Teaching Directional Couplers and Directional Bridges

Directional couplers and bridges are used as components of measurement chains to monitor forward and reflected power in conducted and radiated measurements or as building blocks of measuring instruments like scalar and vector network analyzers. In my experience as a master’s degree and PhD student these devices were presented, at most, for the purpose of illustrating the principle of operation but without any mention of their metrological characteristics and limits of accuracy. The scope of this contribution is to discuss, after having introduced the principle of operation, the essential metrological characteristics of coaxial directional couplers and resistive directional bridges, namely coupling, isolation, directivity, insertion loss and mismatch and provide the basic equations useful to quantify errors in measurement of forward and reflected power.