--- 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).
Parabolic microphone dish sizes (like the 22" telinga) are
significantly larger than the wavelength of the audio waves to be
received, else there would be little gain.
Wavelengths of radio frequency signals are not as small as you might
think compared to wavelengths of audio due to the difference in speed
of light vs. speed of sound in air.
Assuming 1100 ft/sec for sound, a 30hz audio wave is the same length
as a 27mhz rf wave (11 meters or 36 feet). A 15,460 hz soundwave is the
same length as a 13.84 Ghz rf wave.=20
A parabolic antenna for 2.3Ghz- 2.54Ghz (rf wavelength .43ft to .39ft,
about 5 inches, equal to audio wave of 2,572hz to 2,840 hz) are
available in dish sizes from 4.72 inches to 32.3 inches from this
vendor: http://www.ssbusa.com/dishes.html
So, in the example of these rf parabolic antennae, the dish diameter is
anywhere from about 1 wavelength to about 6.4 wavelengths of the rf to
be transmitted or received.=20
Converting this to audio waves, for a 22" inch parabolic mic dish,
considering the diameter at 1 wavelength the audio frequency is 598hz.
A 22" audio parabolic dish diameter is 6.4 times the wavelength of a
3,865hz audio wave. At 15,460hz audio wave, a 22" mic parabolic dish
diameter is 25.7 times the wavelength of the audio wave (.85 inches
wavelength).=20
The audio parabolic dish diameter IS many times larger in diameter than
the audio wavelengths. Even more so than the rf parabolic dishes
indicated above for their rf wavelengths.=20
So, if your statement that the formuala I used for parabolic gain is
only appropriate where the diameter of the dish is significantly larger
the wavelength of the signal, then the formula should be even more
appropriate for audio parabolics, than rf parabolics like in the link
above.
> 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
So, what is that more complicated formula for parabolic gain?=20
And why don't rf engineers use it?
http://www.phasorlabs.com/Vol2002K.pdf
http://www2.gvsu.edu/~w8gvu/geo/geo2.html
http://sina.sharif.ac.ir/~barkeshli/antennas/review/9510_007.htm
http://wireless.ictp.trieste.it/handbook/C4.pdf
All of these sources (and the previously posted links on parabolic dish
gain formula) show parabolic gain to be the same formula I used.=20
I have demonstrated above that the the ratio of dish diameter to
wavelength in common audio dishes are equal or greater than this ratio
for common rf dishes.
Please show me another more complicated formula for parabolic gain. I
am all ears :-)
bret
KC0GWS
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