A diplexer, which is a form of duplexer, is a three-port frequency-dependent device used either for transmitting or receiving purposes.

It can take different frequency bands and multiplex and de-multiplex two ports and put them onto one port on a single coaxial cable. Or a diplexer can route signals based on frequency that have a sufficient wideband and able to and transfer them to the air efficiently. A diplexer essentially combines signals onto one download or perhaps link those being passed through it onto radio transmitters.

This passive device can also split two signals apart from being on the same receive path like audio and data in Digital Subscriber Line (DSL) telephone systems or combine two signals into one single output on the same transmit path. It's also capable of forcing constant impedance to occupy a single broadcast frequency or enable one transmitter used by a broadcast station to operate and transmit multiple frequencies at the same time off one RF antenna.

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In addition, an antenna diplexer is an electronic component that can also be used to transmit and receive simultaneously using the same antenna - or it can enable a single antenna to be used while preventing the output of the other or else. They allow signals such as from an antenna and a satellite TV dish to share the same feed. Even more, it can separate signals according to the frequency they use.

On the downside, diplexers are not for signals that have overlapping frequency ranges. And if transmitters have frequencies too close together they cannot be combined successfully by a diplexer.

Typically, diplexers are used for high frequency satellite signals and low frequency CATV (cable television) / OTA (over-the-air) antenna signals. They can be also be found as part of a front end module for a mobile telecommunication system.

Diplexers are essential when trying to overcome the problem of the ban on building new transmitting towers in some areas. It is very hard nowadays to get permits to build new transmitting towers in many places as there are laws that say one can no longer build towers. Federal Aviation Administration (FAA) and Federal Communications Commission (FCC) rules related to tower erection restrictions and local zoning regulations drive many of these restrictions.

Although there are exceptions for businesses categorized as "public utilities" to build even in zones where it is explicitly prohibited, in reality constructing new towers is nowadays almost impossible.