Keysight Technologies N5244/5B Service Manual page 214

Theory of Operations
Signal Separation Group Operation
The coupled arm of the couplers carries the signal reflected from or
transmitted through the DUT, to the receiver for measurement (through front
panel jumpers), as inputs A, B, C, and D for 4-port models or inputs A and B for
2-port models. The coupling coefficient of the directional couplers is nominally
15 dB for all frequencies above 500 MHz. The coupling coefficient increases for
frequencies below 500 MHz.
5-46 Keysight N5244B/45B Service Guide