As of my last update in September 2021, the term "unilateralization" doesn't appear to be a widely recognized concept in the field of microwave engineering, particularly in the context of S-parameters. However, I can still provide some insights based on common microwave engineering principles and the understanding of S-parameters.
S-Parameters:
S-parameters (Scattering parameters) are essential in microwave engineering for characterizing the behavior of linear, multi-port microwave devices and networks. These parameters describe how electromagnetic waves interact with the device under test. The most common S-parameters are S11, S12, S21, and S22, which represent the reflection and transmission coefficients for a two-port network.
Unilateral Devices:
In the context of electronic circuits, a "unilateral" device is one that only allows signals to flow in one direction, meaning it exhibits no reciprocity. In unilateral devices, the flow of energy is predominantly from the input to the output, and there is a significant difference in their input and output characteristics.
Impact on S-Parameters:
When dealing with bilateral (reciprocal) devices, the S-parameters maintain symmetry, i.e., S12 equals S21, and S11 equals S22. This symmetry is due to the device's behavior being the same when the input and output ports are interchanged.
However, for unilateral devices, this symmetry is broken. The S-parameters of a unilateral device are different when you reverse the input and output ports. This is primarily because the device's internal characteristics result in different interactions with the input and output signals.
In practice, when characterizing a unilateral device using S-parameters, you need to take this asymmetry into account. It means that if you interchange the input and output ports of a unilateral device and measure the S-parameters, you'll get different values than when measured in the original orientation.
To sum up, the concept of "unilateralization" is related to the behavior of unilateral (non-reciprocal) devices and how it affects the S-parameters. The S-parameters of unilateral devices are not symmetric and change when the input and output ports are interchanged, making it crucial to consider this asymmetry in practical applications involving such devices.