How do circulators and isolators work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

A circulator is a non-reciprocal three-port that routes power 1→2→3→1 in one direction only; it uses magnetized ferrite whose permeability depends on rotation sense. An isolator is a circulator with its third port terminated, passing power forward while absorbing reflections. Being non-reciprocal, their S-matrices are not symmetric.^[1]

What are circulators used for?

Separating transmit and receive paths on a shared antenna (radar, some full-duplex systems).
Protecting PAs from load mismatch (isolator at the output).
Reflection amplifiers and measurement setups.

What are the key specifications?

Insertion loss (often a few tenths of a dB), isolation (commonly 20 dB or more within band), return loss, bandwidth, power handling, and temperature range. Ferrite devices can generate passive intermodulation and are sensitive to external magnetic fields.

Are there non-ferrite circulators?

Active circulators using transistors, and time-modulated (magnet-free) circulators using switched or modulated networks, have been demonstrated for integration; they trade noise, linearity and power handling against size.

Frequently asked questions

Why are circulators non-reciprocal?

Biased ferrite breaks time-reversal symmetry, so S12 ≠ S21.

Does an isolator have insertion loss?

Yes, typically a few tenths of a dB forward.

Can an isolator handle full reflected power?

Only if its termination is rated for it.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.

Categories: Filters and passives

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