The Pahvan transmitter was a massive crystalline tower on the surface of Pahvo. It broadcasted the unique "music" produced by the planet into space, and generated interference that blocked transporter use in a radius of thirty kilometers.
On stardate 1308.9, the USS Discovery sent an away team to connect a signal inhibitor to the transmitter, so as to modify the electromagnetic frequency of its signal and use it as a kind of SONAR to detect cloaked Klingon ships. Michael Burnham later connected the inhibitor to the transmitter in an effort to contact the USS Discovery after Saru became compromised, but Saru damaged the inhibitor.
The Pahvans repaired the signal inhibitor and allowed the Discovery to beam back its away team. They then increased their transmitter signal strength by 10^12 power and sent an invitation to Pahvo on Federation and Klingon subspace bands. The message was detected by Kol, who took the Sarcophagus to Pahvo. (DIS: "Si Vis Pacem, Para Bellum")