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 .