S-parameter

[aelster]

In just a few words as requested....

The letter "S" stands for "Scattering". Scattering parameters are widely used in electrical engineering to describe how a network responds to various input signals. The "network" in this case is the transmit array coil itself - an interconnected circuit of resistors, capacitors, inductors, amplifiers, etc.

In the S-parameter method, the network is assumed to have a linear response to small input signals. The network to be described by the S-parameters is connected to the outside world by "ports". Each port can be thought of as a pair of terminals through which electrical currents may flow.

A very simple network might have one input and one output port, which we might label Port 1 and Port 2. In this system there would be a matrix of 4 possible S-parameters, labelled S11, S12, S21, and S22.

The simplified meaning of the S-parameters are as follows:
S11: the reflected power when a signal is applied at various amplitudes and frequencies to Port 1
S22: the reflected power when a signal is applied at various amplitudes and frequencies to Port 2.
S12: the transmitted power recorded at Port 2 when a signal is applied to Port 1.
S21: the transmitted power recorded at Port 1 when a signal is applied to Port 2.

The S-parameters are complex numbers. As complex numbers they have magnitudes and phases. So the response of the system to various input combinations will vary depending on the magnitude, frequency and phase of the inputs. A transmit RF coil, for example, might have a different output response to an input signal at 64 MHz vs 128 MHz.

The model may be extended to an arbitrary number (n) of ports, requiring description by a matrix of (n x n) S-parameters. Such analysis is commonly used in antenna theory and analysis of other electrical systems.

Hope this helps.