When all port terminals of an arbitrary multi-port network are matched, the incident traveling wave an input by the nth port will be scattered to all other ports and emitted.
The vector network analyzer comes with a signal generator that can frequency scan a frequency band. If it is a single-port measurement, add the excitation signal to the port and measure the amplitude and phase of the reflected signal. Determine the impedance or reflection. For dual-port measurement, you can also measure the transmission parameters. Because it is obviously affected by distribution parameters, etc., the network analyzer must be calibrated before use.
