The AV-587 antenna coupler is an antenna signal splitter/diplexer that provides three outputs for antennas with only a single output port. One output provides a combined VOR/LOC/GS signal, a second provides a VOR/LOC signal, and a third output provides the GS signal.
The AV-587 is designed for aircraft instrument panels that include a newer navigational radio and a second, older navigational radio. Newer radios have only one port to receive all three (VOR/LOC/GS) signals while legacy radios typically have separate inputs for VOR/LOC and GS signals. The AV-587 diplexer provides the necessary signals to both the newer and older radio using a single antenna.