What is a Diplexer?

A diplexer, short for “dual-plexer,” is a passive electronic device used in RF (radio frequency) and microwave engineering to combine or separate signals in two different frequency bands.

It allows signals from two distinct frequency ranges to share a common transmission medium, such as a coaxial cable or an antenna, without interfering with each other. Diplexers are commonly used in various applications, including telecommunications, broadcasting, and RF engineering.

Diplexers typically consist of passive components, such as filters and impedance-matching networks, that are carefully designed to achieve the desired signal combining or separating functions. These components ensure that signals in the specified frequency bands are routed to their respective paths while minimizing signal loss and cross-interference.

Block Diagram

A block diagram is shown below. It has three terminals. Port 1 is for the lower frequency band, Port 2 is for the higher frequency band. The third terminal is a common port. An antenna can be connected to this.


There are two primary functions of a diplexer:

  • Signal Combining: In some applications, a diplexer is used to combine signals from two separate frequency bands onto a single transmission line or antenna. As an example a Wi-Fi diplexer combines signals from both ISM bands: 2.4 GHz and 5 GHz. By using a diplexer, the transmitter and receiver can share a common antenna while operating at different frequency bands.
  • Signal Separation: In other scenarios, a diplexer is used to separate incoming signals from a shared transmission line or antenna into two different frequency bands. This separation is necessary when different receivers or devices need to process signals in distinct frequency ranges without interference. For example, in a satellite TV system, a diplexer separates the incoming signals into two bands—one for satellite downlink and another for satellite uplink.


Referring to the block diagram above, the diplexer is specified in terms of the insertion loss, return loss and rejection for both Port 1 and Port 2. All these quantities are specified over the operating frequency range. It also has a max power rating.


Diplexers find applications in various areas, including:

  • Telecommunications: To combine or separate transmit and receive signals in mobile communication systems.
  • Broadcasting: In TV and radio broadcasting, to separate the transmitter output from the antenna into different frequency bands.
  • Satellite Communication: In satellite TV and VSAT (Very Small Aperture Terminal) systems to route signals between the satellite dish and the satellite modem.
  • Radar Systems: In radar applications, to separate signals for transmitting and receiving in radar antennas.


Diplexers are valuable components in RF and microwave systems, as they help efficiently manage and allocate available frequency bands for different functions, making it possible for multiple systems to coexist and operate without interference.

Related Posts

  • What’s the difference between a Diplexer and a Duplexer?
  • What’s the difference between a Diplexer and a Combiner?

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