But the capacitance of the open should never be ignored, and for frequencies above a few hundred MHz, it is necessary to take into account the variation of capacitance with frequency. However, for many applications the inductance of the short may be ignored. Just to make life complicated, neither the fringing capacitance of the open, nor the inductance of the short are independent of frequency. This again causes further delay, although not as much as the capacitance of the open standard. The open circuit has some capacitance between the inner and outer conductors.However, there are two other effects too. This length of transmission line adds a delay, so the phases are no longer 0° and 180°. Each calibration standard has a length of transmission line, after which the inner conductor is either left unconnected (an open standard) or shorted to the outer conductor (a short standard). We will explain why this happens, and why it is to normal.Īlthough the calibration standards of a SOLT (short, open, load, thru) calibration kits are usually called opens and shorts, a more accurate name for almost all calibration standards would be offset open and offset short. Although this is not shown, if the frequency is increased to approximately 7.9 GHz, the short appears as an open, and the open appears as a short! In fact, on a 26.5 GHz 3.5 mm kit, the arcs would have gone around the Smith Chart more than once! Many people do not expect this, so are confused, and believe there's a problem.

As you can see, they are arcs, even on this very expensive Keysight 85052B 26.5 GHz 3.5 mm calibration kit. The cyan trace shows the result of measuring the open circuit which is stored in memory. The sweep range has been limited to 7 GHz, with markers at the lowest frequency (50 MHz), as well as 500 MHz, 2 GHz, 4 GHz and 7 GHz. S-parameters and think they have a problem as after measuring the opens and shorts are shown as arcs, not dots as they expected! The photograph below shows the HP 8720D VNA screen after the calibration standards of an expensive HP 85052B (26.5 GHz 3.5 mm) were measured. In any text book on transmission lines which show a Smith Chart, an open circuit is shown on the right (phase angle = 0°) and short circuit on the left (phase angle = 180°), as shown on the following Smith ChartĪfter performing a calibration of a VNA, many people put the open and short calibration standards on the VNA, measure their

Why measuring the calibration standards on a VNA results in arcs, not dots on a Smith Chart as most people expect.

Why VNA calibration standards are arcs on a Smith Chart, not dots

Two port thru calibration smith chart