the parabola


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 Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce

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