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Re: Making a parabola (was Swallow roost)

Subject: Re: Making a parabola (was Swallow roost)
From: "freitojos" freitojos
Date: Wed Sep 19, 2012 6:06 am ((PDT))
Hi Martin,
This site uses one exel to make parabolic reflectors using plane segments, =
I have used this exel in my two parabolic microphones one with 40cm diamete=
r another with 60cm.
You put in the exel the diameter the number of raws, (more raws more precio=
n I have used 10 raws), and the focal distance. After that you design in a =
cardboard one section of the parabola. Next step you cut in cardboard all t=
he sections.Then you unify all the sections with tape. Next step you put th=
e paper parabola in a place over sand to sustain it, then you put inside cl=
ay and let it dry over a day.
    Atention, with larger than 60 cm parabolas cardboard, I suspect is not =
a good choice,80 cm is a huge parabola and the clay is heavy, aluminium foi=
l or a more rigid material maybe, is better.
At last you use a straight knife with two hands over the surface to cut out=
 the paper and make the interior surface in clay.
Now you have the clay mold, then you must put over it the fiberglass, at le=
ast tree times.
After you must destroy the mold,unfortunately, at least in my experience, y=
ou have the parabola.

--- In  "MARTIN" <> wrot=
> Hi Jose and David,
> I'd be really interested to hear about your project making an 80cm diamet=
er parabola. I'd like a large enough parabola to successfully record cormor=
ants nesting on a tall cliff not far from me here in north Wales. The cormo=
rants 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 adv=
ice much appreciated!
> Best wishes,
> Martin
> --- In  "freitojos" <josefreitas81@> wro=
> >
> > David,
> > Your technical points are never boring. To me it is very important to f=
ully understant what I am doing. Maybe you could said something (tchnical o=
r not) about my project to construct in fiberglass one 80cm diameter parabo=
> > Regards,
> > Jos=E9
> >
> >
> > --- In  "Avocet" <brini@> 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 a=
> > > 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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