OK, I should have been more precise...
>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.
Yes and no. There will be only little gain at low frequencies below about
500 kHz. Fortunately, most bird songs are higher-pitched and a parabola can
really provide significant gain at these higher frequencies.
>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.
You are correct, the rf parabolics you mentioned are not much larger than
the wavelengths. However, this does not mean that your simple formula is
absolutely precise in the lower frequency ranges. For the practical work of=
an rf
engineer, it is perhaps not very important to know the exact gain at the lo=
wer
frequencies. In contrast to sound recording, the rf parabolas are operated
at very narrow frequency bands only (2.3Ghz- 2.54Ghz is much less than an
octave). Therefore, the 'coloration' caused by the small dimensions of port=
able
rf parabolics will be probably no problem at all.
Addionally, the degree of directivity (the beamwidth) of a reflector also
depends on the ratio between wavelength and size. A smaller reflector is le=
ss
directional than a larger one. In fact, a large reflector having a narrow
beamwidth can be become a serious problem. It would be extremely difficult =
to aim
the parabola precisely on the target. This is very important because the
off-axis coloration may be worse (more irregular) than on-axis. The link yo=
u
provided (thanks for that) describes these effects:
http://www.cecer.army.mil/TechReports/pat_mike/pat_mike.post.pdf
The explicitly desired low degree of directionality might be one reason why
portable rf parabolas are so small.
>Please show me another more complicated formula for parabolic
>gain. I am all ears :-)
You should look at the Wahlstrom paper. There you can find a more precise
computational model. The formula contains complex numbers. So, it would be =
very
difficult to write it in plain ASCII text...
Regards,
Raimund
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