Thanks to onioncrush (PDF Investigation) found the Wiki for https://en.wikipedia.org/wiki/Tachyonic_antitelephone.
My guess now because of the key words Tachyonic Antitelephone, High-powered Signal Generator, Mass-Density Data Transmissions, Interstellar Listening (all things needed if you want to send a message for help into the past) post that what we are getting is messages from the future/different multiverse that these radio stations are picking up on. Perhaps because the NMS tapes have the word portal that it has something to do with multi-universes are opening (in NMS)up due to the portals? These are all guesses. Hope this helps. (and btw this is fun!) This might have something to more to due with NMS then we thought? ( I could be wrong big time thought!)
Oh add these to it!
Well if this signal gets into the NMS multiple universes we are screwed! Explains all those abandoned sites with the corrupted buildings! https://goo.gl/images/Bd2HHQ
"Reports indicated advanced technology at the sight apon registered long range scans even before I authorised the expedition exactly where the singnal originated. hypothesis: singnal uses the promise of tech to lure unsuspecting explorers to sight location, indicated by test results of the biomass which showed a similarity to existing DNA strands in all known sentient species, indicating that at some point these culture samples where not always apart of this biomass and might have been fellow adventures."
Myspace524 post also stated.
Echo-64.com: echo gives a voice to the past
multiverse-75.com: Break the laws of physics
myriad-70.com: sky is not the limit
superlumina-6c.com: help us make new discoveries
atlas-65.com: echo gives a voice to the past