How would you define and describe a radio signal from near or within the solar system, omnidirectional, very complex and containing intricate but not necessarily artificial patterns? Invent a notional signal that could meet these parameters. You don't need to worry about a source, but it would be nice if it could be mistaken for something natural.
Not a radio astronomer (I've so far stuck to optical in my journey to my degrees), but I did stay at a Holiday Inn Express last night! By which I mean, I had to do a radio lab for my astronomy lab last quarter. Anyway, I would describe it as having very complex modulation with a packed frequency spectrum, lots of power everywhere.
This will not be mistaken for something natural, IF it is as you describe it (more on that a bit later). No way in hell if it's as complex as you say, especially if it keeps repeating. All the natural solar system radio sources look like a big burst of noise, even if they do have periodicities in them. In astronomer speak, their power spectra tend to be very broad, no definite sharp peaks anywhere. This signal would have a definite peak that stood out above the others, the carrier wave, which would be the strongest. However, like Nuke mentioned, you can easily make an artificial signal almost completely indistinguishable from natural sources if you apply enough modulation to it; most modern radio signals are this way, such that they look like random noise until you apply the correct demodulation. Since this is a high tech artificial device, I would imagine that it would be much the same way, and indistinguishable from natural sources.
But what is the frequency(ies) this is broadcasting on? In certain bands, it would be a dead giveaway that it's artificial, because nothing natural (that we know of) emits at those frequencies. For example, if it emits at 21 cm, it'll be immediately known that something funny is going on, because the only natural source is neutral hydrogen, and the only appreciable emission is on a galactic scale. Seeing high flux from inside the Solar System would raise immediate "WTF?" looks and exclamations. If, on the other hand, it emits around 20 MHz, well, Jupiter's magnetosphere, the Sun, the Milky Way, and quite a lot else radiates at that. So it could be disguised for a long time if the modulation is such that it can fight across the noise produced by all the stuff around it.
In any case, Mika provided a good overview of some radio telescope basics; I'll expand on what he's written. A radio telescope doesn't have appreciable side lobes by design; to achieve as much resolution as possible without resorting to interferometry, the current design of a big dish and shielded antenna kinda necessitates itself. The dish also obviously helps gather more radiation. A typical radio telescope will have multiple antennas, each of which is called a "feed," each of which is tuned to a different frequency range, along with a single polarization. The signal processing is fairly simple; the data is read in from each feed, amplified, down-converted, and separated out by Fourier analysis to get the amount of power at each frequency, and also to read the radiation's phase. Since this is usually all radio astronomers care about, this is where it usually stops. If it's a transient search, more Fourier stuff can be used to get power spectra over time, etc.
Mika, thank you, that was a good read. If it's not too much bother, what would you say if the recipient was a distributed antenna, as in radio interferometry?
Really, not much changes. With interferometry, you get better resolution, and that's really it. Unless you are using VLBI (Very Long Baseline Interferometry), you won't be able to tell if it's a focused tight beam or an omnidirectional emitter; depending on its distance, maybe not even then. If the source is on Jupiter and you are standing on Earth, even if the source is focusing the radiation as tightly as possible (a radio laser, if you will), the beam will still probably cover the planet. Basically, you need to be able to observe at significantly different angles with respect to the source. This may not be possible except with ships, and they probably won't be set up for the wavelengths that this thing would use to disguise itself, so they may not even be able to see it.
By the way, if you want me to vet any astronomical dialog or anything once it's written, I'd be glad to help.
