What three test loads are used to calibrate an RF vector network analyzer?

Calibrating an RF vector network analyzer (VNA) removes errors caused by the internal circuitry and the cables connecting it to the device under test. This calibration typically relies on a known set of standards known as 'SOL' or 'SOLT' (Short, Open, Load, Through). By measuring an ideal short circuit (0 ohms), an ideal open circuit (infinite ohms), and a perfect 50-ohm reference load, the VNA software can calculate mathematical error terms to compensate for phase and amplitude losses in the test setup. Other combinations, like different impedances or various lengths of coaxial cables, are incorrect because they do not provide the exact extremes (zero and infinity) and the baseline target (50 ohms) needed to map the Smith chart accurately.