4-Port Circulator vs. 3-Port Circulator: Key Differences

A circulator is a multiport device that allows electromagnetic waves to travel from one port to the next immediate port in a single direction. 3-port and 4-port circulators are the most commonly used types. Here, we will explore the key differences between these two types.

4-Port Circulator

A 4-port circulator can be constructed using two magic-T junctions and a non-reciprocal 180-degree phase shifter. Alternatively, it can be built using a combination of two 3dB side-hole directional couplers with two non-reciprocal phase shifters.

The S-matrix (scattering matrix) of a perfectly matched, lossless, non-reciprocal 4-port circulator is defined as follows:

[S] = |0 0 0 1 | |1 0 0 0 | |0 1 0 0 | |0 0 1 0 |

## 3-Port Circulator

![3 port circulator](../../assets/3-port-circulator.jpg)

A 3-port circulator is typically formed using a 120-degree H-plane waveguide or by employing a stripline symmetrical Y-junction with a central ferrite disc.

The S-matrix of a perfectly matched, lossless, non-reciprocal 3-port circulator is defined as follows:

[S] = |0 0 S13| |S21 0 0 | |0 S32 0 |