The obvious advantage of having a broad bandwidth is that a greater frequency range will be covered at a given disposal of channels. Choosing a very narrow bandwidth , on the other hand, would make a broad transmission hard to intercept, since only a small piece of it would be registered in each channel. So choosing bandpass often comes down to personal taste, depending on which kind of signal you anticipate. Once the above problem has been settled, the big question remaining is what range in the radio spectrum to search. Photons at 1-10 GHz are referred to as microwaves , and this range has received most attention from SETI scientists.
Microwaves seem, for numerous reasons, to be the best means for intergalactic communication.
First, this is where the atmospheric interference and the galactic background noise are at their lowest levels, providing the perfect setting for an artificial signal to be detected. Second, microwaves have great penetration capability and are therefore not easily absorbed or scattered by interstellar stuff coming in their way. The left line of the waterhole marks out the natural emission frequency of hydrogen , at 1421 MHz . Now, since hydrogen is the most common atom in the whole universe, many people argue that its characteristic frequency must be the only logical meeting place for intergalactic civilizations. Therefore most SETI projects so far have chosen a scanning range centered at 1.42 GHz.