Ali, I have often thought that looking for data at that 'water hole' on the 21 CM band may be a waste of time. After all do we send 21CM data at the 1000 nearest starts?, no we have radar, TV , AM, FM and spread spectrum transmissions. In fact as we make more and more use of SS we make our transmissions merge witgh the noise. True the power bandwidth changes, but not grossly. I suspect an advanced civilization may use SS so extensively that they are not detectable, to say nothing of fiber pathways.
I have thought for some time that the most powerful transmitters on earth are the 50 cycle and 60 cycle grids on various continents. These phase coherent grids will generate powerful signals that will couple through the atmosphere into space capacitively via the ionosphere. True they are long wavelength, but they have a power advantage by
many orders of magnitude over all other continuous sources.
In a similar vein AC power and transmission may well have been discovered and used by the ET races we seek. So possibly a search from 30 hertz to 1000(or more) with a long wavelength antenna array might find such a system radiating coherently with their AC grid. Wavelengths are long, at 100 hertz = 3,000,000 meters so a 1/4 wave is 750KM, which is big and hard to steer. In high orbit, or at a lagrange point such an antenna is manageable and could be made from light weight materials. Imagine a broad band yagi based on this with 4 elements. Once assembled of suitably stiff conductive materials it would be able to inspect stars one at a time over the 30-2000Hertz range and could be slowly steered from star to star with small thrusters under earth control, in fact with the proper choice of targets you will have a search spiral as you move slightly from one to the other. Such a small number of elements will have wide lobes so any detected signal will have to be narrowed by some experimental methods like sweeping across the signal in several angles of attack.
Such an AC signal from an entire civilization will be easily detectable, far more so that what SETI is looking for, and the planet will emit it as a matter of course, the same way we 'shine' at 50 and 60 hertz.
Bill |
