Many years ago TV sets had separate inputs for VHF and UHF. The
UHF/
VHF
Separator
Joiner was developed to separate the UHF and VHF signals coming from the antenna to feed the separate inputs for two tuners. There were also UVSJs that would combine two 300 ohm antennas for a 75 ohm input TV. Some TVs had two 2-screw 300 ohm inputs, and some had a 75 ohm coax input for VHF and a 2-screw input for UHF where you could mount a UHF loop.
Separate tuners for UHF and VHF. To tune to a UHF channel, you would set the VHF channel selector to U, and then rotate the UHF dial. The output of the UHF tuner was sent to the input of the VHF tuner:
The 2 screws in the center are for VHF and have the internal antenna connected. The 2 screws on the right are for a UHF antenna:
This UVSJ combines a UHF and a VHF antenna for a 75 ohm coax input TV:
This UVSJ separates the UHF and VHF signals from a 75 ohm all-band antenna for a TV with separate 300 ohm UHF and VHF inputs:
The common use these days is to combine a UHF antenna with a VHF antenna for the single input of current TVs, but the UVSJ is a reciprocal device that can be used to combine UHF and VHF signals OR separate UHF and VHF signals.
The UVSJ UHF/VHF Separator-Joiner belongs to the filter family of Diplexers. All UVSJs are Diplexers, but not all Diplexers are UVSJs; Diplexer is the more general term. A HLSJ is also a Diplexer, but for VHF-Low and VHF-High.