On May 7, 4:08*am, DougC wrote:
I have been wanting to try some silly experiments using a digital TV
dish to make a super-hearing type thing for a while, and finally got
ahold of one from someone's trash.

The one I found has the mounting pole and horn bracket, but the feed
horn is missing. Even so just looking at photos of them online, it
doesn't appear to me that the horn is at the geometric focal point.
Also--even considering it's an offset dish--the dish appears rather
shallow and has a round profile, not a parabola at all. Do RF waves
behave differently than sound waves in this regard? (Would RF waves have
a shorter F-length than simple geometry would indicate?)

Also just for trivia purposes I am curious as to what the typical dB
gain is for this sort of dish (when used for RF), and if that equates at
all to sound waves...
~

The feed horn is of course mounted exactly at the focal point, but the
dish is not used perpendicular to the satellite. Combined with the
offset mount for the feed horn, it all works out very fine - thank
you.

You wouldn't be able to discern the difference between a spherical
section anntena or a true parabola without the abilty to measure
within thousandths of an inch to the focal point of various diameters
of the 18" dish. 10 foot C band are spherical-ish at best and no way
to even measure the difference between perfection and what you get for
$50 used. You are chasing much less than one tenth decible difference
when the dish has 38 db gain at C band 3.7 Ghz - no point at all to
such silly behavior. Toss the Parabola dreams where they belong.

Exact db gain has to do with dish size and wavelength so no, it can't
possibly relate to sound waves with 30 feet wavelengths compared to
half inch RF wavelengths. I dont' have the formula at my fingertips
but it's not a national secret, google for it yourself.

There is a reason you don't see them much outside of a Monday night
football game - they don't work as advertised. Better designs enclose
or shut out sounds entering from the sides so the old parabola has to
go right off. I think a 5 gallon plastic bucket would work just a
good if not better just on extraneous sound rejection principles. You
are dealing with a decided mismatch in wavelenths to focal point
distances, a portable dish will never work out well for sound
frequencies.

Of more interest to me would be constructing an array of canceling
microphones to extract the "aimed at" sound of desire - at least this
one is plausible.