Preferred frequencies for SETI observations

SETI observational programs conducted over the last two decades, and most of those planned for the near future, have concentrated on searching for signals at microwave frequencies. Considerations of signal-to-noise ratio at the receiving end indicate that this is the correct approach if the broadcasting society is not concerned with directionality and transmits into a fairly large solid angle. However, if that society desires to transmit only a highly directional beacon, then it is not now possible, given our lack of knowledge of advanced space technology, to predict reliably whether microwave or infrared wavelengths are to be preferred in an optimum search program. Given the realities of current terrestrial technology, either the centimeter or millimeter domain is to be preferred to the infrared, independent of considerations of directionality. In any event, there does not appear to be any cosmically unique (“magic”) frequency at which to conduct SETI.