Andrew Nowicki <andrew@nospam.com> writes:

Louis Scheffer wrote:

As a first guess, cover the planetary zones of the
nearest million stars or so, with beams that are on
all the time  [Think of the beams as looking like a pincushion].
Modern phased array transmitters can generate this
number of beams, and keep them focused on the stars,
with little problem.  And this is roughly the number of stars
for which we have adequate proper motion and distance measurements.
(You need both since the star will be in a different place when the
beam gets there.)

This is the first clear signal in this noisy thread. :-)
We need to flesh out details. 

This has already been done.  See Appendix B, pages 303-320, of the
book SETI 2020.

If your message lasts 10
seconds and you are sending it to one million stars,
the same star receives the message about once every
3 months. If their SETI program works like our program,
they will probably miss our message.

Perhaps I was not clear.  All the beams, shining on all the stars,
are on all the time, just like your hologram proposal below.  
Whenever the receiver looks, they will see it.  This
is precisely to avoid the problem you just stated.

I have a better idea: a parallel, modulated microwave
beam is made by a conventional transmitter. A microwave
hologram splits the beam into thousands of narrow beams
aimed at the stars.

This is *exactly* what a phased array transmitter does,
except it generates the radiation locally at each point,
rather than by scattering coherent radiation from a central
source.  The math is identical.  However, the phased array
is able to cope with changes much better (as the stars move
with respect to each other, or you add more stars, you 
would need to change the hologram.)  With a phased array
you just update the coefficients with no physical changes.

    Lou Scheffer