Workman TAT CB / Ham Radio "T" Connector (Female-Male-Female) Coax Cable Adapter1 Male PL-259 Connector to 2 Female SO-239 Connectors
The T-connector has a couple of main uses.
First, if you want to run dual no-ground-plane antennas you add the T-connector to the back of the radio
and connect the coax from the two antenna leads to the ends of the "T".
Likewise, if you have a standard co-phase installation you can use individual runs of RG-59 A/U to each antenna instead of a pre-made co-phasing harness.
Workman Model Number TA-T