Since they can influence return loss measurements more than we may wish.
Also, the treatment of measurements errors is mandatory. Therefore, these devices are studied in this article as well, including the measurement setup which is typically used. Usually, return loss is measured using a directional coupler. For one port devices, it is the only existing scattering parameter too!
From it, impedance levels can be calculated and amplitude variations in transmission lines predicted. It is a measure of the amount of power reflected at a port. Return loss is an important parameters of any RF (Radio Frequency) device. Measuring reflection with a directional coupler