Echo Two was the second subspace amplifier deployed by the United Earth starship Enterprise.
In September of 2151 Echo Two was destroyed, along with Echo One, by an unknown alien vessel shortly after deployment. (ENT: "Silent Enemy")
Echo Two was a CGI model designed by John Eaves and built at Eden FX by Fred Pienkos, who commented, "I love modelling, but I rarely get a chance to do it. I actually got to build Echo Two from scratch. That was fun, because they gave me a lot of freedom; the shape was in the concept drawings, but I got to add all the technology that was built into it. Then I did the sequence where it is dropped." (Star Trek: The Magazine Volume 3, Issue 7, p.52)
According to Star Trek: Star Charts (p. 60) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Echo Two was located in the Beta Quadrant.