IIRC, it is through a UHF signal.
We added our first of what we hope are many duplex links in the system in 1995. The Mt. Thorodin site (145.310 MHz) moved into a new building. With this move, a UHF linking repeater was added at that site. East and west slope users are now linked via UHF between Mt. Thorodin and the Mt. Baldy (146.790 MHz) site. New linking methods improved audio quality to the best we've ever seen. Of note, if you hear a fast ‘K' proceed signal, you'll know signal was generated by direct VHF input to the 145.310 MHz repeater or a repeater that links to the 310 via VHF. If you hear a slow ‘K' proceed signal, it was generated because the user was linking to the Mt. Thorodin site via a UHF link.