A directional coupler is an accessory with which engineers can combine signals, isolate them, or even eliminate them. Directional couplers diverge the input signal of a microwave or radio frequency-generating device from its distributed power, splitting the input power between both the coupled and output ports to achieve a desired coupling ratio. Each circuit in a directional coupler is composed of four ports: coupled, input, transmitted and isolated. When choosing a coupler for testing purposes, engineers seek out those with high directivity, stalwart impedance and a wide operational bandwidth.