40 dB directional coupler: An RF passive component designed to extract an extremely small fraction of the signal from a main transmission line, with a coupling factor of 40 dB—meaning only about 0.01% of the input power is coupled to the secondary port while nearly all the power (~99.99%) continues through the main path. Commonly used for ultra-low-level signal monitoring, precision measurement, calibration, and test setups in RF and microwave systems, it offers minimal insertion loss on the through line, excellent directivity, and stable performance across wide frequency ranges—ideal for sensitive lab and production environments.