In article <T1059813797@djwhome.demon.co.uk>,
David Woolley <david@djwhome.demon.co.uk> wrote:

I'm slightly surprised that people aren't doing that already; or is
it not reasonable to detect at a couple of parsecs out a signal beamed
with a three-kilowatt signal from an antenna costing $5000 (figures picked

It's not reasonable.  The SETI@Home sensitivity is around 1GW EIRP
at 1 parsec, so you would need an antenna gain of around 60dB.

That's roughly what I expected; though I hadn't quite realised SETI
was such an Arecibo-shall-speak-unto-Arecibo project.  I would have
picked a frequency a good deal less than 23cm to avoid the enormous-
dish issue; but I know sufficiently little about radio that I have
no idea whether transmitting at 1cm = 300GHz is reasonable.  I
suspect not, especially not in a 10^-10 frequency band.

Umm.  I know an amateur astronomer with a 90cm reflecting telescope,
which is starting to get quite exciting antenna gains at 500nm, but
whilst a kilowatt of light is trivial to produce, a kilowatt of light
in a specific tenth-of-an-Angstrom band isn't (and you probably need
much sharper than a tenth of an Angstom, even if you transmit straight
in the middle of a Fraunhofer line).  Also the signalling becomes
rather obvious through scattering off atmospheric dust, and given the
treaty issue it's rather unwise to signal in a way
green-pillar-in-the-sky obvious to the entire population of Perth.

I have no idea how elaborate the hardware used for laser ranging to
the Moon was, though I don't recall the telescope being that enormous.

Tom