I am assuming the signal would be a tight beam aimed at a particular target star, in which case the frequency shift caused by the relative motion of both the transmitter and receiver would be taken into account, and removed from the signal. Of course, if ET is broadcasting omnidirectionally, or into a broad area of the sky, he would have no way of knowing how to Doppler-correct the signal for motion.

But I still think that using a harmonic or an transcendental number multiple of the frequency would satisfy both criteria: that the signal be obviously artificial, but still related to natural cosmic frequency. Of course, any motion, whether around a planet’s axis, a planetary orbit, or even the orbit of the star around the galaxy, would Doppler- shift the signal and require a broad-band strategy at the receiver.

Another useful concept is the “waterhole”, the space between the natural emission lines of the H+ and OH- radicals, also a common signal in the molecular clouds in the galactic plane. Both these emission lines straddle the optimum low-noise saddle in the radio background, so they would be a good place to listen. Besides, all living things gather around the waterhole, don’t they?

Yeah, its a needle in a haystack problem, all right.

At what frequency shall we look? A long spectrum search for a weak signal of unknown frequency is difficult. But, just in the most favoured radio region there lies a unique, objective standard of frequency, which must be known to every observer in the universe: the outstanding radio emission line at 1,420 Me./s. (21 centimeter wavelength) of neutral hydrogen. It is reasonable to expect that sensitive receivers for this frequency will be made at an early stage of the development of radio-astronomy. That would be the expectation of the operators of the assumed source, and the present state of terrestrial instruments indeed justifies the expectation. Therefore we think it most promising to search in the neighborhood of 1,420 Mc./s.

In all directions outside the plane of the galaxy the 21-cm. emission line does not emerge from the general background. For stars in directions far from the galactic plane search should then be made around that wavelength. However, the unknown Doppler shifts which arise from the motion of unseen planets suggest that the observed emission might be shifted up or down from the natural co-moving atomic frequency by plus or minus approximately 300 kilocycles per second (corresponding to velocities of plus or minus 100 kilometers per second). Closer to the galactic plane, where the 21-cm. line is strong, the source frequency would presumably move off to the wing of the natural line background as observed from the direction of the Sun.

Pyotr Makovetsky proposed a frequency = pi*1420Mc/s (obviously you couldn’t get EXACTLY that frequency) to distinguish the signal as artificial and remove interference from the natural H line.

This occurs in monatomic hydrogen, not the more familiar hydrogen molecule (H2) we are likely to encounter here on earth. In deep space, there is no shortage of H1 (in fact, most of the galaxy’s mass may be H1) and even though this is a very rare transition, it occurs often enough that the vast clouds of H1 in the galaxy give out a constant hiss at this frequency. In fact, the 21 cm line was the first to be detected by radio astronomers, and we have been using that wavelength to map galactic structure ever since.

Because of its ubiquity, and its location in a region of the microwave spectrum where there is little interference from other natural transmissions and little quantum noise in radio receivers, it has also been a frequency favored by SETI researchers. The reasoning is that any technically advanced species would be familiar with this wavelength, and presumably, they would know anyone capable of building radio telescopes would be listening on it during the course of their astronomy research.

But why transmit at a frequency where most of the galaxy’s hydrogen is emitting, potentially drowning out a faint signal? It seems more reasonable to transmit at some harmonic (twice or half) of the 21 cm line, providing that still is in the low-noise region of the radio background. Another strategy would be to transmit at some frequency formed by the product of 1420 MHz and some natural constant like pi or e. This could only be an artificially produced signal and would immediately be recognized as such. If I were doing active SETI, that’s where I would transmit, and if I were doing passive SETI, that’s where I’d listen. It always seemed to me a 21cm transmission would not be a good choice for SETI purposes.

As for a cometary origin for this signal, I’m not sure I’m convinced.
Is H1 found in the solar system at all? Would the amount carried aboard a comet (presumably from ionized water, methane or ammonia) be enough to be detectable? Granted, the comet is a lot closer than the neutral hydrogen clouds of the spiral arms, but there isn’t much matter there. I don’t know enough about this to do the numbers.