Fall 2008 Issue

DR. SETI’s STARSHIP

Beckoning Beacons

By Dr. H. Paul Shuch, N6TX

Figure 1. Drift-scan sweep of Quasar 3C273, about 3 dB out
of the noise.

How do you know if your rig is working? “Easy,” you say. “Just call CQ and see who answers.” True, but if the nearest DX is light years away, you can grow old, cold, and lonely awaiting that “QRZ” from Beyond. Such is the dilemma facing those radio amateurs pursuing interstellar DX, a practice otherwise known as SETI.

Maybe you’re not trying to work DX at all, but are just an SWL. This, in truth, is more the case for the hundreds of amateur observers in the grass-roots, nonprofit SETI League who build sensitive microwave receiving stations, seeking radio evidence of technological civilizations out there among the stars. A receiving station is less costly than one that also transmits, for two reasons. The obvious reason is that a shortwave listener need not invest in a transmitter. However, beyond that truism, on a galactic scale (where transmitters need to radiate power levels that boggle the imagination), being a passive listener puts the burden of generating gigawatts right where it belongs —squarely on the shoulders of our (presumably older, wiser, and wealthier) cosmic companions. Earth is, after all, a young planet orbiting a young star. Other species, if they exist, are likely to be more ancient. If their planet has an expanding economy (a principle terrestrial economists call “inflation”), then they can afford better than we to radiate incredibly strong beacons, which just might reach our modest receivers as incredibly weak noise.

Now, receiving those feeble signals on Earth is no easy task. It requires searching through the quietest part of the spectrum, with the highest gain antennas, the most efficient feeds, the lowest noise receivers, and the cleverest digital signal processors we can muster. Thus, radio astronomers (whether professional or amateur) and those engaged in the scientific Search for Extra-Terrestrial Intelligence go to great pains for that extra tenth of a dB of sensitivity, as do weak-signal microwave DXers and moonbouncers. In fact, the SETI and EME communities have such a commonality of purpose that it makes good sense for them to share their technology, which is where our story begins.

Natural Calibration Beacons

“You can’t work ’em if you can’t hear ’em,” the saying goes. But how do you know if you can hear ’em, considering you don’t even know for sure that they exist?
Forget ET for just a moment, and consider that the universe is full of natural radio emissions. Stars, planets, moons, pulsars, quasars, supernova remnants, and even the chemicals that populate the black void between the stars all emit microwave radiation. Since its earliest days, radio astronomy has sought to study these emissions. Some are of known power, so you can calibrate your receiver’s sensitivity on them. My favorite quasar, for example, 3C273, is known to emit +46 Janskys (a linear measure of flux density) on Earth at a frequency of 1420 MHz. Therefore, when I receive it 3 dB out of the background noise (see figure 1), I know the sensitivity of my receive station is half that level, or +23 Janskys. (This is, incidentally, a level of sensitivity typical of amateur radio telescopes and 23-cm moonbounce stations.) If I monitor 3C273 after tweaking my station and the received signal strength increases, I know my receiver is working better. If signal strength goes down, I know I should have left well enough alone.

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