Low-cost microwave/millimeter-wave impedance measuring scheme using a three-probe microstrip circuit

A simple, compact, and low-cost three-probe microstrip impedance measuring scheme has been developed. The impedance of an unknown load can be determined by measuring the coupling power levels at the three probes. The algorithm, formulas, and bandwidth criteria developed are very simple and can be easily used. The power coupling is arbitrary, and no precise directional coupler is required. Using a switch, only one power meter is needed to detect the output power levels of the three probes. The phase information is obtained by amplitude measurement. A detailed analysis and calibration method are developed. The calibration is based on three known standards (a short, an open, and a matched load). The reflection coefficient is calculated from power measurements at three locations along a uniform line terminated by an unknown load. The operating frequency bandwidth depends on the selection of probe locations. A prototype device has been built at X-band