This document discusses the analysis of an ideal “tandem” directional coupler using lumped element circuit theory.
A directional coupler is a four port device that is used to sample the forward and reflected wave in a transmission line. It is a key component of a VNA and can be used to measure power flowing in a transmission line, but it can have countless possible uses. Generally they are built using distributed elements structures, but there are also versions built using lumped elements.
The tandem directional coupler, or also tandem bridge, is one of those directional couplers that is built using a lumped element approach. This kind of directional coupler, famous for radio-hams, has inherently large bandwidth and can handle a lot of power. It is also well suited for low frequency scenarios, down to the kHz range. It is composed of two current transformers, or two transformers with a high turn ratio, and two termination resistors with the desired characteristic impedance \(Z_0\), which can be arbitrary but usually 50\(\Omega\).
