An RF duplexer is a key component in radio communication systems that enables a single antenna to transmit and receive signals simultaneously without interference, even when transmit and receive frequencies are close to each other. This functionality is essential in devices like mobile phones, two-way radios, base stations, and radar systems, where using separate antennas for transmission and reception is impractical due to size, cost, or space constraints.

The RF duplexer operates by leveraging frequency selectivity: it allows transmit signals (Tx) to pass from the transmitter to the antenna while blocking them from reaching the receiver, and simultaneously permits receive signals (Rx) to travel from the antenna to the receiver while preventing them from entering the transmitter. This is achieved using two sets of filters—one for the transmit frequency band and one for the receive band—connected to a common antenna port. The filters are designed to have high attenuation (typically 60 dB or more) in the opposite band, ensuring isolation between the transmitter and receiver.

There are two main types of RF duplexers: cavity duplexers and hybrid duplexers. Cavity duplexers use resonant cavities (hollow metal enclosures) to create narrowband filters, offering high Q-factor (sharp frequency selectivity) and high power handling (up to several kilowatts), making them ideal for base stations and high-power transmitters. Hybrid duplexers, on the other hand, use lumped components (inductors, capacitors) and hybrid couplers, providing a more compact design suitable for low-power devices like mobile phones, though with slightly lower isolation than cavity models.

Isolation between the transmit and receive ports is a critical specification, as even small amounts of transmit signal leaking into the receiver can overwhelm its sensitive circuitry, causing desensitization (reduced ability to detect weak signals). Insertion loss is another key parameter—the duplexer should introduce minimal loss (less than 1 dB) in the desired frequency bands to preserve signal strength. Power handling is also important for transmit paths, as duplexers must withstand the high power output of transmitters (from watts to kilowatts) without damage.

In cellular networks, RF duplexers enable frequency-division duplexing (FDD), where uplink (mobile to base station) and downlink (base station to mobile) signals use separate frequency bands. For example, in 4G LTE, a duplexer allows a smartphone to transmit on the uplink band while receiving on the downlink band simultaneously. In radar systems, duplexers protect the receiver from the high-power transmitted pulse, which is millions of times stronger than the reflected echo signal. By enabling efficient shared antenna use, RF duplexers reduce system complexity, cost, and size while maintaining reliable two-way communication.

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