The ports described in the S-Parameters and Ports section require a detailed specification of the mode, including the propagation constant and field profile. In situations with the mode being TEM, a lumped port might be a better choice. It also allows for connecting to an electrical circuit. It is not as accurate as the ordinary port in terms of calculating S-parameters, but it is easier to use. Lumped ports are used to model a connecting transmission line or as a voltage or current source applied between electrodes. For example, apply a lumped port directly to a printed circuit board or to the transmission line feed of a device. The lumped port must be applied between two metallic objects separated by a distance much smaller than the wavelength, that is a local quasi-static approximation must be justified. This is because the concept of port or gap voltage breaks down unless the gap is much smaller than the local wavelength.

A lumped port specified as an input port calculates the impedance, Zport, and S11 S-parameter for that port. The parameters are directly given by the relations

where Vport is the extracted voltage for the port given by the electric field line integral between the terminals averaged over the entire port. The current Iport is the averaged total current over all cross sections parallel to the terminals. Ports not specified as input ports only return the extracted voltage and current.