archiv~1: SETI Multipath

SETI Multipath

David Woolley ( david@djwhome.demon.co.uk )
Sat, 21 Nov 1998 00:58:42 +0000 (GMT)

> but wide-spectrum ones do not? What exactly are your monochromatic
> signals bouncing off of to create the multipath and why don't the wide
> spectrum ones behave the same way?

I think what he is talking about is called frequency diversity in
short wave terrestial radio systems; you transmit on several frequencies
and then select the strongest one as the one to output in the receiver.
Although its not quite the same thing as short term fading, short wave
broadcast stations transmit on many bands simultaneously to ensure that
at least one will get through to its audience (which is not to say that
you couldn't design a frequency diversity system, only that it would
be overengineering for domestic use).

The other approach in terrestial systems is spatial diversity, where
you have multiple receive antennas, on the basis that when the
multipath signals cancel at one antenna they will add at another.
I suspect the very narrow angular spread of the signal means that you
would need impossibly large spatial separations to use this in a
SETI context.

The trivial method is time diversity; repeating the same transmission
over and over.