Re: OT: Aliens in the heavans (was Re: seti@home)
On Thu, 07 Mar 2002 15:26:29 +1030, Tom Cook wrote:
>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)
<snip>
>
>Thankyou for that clarification.
You're welcome. I guess that Ham ticket is good for something ;-}
>
>[snip]
>> Which brings us to power density at the receiving end--there ain't much.
<snip>
>>...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.
Too true. However the sampling methods and applied math beyond my
simple algebra, differential and integral calculus are designed to
filter out the noise, leaving only concocted signals (hopefully).
>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.
Actually, this is probably the simplest problem to solve. For example,
phase differences among receiving points in the array can fine tune the
direction. Another method is to simulate Doppler shifts. Added to this
is sampling, where values are compared for adjacent readings. Probably
more important is the ability to slice the spectrum into *very* narrow
bands. For the seti project, bandwidths on the order of 0.075 Hertz are
used. Narrow beam width is a product of antenna gain, and not always
desired.
>
>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
What? The people working in Puerto Rico and UCB aren't buying food with
their paychecks?
> * Make sustainable industry economically viable
ALL scientific research has benefitted society in the long run--ALL.
> * Devise an atmosphere cleaner
Find a project you like, dig into your pocket, and support it.
> * Land mine clearing programmes
Ditto
> * Better and more accessable education
I thought this did benefit education. As above, find a poor kid and
send him to school on your nickel.
>
>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...
Right, and these are not affected directly by working at a radio
telescope any more than by working at a public health clinic.
>
>Spare CPU cycles:
> * I dunno, play quake or something.
Or maybe crunch numbers for seti?
>
>
>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.
Uh, not strictly true. In a loop antenna of 1 wavelength, the max
propagation is normal to the plane of the loop. Small loops, say 0.1
waves, propagate primarily parallel to the plane of the loop.
>>
>> 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 ;-)
Graph a sine function on polar coordinates. You will see a figure 8,
where the wire dipole is on the X-axis. Now rotate that figure 8 about
the X-axis. You will see a 3D annulus, or donut. This is the pattern
of an ideal half-wave dipole in free space. Note that signal strength
approaches zero as the angle of interest approaches 0 and pi radians.
You don't want to know about near earth patterns :-), but basically it
cuts off the part of the donut that would be buried in the ground and is
augmented +/- in a phased manner according to height and wave angle
above ground. Whew.
Jeez, two days, two recitations.
--
gt
Yes I fear I am living beyond my mental means--Nash
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