Also known as: isolator, ferrite isolator
An isolator is a two-port passive device that lets a signal pass in one direction while strongly absorbing anything travelling the other way.1 It is, in effect, a circulator with one of its three ports permanently terminated in a matched load — so power reflected back toward the source is dumped into that load instead of returning. Its usual job is to protect a source, most often a power amplifier, from reflected power.
Overview
An isolator solves a specific problem: what happens when a source drives a load that is not perfectly matched. A poorly matched antenna or a switching filter reflects part of the incident power back toward the transmitter. That reflected wave can pull an oscillator off frequency, distort an amplifier, or in the worst case damage the output device. An isolator interposes a one-way path so the source always sees a good match and never sees the reflection.
How it works
Like a circulator, an isolator relies on a ferrite element in a static magnetic field, giving a direction-dependent phase shift. Forward-travelling energy passes port 1 to port 2 with low insertion loss; reverse-travelling energy is routed toward the third (internal) port, where a matched load absorbs it as heat. The device is rated by its isolation (how much reverse power it blocks, often 20–30 dB) and its forward insertion loss (ideally a few tenths of a dB). Because the reflected energy is dissipated rather than returned, the source sees a clean, near-constant load regardless of what the antenna is doing.
The amount of reflected power an isolator has to absorb is set by the load match — described equivalently by return loss or standing-wave ratio. A worse match means more energy heading back into the isolator’s load, which is why power-rated isolators have substantial heat-sinking.
Relevance to SDR
Isolators are standard on transmit chains: they guard base-station and repeater PAs against antenna mismatch and are common in test setups to keep reflections from corrupting measurements. They are the terminated cousin of the circulator and serve a different aim than a duplexer, which separates TX and RX by frequency rather than protecting a source by direction.
For GopherTrunk an isolator is transmit-side hardware and out of scope — GT is a receive-only decoder that does not transmit. It is worth knowing about only as part of the transmitter and repeater equipment whose signals GT ultimately decodes.
Sources
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Isolator (microwave) — Wikipedia, on ferrite isolators as terminated circulators that protect sources from reflected power. ↩