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Message: <blockquote data-quote="spiney" data-source="post: 247072" data-attributes="member: 192438">In free space, all radio frequencies will - of course - propogate equally well!DIfficulties are various absorbtions in the earth's atmosphere, then - once outside that - in interstellar space, where there's vast amounts of molecular and ionised hygrogen. Although maybe just 1 particle per cubic meter (!), over astromnomical distances that comes to a lot of stuff!What the excitation modes of these clouds are, I don't know offhand but a lot will be in microwaves region (translational/rotational modes), no doubt various SETI and other (microwave spectroscopy) websites list the frequencies.</blockquote>