A duplexer is a tunable filter that allows a repeater’s
transmit and receive frequencies to share one common antenna. Without it, the
same repeater would require two antenna systems, one for the transmitter and
one for the receiver.
A duplexer is necessary when working in DIGITAL mode. From
experience, a repeater in ANALOG mode will work fine when using one antenna
system for the receiver and one for the transmitter. In DIGITAL mode, however,
the repeater will work erratically without a duplexer. Sometimes it will repeat
and other times it will not.