In article <40990158.7040305@somewhere.com>,
"Rich <someone@somewhere.com>" <> (improbable address) wrote:

Do you mean current (or past) emissions, or beamed transmissions of the

Planetary radar is detectable to something like 1,000 LY with current
systems and relatively short integration times.  We've been doing that
sort of transmission for quite some time, and even decades before that,
a reasonable extrapolation of technology would have predicted that it
would be possible.  Planetary radar will tend to give false negatives at
the confirmation stage because it won't be repeatable.

The Allen array is claimed to have the ability to detect the equivalent of
our analogue TV carriers from local stars.  (I suspect this is with
a long observation time.)

to look, and you need to know what frequency at which to look.

Most searches cover a wide range of frequencies, so frequency is not
critical.  In any case that they are difficult to detect, whilst
possible, is a justification for continuing, as it is reasonable
grounds for assuming that no detection doesn't indicate no ETI's.

Above you say "interstellar", now you talk about galactic sources. Do
you think we could detect civilizations across the milky way?

Arecibo is detectable by Arecibo to the best part of 1000LY, in a quite
conservative processing configuration.  Detecting itself at the nearest
point on the effective edge of the disk ought to require significantly
less than 1 day's (around an hour, I think) integration time (we are
nearly on the centre plane), so current technology is theoretically
capable of making detections over galactically significant distances.

Finding something within about 100LY is, of course, more attractive,
and what targetted SETI aims for.

It has been claimed that a 1 watt transmitter would be sufficient
with modern receivers.

I don't know where you got that figure.  You need very approximately
1GW EIRP from the nearest star to get the S@H threshold of 22 times mean
noise power in 0.075Hz and time * bandwidth = 1.  To detect 1W EIRP at
that distance, you would need an observation time of about 1E19 seconds!
1 Watt feed point power is detectable from local stars if the transmit
aperture is similar to Arecibo, although it will require around 20
minutes observation.