david@djwhome.demon.co.uk (David Woolley) writes:

In article <4095e404$1@news.cadence.com>,
Louis Scheffer <lou@cadence.com> wrote:

This is something we can do with existing technology for about
$200M, for a beam bright enough that we ourselves can detect it.

Which is probably several times the total formal spend on SETI to date.
SETI is a very low budget activity compared with other space science, 
and is almost exclusively privately funded.

True, this is much more than current SETI spending.  But we don't need
to start with a system that broadcasts to 1M stars - we could start with
1K stars and work up (phased arrays can be expanded after they are built).
This would bring the budget in line with current SETI expenditures.

Active SETI is politically sensitive, both on cost grounds and because
of the fear that one interest group will transmit a biassed view
of earth (particularly on political or religious issues) or that ETIs
will launch a pre-emptive strike against anyone they detect.

Very true, but when the cost comes down so much it can easily be done by
private groups, it may be hard to prevent.  For example, there is 
already a company that will send a message of your choosing from a 
radio telescope in Russia.

http://www.callyourstar.com/cosmiccall.html

And this cost will come down further as a consequence of Moore's
law.

There is a significant energy cost that is not subject to Moore's law.

This seems intuitively reasonable, but is not true.  In a phased array,
replacing modules with smaller cheaper modules saves power.  This is
because the EIRP goes as N^2, so for constant EIRP, the power per module 
goes down faster than the number of modules goes up.  

For example, imagine a transmitter with 1M modules of 1 watt each.  
If transmitters become cheaper, you can replace this with 10M 
modules of 10mw each, for a total power savings of a factor of 10.  
(The EIRP stays the same since it goes at the number of
elements squared.)  Making large numbers of smaller transmitters is
exactly what Moore's law tells us will be cheaper in the future.

For current, non-arrayed systems, transmit tubes don't last long and
represent a signicant cost.

True, but no rational SETI transmitter would use vacuum tubes any more,
since they don't last very long and represent a significant cost.

     Lou Scheffer