Re: OT: Aliens in the heavans (was Re: seti@home)
Wow, people do read my posts. I should be more careful about what I
write.
Gary Turner wrote:
[snip]
> While it is true that the EMF, or voltage is inversely proportional to
> the distance, the power is reduced by the square of the distance.
> (P=e^2/r, P=i^2*r, or P=e*i) Thus a signal with a power density of 1
> Watt per sq meter at 10 meters distance, will have a power density of
> .01 Watts per sq meter at 100 meter's distance (in a lossless system).
Thankyou for that clarification.
[snip]
> Which brings us to power density at the receiving end--there ain't much.
> Given that the W/sq meter is minuscule, an antenna such as the one the
> seti project uses has an effective aperture on the order of 10's of
> thousands of sq meters. And that helps. The small beam angle acts to
> remove all signals not in the desired direction, so the noise level is
> reduced. VLNAs bring sensitivity to a level that a signal energy level
> only a few degrees above abs 0 is detectable.
Levels a few degrees above abs 0 may be detectable, but that does not
imply intelligable. There are some very strong radiating bodies out
there; what fraction of a degree of the sky can you pick out with your
antenna? A source does not need to be close (in a distance sense) to
another source for them to appear close (in a directional sense) from
earth.
Anthony Campbell <ac@acampbell.org.uk> worte:
[snip]
> > I can think of a number of better ways of spending the money/time/spare
> > CPU cycles.
>
> For example?
Money:
* Feeding people
* Make sustainable industry economically viable
* Devise an atmosphere cleaner
* Land mine clearing programmes
* Better and more accessable education
Time:
* Looking after family
* Getting more excercise
* Learn another language
* Build a boat
* Strip a friend's car down
* Port octave to be a real win32 app
* I have a list of projects here somewhere...
Spare CPU cycles:
* I dunno, play quake or something.
John Hasler <john@dhh.gt.org> wrote:
> > ...since no-one has yet devised an antenna which radiates very
> > well in all directions...
>
> That's completely irrelevant.
>
> > The direction of propagation is perpendicular to the direction of motion
> > of the exciting charges (aren't they exciting? ;-) and so the wave
> > propagates in the horizontal plane (assuming that your antenna is
> > oriented that way.
>
> The radiation propagates in all directions (though the intensity varies
> around the antenna patern). I think you are confounding polarization and
> propagation.
Is that right? I thought that an ideal dipole would radiate only in a
plane. Obviously we don't have ideal dipoles, but that's what I thought
the theory said. I am more than happy to take correction here; I have
an exam on this stuff in not so many weeks time ;-)
Tom
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