A directional coupler is a passive RF component that allows a controlled amount of signal power to be tapped from a transmission line without significantly disturbing the main signal flow. It typically has four ports: input, output, coupled, and isolated. When a signal enters the input port, most of it continues to the output, while a small, defined portion—determined by the coupling factor (e.g., 10 dB, 20 dB)—is diverted to the coupled port. The isolated port ideally receives no power, ensuring that only signals traveling in one direction are sampled. These devices are engineered for high directivity (often ≥ 20 dB), meaning they can distinguish between forward and reflected signals, which is essential for accurate power monitoring and VSWR measurements. With low insertion loss (typically < 0.5 dB) and excellent impedance matching (VSWR ≤ 1.2), directional couplers are widely used in RF testing, signal monitoring, amplifier feedback loops, and communication systems. Their construction can vary—ranging from stripline and microstrip formats to waveguide and coaxial designs—depending on frequency range, power handling, and application.