Re: OT: Aliens in the heavans (was Re: seti@home)
"Gary Turner" <kk5st@swbell.net> writes:
> On Thu, 07 Mar 2002 15:26:29 +1030, Tom Cook wrote:
> >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.
Of course Gary is absolutely correct. But there's also another minor
point. The surface of the Earth is not a plane. It's spherical. Even
if we could devise an antenna that only radiated in a perfect plane
(we can't) that signal is either going to be reflected by the
atmosphere, blocked by some terrain feature or radiate out into space
(neglecting the minute effects of gravity). So you'd still be sending
some signal out into space, which was my original counterpoint.
Now whether or not an antenna 10's-100's of light years way can detect
such signals I'll have to leave that to someone with actual practical
knowledge.
Gary
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