5 dB directional coupler: An RF passive component designed to sample or split off a small portion of the signal from a main transmission line, with a coupling value of 5 dB. This means approximately 31% of the input power is coupled to the secondary port while the rest continues through the main line. Widely used in RF and microwave systems for monitoring, signal measurement, feedback, or combining signals, it offers low insertion loss on the main line, good directivity, and stable performance over a broad frequency range.