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A precision slotted-line measurement system is described that provides rapid impedance measurements over a wide dynamic range for both active and passive devices. Phase accuracy of 0.1Â° at 7.0 GHz and magnitude accuracy of 0.5 percent are obtainable for high VSWR measurements. Phase-locked sources used for both the slotted-line drive and the receiver local oscillator provide greater than 70 dB of dynamic range for an incident power level of -50 dBm at the unknown. A micrometer positioner for the slotted-line probe provides an order of magnitude improvement in probe-position accuracy over that obtainable using the conventional slotted-line scale and micrometer. A computer program simulates the slotted line and its associated connector by several sections of lossy transmission line together with appropriate discontinuity capacitance and contact loss. This form of model has the advantage that frequency dependence is implicitly included. Impedance measurement is simple and rapid. The only data required are the frequency, the position of an unknown null, either the width of the minimum or the VSWR, and the desired reference plane extension. The corrected data may be plotted directly on a Smith chart. Printout is in a convenient format for data evaluation.