When two signals are taken from two different cables and logically put on the same cable, the device used is known as a diplexer. If you wanted to add an antenna signal to an existing cable, you would use a diplexer. Note that there are various types of diplexers.

What is the purpose of a diplexer?

A diplexer is a passive (RF) filter component with three ports, which enables the sharing of a common antenna between two distinct frequency bands. This technology allows transmitters operating on different frequencies to use the same antenna and each band may both transmit and/or receive.

Is a diplexer the same as a combiner?

The diplexer is a different device than a passive combiner or splitter. The ports of a diplexer are frequency selective; the ports of a combiner are not. There is also a power "loss" difference - a combiner takes all the power delivered to the S port and equally divides it between the A and B ports.