Hi Jose,
Thanks very much for your detailed reply. It is interesting and useful to k=
now how to make a parabola, but it looks a bit too complicated for me with =
my rather poor practical skills!
Best wishes,
Martin
--- In "freitojos" <> wr=
ote:
>
> Hi Martin,
> http://vzone.virgin.net/ljmayes.mal/var/parabola.htm
> 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 diame=
ter another with 60cm.
> You put in the exel the diameter the number of raws, (more raws more prec=
ion 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=
the sections.Then you unify all the sections with tape. Next step you put =
the paper parabola in a place over sand to sustain it, then you put inside =
clay and let it dry over a day.
> Atention, with larger than 60 cm parabolas cardboard, I suspect is no=
t a good choice,80 cm is a huge parabola and the clay is heavy, aluminium f=
oil or a more rigid material maybe, is better.
> At last you use a straight knife with two hands over the surface to cut o=
ut the paper and make the interior surface in clay.
> Now you have the clay mold, then you must put over it the fiberglass, at =
least tree times.
> After you must destroy the mold,unfortunately, at least in my experience,=
you have the parabola.
> Regards,
> Jos=E9
>
> --- In "MARTIN" <annieandmartin@> wrote=
:
> >
> >
> >
> >
> >
> >
> > Hi Jose and David,
> >
> > I'd be really interested to hear about your project making an 80cm diam=
eter parabola. I'd like a large enough parabola to successfully record corm=
orants nesting on a tall cliff not far from me here in north Wales. The cor=
morants have quite low frequency calls starting at about 400Hz. Any ideas a=
s to how large a dish I'd need?
> >
> > Is it easy to make one using fibreglass? I'm not very practical - any a=
dvice much appreciated!
> >
> > Best wishes,
> >
> > Martin
> >
> >
> >
> > --- In "freitojos" <josefreitas81@> w=
rote:
> > >
> > > David,
> > > 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 not) about my project to construct in fiberglass one 80cm diameter para=
bola?
> > > 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 parabol=
a
> > > >
> > > > Jos=E9,
> > > >
> > > > Just to make a boring technical point, the diameter of most parabol=
ic
> > > > reflectors is the point where the slope is at 45 degrees. With shor=
t
> > > > 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 leng=
th.
> > > >
> > > > 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, an=
d
> > > > the radius is twice that, and the diameter four times.
> > > >
> > > > David
> > > >
> > > > David Brinicombe
> > > > North Devon, UK
> > > > Cogito cogito ergo cogito sum - Ambrose Bierce
> > > >
> > >
> >
>
|