---
title: "Short note: Microwave circulators"
subject: "RF and Microwave Engineering"
chapter: 4
key: "semester_7/rf_and_microwave_engineering:circulators-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/circulators-note
site: BCT Engineering
---

# Short note: Microwave circulators

RF and Microwave Engineering · Chapter 4: RF and Microwave Components and Devices

- Asked in: 2073 Magh, 5 marks; 2072 Asoj, 5 marks; 2070 Bhadra, 5 marks (asked 3 times)
- Video: [Circulator - Faraday Rotation - Microwave Engineering - UNIT V](https://www.youtube.com/watch?v=FVAb_tGLfYs) — Dr.P.Prasanna Murali krishna
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/circulators-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-4

## Question

Write a short note on microwave circulators.

## Short answer

A circulator is a non-reciprocal multi-port ferrite device that routes power from port 1→2→3→1 with all ports matched. An ideal 3-port S-matrix has zeros on the diagonal and unity coupling to the next port. Applications include duplexers, isolators (with a terminated port), and reflection amplifiers.

## Answer

A **circulator** is a multi-port (usually 3- or 4-port), **non-reciprocal** microwave device in which power entering any port is transmitted only to the next port in a fixed rotation (1 → 2 → 3 → 1) and not to the others. Non-reciprocity is produced by a **magnetized ferrite** placed at the junction.

```text
           port 2
             |
          +--+--+
          | /-> |
 port 1 --| | F | --- port 3
          | <-/ |
          +-----+
     F = biased ferrite disc
     1 -> 2 -> 3 -> 1
```

**S-matrix of an ideal 3-port circulator:**
```text
       | 0  0  1 |
 [S] = | 1  0  0 |
       | 0  1  0 |
```
- All ports matched (Sᵢᵢ = 0), lossless (unitary) and non-reciprocal (S21 ≠ S12).
- This agrees with the rule that a matched, lossless 3-port must be non-reciprocal.

**Working principle:** In the Y-junction circulator, the ferrite disc biased by a dc magnetic field splits the field into two counter-rotating modes with different propagation constants (because the ferrite permeability differs for the two senses of rotation). The disc size and bias field are chosen so that the two modes add at the next port and cancel at the remaining port. A 4-port circulator can be made with two magic tees and a non-reciprocal (Faraday rotation) phase shifter.

**Performance parameters:** insertion loss (typ. 0.3–0.5 dB), isolation (20–30 dB), VSWR and bandwidth.

**Applications:**
- **Duplexer** in radar and transceivers: transmitter → antenna → receiver using one antenna.
- **Isolator:** a circulator with port 3 terminated in a matched load, used to protect sources from reflections.
- Reflection-type (negative resistance) amplifiers such as tunnel diode and parametric amplifiers.
- Separating incident and reflected signals in measurement systems.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What produces non-reciprocity in a circulator?
   A. A magnetized ferrite at the junction
   B. A metal septum
   C. A resistive card
   D. A quarter-wave stub
   Answer: A. A dc-biased ferrite has different permeability for the two senses of rotation. (Key point: Non-reciprocity source)

2. An ideal 3-port circulator is
   A. Matched, lossless and non-reciprocal
   B. Reciprocal and lossless with one mismatched port
   C. Lossy and reciprocal
   D. Matched and reciprocal
   Answer: A. Sii = 0, the matrix is unitary but not symmetric, as required by the 3-port theorem. (Key point: Ideal S-matrix)

3. How can an isolator be made from a circulator?
   A. Short port 3
   B. Terminate port 3 in a matched load
   C. Open port 1
   D. Add a fourth port
   Answer: B. The matched load on port 3 absorbs reflections coming back, protecting the source. (Key point: Isolator)