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 "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 "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 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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