A diplexer (sometimes called a combiner) is a device that combines the radio frequency output of two or more radio transmitters into a single output. This is extremely useful in reducing the number of radio antennas on a tower, reducing the weight and loading from wind and potential ice, as well as the necessary size of the tower itself. Diplexers must be carefully designed and tuned to prevent intermodulation and keep reflected power (VSWR) to a minimum for each input transmitter and frequency. While diplexers can combine a relatively wide bandwidth, the major limitation comes with the antenna itself, which must be sufficiently wideband to accept all of the signals being passed through it, and transfer them to the air efficiently.

One of the most massive diplexers in use is atop the Empire State Building in New York, where over a dozen radio stations transmit through one four-panel antenna . Diplexers are also used to combine the audio and video carriers for television, and also for non-broadcast stations such as amateur radio.

Small diplexers are also used in the home, allowing direct broadcast satellite TV signals from the dish to the receiver to piggyback on one regular coaxial cable, along with lower-frequency signals from an outdoor terrestrial TV antenna for local channels. This is extremely useful in homes which are already pre-wired, as it eliminates the need for the difficult installation of unsightly extra cables. However, the cables must be the higher-quality RG6, as the less-expensive RG59 will not pass the high intermediate frequency (usually 950 to 1450MHz) which the LNB outputs from the feedhorn.

In this case, one diplexer joins the two signals together. Another diplexer then separates the signals to the receiver of the TV set, and the IRD of the DBS set-top box. More complex systems have a distribution amplifier, which allows each IRD to access multiple LNBs with different antenna polarizations. These usually have an antenna input and a diplexer, so that the antenna signal is also distributed along with the satellite.

See also: multiplexing