An RF (Radio Frequency) coupler is a passive device used in RF and microwave systems to divide or combine electromagnetic signals in transmission lines. It is designed to allow a portion of the power in a main transmission line to be coupled to another port, which can be used for various purposes such as signal monitoring, testing, or providing power to another device. RF couplers come in various types, including directional couplers, hybrid couplers, and quadrature couplers, each serving specific applications. Directional couplers, for example, allow the measurement of power levels in a transmission line without disrupting the main signal flow.