20 dB directional coupler: An RF passive device designed to extract a very small portion of the signal from a main transmission line, with a coupling factor of 20 dB—meaning only about 1% of the input power is coupled to the secondary port while ~99% continues through the main line. Commonly used for low-level signal sampling, monitoring, measurement, and feedback in RF and microwave systems, it ensures very low insertion loss on the through path, good directivity, and stable performance across wide frequency ranges, supporting applications like GSM, LTE, and broadband wireless networks.