I have a 3D simulation in CST microwave studio with a discrete port (S-parameter) setup near to the simulation but not physically connected to any of it. Upon running the simulation with a plane wave source, in the frequency domain solver, the resultant port voltage shows coupling to the port, with a complex frequency pattern showing resonances at some frequencies. Why is this? I had assumed that a port is essentially invisible to the simulation and so shouldn't have any coupling.