# Does insertion loss include return loss?

Does insertion loss include return loss? If we take a 3-ports circulator, does insertion loss from port 1 to port 2 consist of return loss of port 1, a power that leaks throug port 3 and power dissipation in the circulator?

Insertion loss is simply output power divided by incoming power, usually expressed in decibels.

It accounts for any effect that affects the ratio of outgoing power to incoming power.

In the case of a multi-port device it is the outgoing power at one particular output port divided by the incoming power at one particular input port.

It doesn't matter if the power is lost to internal heating, radiation, reflection at the input port, or delivered to another output port. If it affects the output power at the port under consideration it is "included" in the determination of the insertion loss.

Insertion loss, let's say the S21 you've asked about, is the ratio of the power delivered by port 2 into a 50 ohm load, divided by the power available from a 50 ohm source connected to port 1, with port 3 terminated in a 50 ohm load.

It's as simple as that, measure it in a perfect 50 ohm system.

Note that it's measured and specified assuming all three ports are connected to a 50 ohm system. The insertion loss will therefore include any the reflections at the circulator ports if they are not 50 ohms (Sxx not equal to 0), and will include any lack of directivity and subsequent reflection at port 3.

If you use the circulator in a system with other than a 50 ohm source and load, or if port 3 is terminated in other than 50 ohms, then the power loss you actually measure will be different. If you know your termination impedances, and all the other circulator S parameters, then you can compute what your measured loss will be.