Please do, I have much to gain :-)
bret
--- Rich Peet <> wrote:
> Or maybe we can talk Randy into posting his work. An electrical
> engineer and one of our very active nature recordists who wrote a
> definitive mathematical analysis of parabolas some time back.
>
> Rich
>
> --- In wrote:
> > Hi Bret,
> >
> > The formula you got is obviously a simplified approximation that is
>
> only
> > valid when the size of the dish is significantly larger than the
> wavelength of
> > the signals to be received (the links you provided refer to very
> short radio
> > or light waves).
> >
> > However, if the wavelengths of the signals are in the range of the
> diameter
> > of the reflector, then a more precise (and more complicated)
> formula is
> > required. Sten Wahlstr=F6m's paper in the Journal of the Audio
> Engineering Society
> > describes the theory of parabolic reflectors for acoustic
> applications:
> >
> > Wahlstr=F6m, S. (1985): The Parabolic Reflector as an Acoustic
> Amplifier. J.
> > Audio Eng. Soc., Vol. 33, No. 6, pp 418.
> >
> > I guess that this paper is not available on the internet.
> >
> > Regards,
> > Raimund=20
> >
> >
> >
> > > :
> > > > From: Bret <>
> > > > > Parabolic reflectors provide gain that has a slope rate of
> 6db per
> > > > > octave (higher freq, higher gain, octave up =3D 6db more gain),
>
> down
> > > > to
> > > > > 0db gain where the wavelength of the signal equals pi times
> the
> > > > > diameter of the parabola (assuming efficiency factor =3D 1).=20
> Below
> > > > that
> > > > > freq. gain is 0db.
> > >
> > > --- Walter Knapp <> wrote
> > > > I'm not sure where you got this figure for the low end of the
> gain,
> > > > but
> > > > it's at considerable odds with Sten Wahlstrom's paper detailing
>
> the
> > > > gain
> > > > of parabolic systems. He clearly states the 0dB gain point as a
> > > > diameter
> > > > 1/64th the wavelength. How rapidly and how cleanly the gain
> rises
> > > > between there and the wavelength and diameter being equal is
> highly
> > > > dependent on the ratio of the focal length to dish depth. But,
> it
> > > > does
> > > > rise as long as your focal length to depth ratio stays above 1.
>
> If
> > > > that's below 1 you get irregularities in the gain rise. Above
> the
> > > > point
> > > > where the wavelength and diameter being equal the rise is the
> 6dB per
> > > > octave. At least in theory.
> > >
> > > I would love to read Sten's paper, if you can provide a link.=20
> > >
> > > As far as where I got the notion of low end of gain, it is from
> the
> > > formula for gain for a parabolic reflector:
> > > Gain =3D 10*log(k*(pi*Diameter/Wavelength)^2)
> > >
> > > This seems to be commonly accepted:
> > > http://www.qsl.net/n1bwt/chap4.pdf (page 4)
> > > http://www.setileague.org/askdr/magnify.htm
> > > http://www.setileague.org/askdr/efficien.htm
> > >
> > > If we accept that formula for gain, then
> > > Gain =3D 0 when
> > > log(k*(pi*diameter/wavelength)^2) =3D 0
> > > For that to equal 0, then
> > > k*(pi*diameter/wavelength)^2 must =3D 1
> > > because log (1) =3D 0
> > >
> > > k is the efficiency factor of the reflector and feed system, to
> > > simplify let's assume it is 1 (it will be less than 1 in reality,
>
> this
> > > will shift the 0 db gain point to a higher frequency)
> > >
> > > If we assume k =3D 1,
> > > then gain =3D 0 when
> > > (pi*diameter/wavelength)^2 =3D 1
> > > Taking the square root of both sides of that equation,
> > > pi*diameter/wavelength =3D square root(1)
> > > pi*diameter/wavelength =3D 1
> > > therefore
> > > gain =3D 0 when pi*diameter=3Dwavelength (when efficiency factor =3D =
1)
> > >
> > > > The Telinga is not Sten Wahlstrom's optimal parabolic. That
> seems to
> > > > go
> > > > for one with a focal length to depth ratio of 4. The Telinga is
>
> > > > something like 1.2 or so. But Sten also notes that most
> practical
> > > > parabolas are of a ratio only slightly greater than 1. It's
> very
> > > > clear
> > > > from what he has that one should avoid parabolas with ratios
> less
> > > > than 1.
> > > >
> > > > Walt
> > > >
> > >
> > > The focal length to depth ration of the parabolic reflector will
> affect
> > > rearward lobing of the polar pattern of gain:
> > > http://www.cecer.army.mil/TechReports/pat_mike/pat_mike.post.pdf
> > > (see parabolic reflector section under microphone systems pages
> 8,9).
> > >
> > > Please tell me where I can find Sten's paper.=20
> > >
> > > bret
> > >
> > > __________________________________
> > > Do you Yahoo!?
> > > Yahoo! Mail SpamGuard - Read only the mail you want.
> > > http://antispam.yahoo.com/tools
> > >
> > >
> > >
> > > "Microphones are not ears,
> > > Loudspeakers are not birds,
> > > A listening room is not nature."
> > > Klas Strandberg
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >=20
> > >
> >
> > --
> > GMX ProMail (250 MB Mailbox, 50 FreeSMS, Virenschutz, 2,99
> EUR/Monat...)
> > jetzt 3 Monate GRATIS + 3x DER SPIEGEL +++
> http://www.gmx.net/derspiegel +++
>
>
>
> "Microphones are not ears,
> Loudspeakers are not birds,
> A listening room is not nature."
> Klas Strandberg
> Yahoo! Groups Links
>
>
>
>=20
>
__________________________________
Do you Yahoo!?
Yahoo! Mail SpamGuard - Read only the mail you want.
http://antispam.yahoo.com/tools
________________________________________________________________________
________________________________________________________________________
|