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## Making a parabola (was Swallow roost)

 Subject: Making a parabola (was Swallow roost) "MARTIN" mcgarnett Wed Sep 19, 2012 4:10 am ((PDT))
 ``` Hi Jose and David, I'd be really interested to hear about your project making an 80cm diameter= parabola. I'd like a large enough parabola to successfully record cormoran= ts nesting on a tall cliff not far from me here in north Wales. The cormora= nts have quite low frequency calls starting at about 400Hz. Any ideas as to= how large a dish I'd need? Is it easy to make one using fibreglass? I'm not very practical - any advic= e much appreciated! Best wishes, Martin --- In m("yahoogroups.com,","naturerecordists"); "freitojos" wr= ote: > > David, > Your technical points are never boring. To me it is very important to ful= ly understant what I am doing. Maybe you could said something (tchnical or = not) about my project to construct in fiberglass one 80cm diameter parabola= ? > Regards, > Jos=E9 > > > --- In m("yahoogroups.com,","naturerecordists"); "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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