Scattering Parameters


S-Parameters are the measurement values derived from a Vector Network Analyzer (VNA). The S-Parameters describe how signals are going through a device. S-Parameters describe the relationship between different ports of the device in terms of amplitude and phase for a defined frequency bandwidth.

The S-parameters can be saved e.g. as  S1P-, S2P– or S4P-file that contains all the combinations of the reflection and transmissions. S4P shows how the DUT behaves with a signal in both forward and reverse directions. For a two port network, SParameters are defined as:

S Parameters

S11 is the input port voltage reflection coefficient
S12 is the reverse voltage gain
S21 is the forward voltage gain
S22 is the output port voltage reflection coefficient

The S-parameter matrix is used to determine the reflection coefficients and transmission gains from both sides of a twoport or analog multi-port network.

S-Parameters allow to determine Gain, Return Loss, VSWR and Insertion Loss values.

Details and formulas can be found on Wikipedia.

Return Loss, Insertion Loss, S-Parameters

Return loss, also known as reflection loss, is a measure of the fraction of power that is not delivered by a source to a load. If the power incident on a load is Pi and the power reflected by the load is Pr, then the return loss in decibels is


The better the load is matched to the source, the lower the reflected power and hence the higher the return loss. RL is a positive quantity if the reflected power is less than the incident power. If the load has a complex reflection coefficient ρ, then


That is, the return loss is the negative of the input reflection coefficient expressed in decibels.

When generalized to a terminated two ports, the return loss is defined with respect to the input reflection coefficient of
the terminated two port. The two port has the input reflection coefficient


where ΓL is the reflection coefficient of the load. Thus the return loss of a terminated two-port is


If the load is matched, i.e. ZL=Z0 (the system reference impedance), then


This return loss is also called the input return loss since the reflection coefficient is calculated at Port 1. The output return loss is calculated looking into Port 2 of the two-port, where now the termination at Port 1 is just the source impedance.

Please tell us about your requirements for measurements in our Anechoic Chamber.

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