A duplex filter (duplexer) is a radio engineering device designed to separate the receiving and transmitting signals during the operation of a duplex radio station. Typically, a duplex filter acts as a combiner, allowing the receiver and transmitter connectors of a duplex radio station to be connected to a single antenna. The main purpose of a duplex filter is to suppress the transmitter’s parasitic signal at the receiver input.
A duplex filter has three ports: for connecting the antenna, the radio reception path (receiver), and the radio transmission path (transmitter). It is widely used in building duplex repeaters.
The operation of a duplexer is based on the principle of passing a signal of one frequency while blocking a signal of another frequency. Thus, in the receiving branch, the useful signal received from the antenna freely passes to the receiver, while the filter blocks the carrier signals of the transmitter. In the opposite branch, the signal from the transmitter freely reaches the antenna, and its noise in the reception frequency range is blocked.
Structurally, a duplex filter consists of a set of LC filters, usually made in the form of cylinders with adjustable electrical capacitance for precise tuning.