A measuring device comprises at least one port, an external cable connected to the at least one port, a transmitter connected to the at least one port through a switch, a receiver connected to the at least one port through the switch and a reference path connecting the transmitter with the receiver through the switch. The cable is connected to calibration elements, e.g., short and open. The switch alternately switch in a manner that in a first state the transmitter and the receiver are connected to the at least one port and that in a second state the transmitter and the receiver are connected via the reference path.