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

A circulator is a clever multi-port device that allows electromagnetic waves to travel from one port to the next, always in the same direction. You’ll most often encounter 3-port and 4-port circulators. Let’s dive into the differences between the two.

4-Port Circulator

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

The S-matrix (scattering matrix) of a perfectly matched, lossless, and 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

The 3-port circulator is typically built using a 120-degree H-plane waveguide. Another method involves using a stripline symmetrical Y-junction with a central ferrite disc.

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

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