[technics256]

If I recall my EE schooling correctly, the 1Hz resolution bandwidth significantly reduces the noise floor of the receiver, and this allows you to resolve smaller signals from the noise. Also, assuming it is sweeping, having a digital signal with greater bandwidth is no problem to find.

[madengr]

Yes, EE here too, but if the signal you're trying to detect is also noise like (pseudo random), it is averaged out with the noise. That's why I'm thinking that any moderately complex signal won't be detectable. We'd have to detect a signal that aliens purposely sent out at a very narrow bandwidth; essentially seconds per bit, or even narrower.

[jacquesm]

And that they didn't bother to compress to save on bandwidth.

To me that's the kicker. There's only a 100 year or so window during which you can catch a civilization's transmissions un-encrypted and un-compressed. After that it might as well be noise.

[tzs]

Even after they have switched to encrypting and compressing their transmissions, they might still have have unencrypted, uncompressed station identification messages transmitted periodically.