S4P

S4P is a term used in RF engineering to denote a four-port S-parameter data file in the Touchstone format. It contains the scattering parameters that describe how a four-port network reflects and transmits signals between its ports. A network with four ports has 16 S-parameters, labeled S11 through S44, which capture the input reflections at each port as well as the coupling between any pair of ports.

The .s4p file extension is part of the Touchstone family of data files used to store frequency-dependent multi-port network data. A typical file begins with a header line that specifies the frequency unit and reference impedance (for example, a line like “# GHz S RI R 50”). Data lines follow, each corresponding to a frequency point and containing the 16 S-parameter values. These values can be presented in rectangular form (real and imaginary parts) or polar form (magnitude and phase), depending on the file’s formatting. The standard ordering for the parameters is S11, S12, S13, S14, S21, S22, S23, S24, S31, S32, S33, S34, S41, S42, S43, S44.

Applications and use cases include modeling and analyzing multi-port RF and microwave components such as multi-port filters, directional couplers, switches, antennas, and network analyzers. S4P files enable engineers to simulate network behavior, cascade components, and compute metrics like insertion loss, return loss, and isolation across frequency.

Notes: S4P files assume a linear, time-invariant network referenced to a specified impedance (commonly 50 ohms). They are one of several extensions in the Touchstone format used for multi-port data, alongside S2P and S3P variants.