Understanding RF Diplexers

An RF diplexer, also called an antenna diplexer, is a passive device that may be used to allow several transmitters to operate on a single antenna. Typically, an RF diplexer allows transmitters operating on two different frequencies to use the same antenna. The single antenna can then transmit on one frequency band while receiving on another band.

Understanding RF Diplexers

(Pixabay / Coyotechnical)

RF diplexers have many uses, such as in a cellular station where they facilitate transmission and reception simultaneously. The RF diplexer allows the antenna system to be used while at the same time preventing the transmitted signal from going into the receiver, which blocks the input. An RF diplexer may also be used by a broadcast station to transmit on several different frequencies simultaneously with the use of the same antenna.

Various types of RF diplexers include a filtered design. This design enables the transmitter and receiver to separate the signals according to the frequency allocation. RF diplexers may use band pass filters, but the use of a low and high pass filter is optimal for attaining the best rejection. This unique configuration results in two things:

  1. Signals with frequencies below the cutoff of the low pass filter go to port 2.
  2. Signals with frequencies above the cutoff of the high pass filter are directed to port 3.

The absence of a path between the two ports prevents signals from the low pass filters from channeling through the high pass filters (or the other way around). Additionally, the low pass and high pass diplexer keep the load impedance matched to the internal impedance of both the receiver and transmitter, which means that the impedance remains constant regardless of the load impedance that is connected to the other port.

The design of an RF diplexer needs to make accommodation for the degree of isolation or rejection that should exist between the high and low frequency transmitter and receiver ports. A lower diplexer performance envelope may be used when the diplexer is for receive mode only, like when FM and TV signals are combined. A step up in performance is needed when the diplexer is used in a transit mode only. The highest performance diplexer is required when one port is used for transmission and the other port is used for receiving signals simultaneously.

RF diplexers are used in many applications, such as military communications and cellular transmissions.