I don't think you can find Sten Wahlstr=F6ms paper on the web, but I'm sure=
I
have it and - if not - I can phone him and get another copy.
Klas.
>:
>> 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). 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
>
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>"Microphones are not ears,
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>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
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