Also known as: duplexer, cavity duplexer
A duplexer lets a transmitter and receiver operate on a single antenna at the same time by separating their two frequencies with sharp filters.1 Its whole purpose is isolation: preventing the powerful transmit signal from reaching — and desensitising or damaging — the receiver only a small frequency offset away. It is the defining component of a repeater.
Overview
A repeater listens on one frequency and re-transmits on another, offset by a fixed amount (the split). Both must use the same antenna, yet the transmitter may put out tens of watts while the receiver is trying to hear microwatts on a frequency perhaps only a few hundred kHz away. Bridging that enormous ratio — often needing 80–100 dB of TX-to-RX isolation — is exactly what a duplexer does, and why it is built from very selective filters.
How it works
A duplexer is a three-port network: antenna, transmit, receive. Between the antenna and each radio sits a filter tuned so that the wanted frequency passes with low loss while the other frequency is deeply rejected. Two common styles:
- Band-pass / band-reject (notch) — each leg has a sharp notch at the opposite frequency, so the TX leg notches out the receive frequency and vice versa. Compact and efficient for moderate splits.
- Band-pass — each leg passes only its own frequency; more insertion loss but broader rejection, useful in RF-dense sites.
The filters are almost always resonant cavity filters — tuned metal cans with very high Q — because only high-Q resonators can be that selective at high power with low loss. The achievable isolation depends on how far apart the two frequencies are: a wider split (or guard band) makes the filtering job easier.
Relevance to SDR
Duplexers are ubiquitous in land-mobile radio: every VHF/UHF repeater, and the base stations of the trunked systems GopherTrunk targets — P25, DMR, and similar — use one to share the tower antenna between transmit and receive. A duplexer separates TX and RX by frequency, which is a different approach from a circulator (separation by direction) and closely related to a diplexer (which splits two bands rather than a TX/RX pair on one service).
GopherTrunk is a receive-only decoder and neither contains nor needs a duplexer. The device matters to GT only as part of the infrastructure it monitors: a mistuned or degraded duplexer at a site raises the transmitter’s noise and can desensitise nearby receivers, degrading the signals GT decodes.