Experimental evidence is presented that shows the conventional expression relating the transducer efficiency of a two-port to measured scattering parameters is incorrect when the characteristic impedance at one of the ports is complex. This evidence is based on the measurement of the power from a microwave source transferred through a probe to a lossy coplanar waveguide. The conventional expression differs from the measurement by up to 20%. An alternative, accounting for the complex characteristic impedance, gives accurate results.
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