Also known as: circulator, ferrite circulator
A circulator is a passive three-port device that routes a signal one way around its ports: power entering port 1 leaves at port 2, power into port 2 leaves at port 3, and power into port 3 leaves at port 1 — but not in reverse.1 This nonreciprocal behaviour lets a transmitter and a receiver share a single antenna while keeping the transmit power out of the receiver.
Overview
A circulator’s usefulness comes entirely from its one-way routing. In a shared-antenna radio the transmitter connects to port 1, the antenna to port 2, and the receiver to port 3. Transmit power flows 1→2 out the antenna; incoming signals flow 2→3 into the receiver; and the huge transmit signal, which would swamp the receiver, is not routed 1→3. It is the nonreciprocity — the fact that the path depends on direction — that makes this possible; you cannot build the same behaviour from ordinary reciprocal components.
How it works
The one-way action comes from a ferrite element held in a static magnetic field from a permanent magnet. Microwave energy passing through a magnetised ferrite experiences a direction-dependent phase shift (a gyromagnetic effect related to Faraday rotation). The ports are arranged so that these phase shifts add constructively toward the “next” port and cancel toward the “previous” one. The result is low insertion loss in the wanted direction (a few tenths of a dB) and high isolation (often 20 dB or more) in the reverse direction.
Real circulators are band-limited and their isolation is finite, so the small leakage that does reach the receiver still matters in high-power systems. The quality of the antenna match also matters: energy reflected from a poorly matched antenna (see return loss) circulates onward to the next port rather than back to the source.
Relevance to SDR
Circulators are common in radar, high-power base stations, and any system where TX and RX must coexist on one antenna, and they are the building block of the closely related isolator — a circulator with one port terminated in a matched load to soak up reflected power and protect an amplifier. They achieve the same TX/RX-sharing goal as a duplexer but by direction rather than by frequency, so unlike a duplexer they let transmit and receive occupy the same frequency.
For GopherTrunk, a circulator is transmit-side infrastructure well outside its scope: GT is a receive-only decoder and does not transmit, so it never needs one. A circulator is relevant only as part of the base-station and repeater hardware that generates the signals GT listens to.
Sources
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Circulator — Wikipedia, on ferrite circulators, nonreciprocal routing, isolation, and shared-antenna use. ↩