How to calculate the values of resistive return loss bridge？

Hi, i am trying to analysis the circuit used in Nanovna. I do feel confused about the resistive return loss bridge in TX
port. I try to simulate in multisim but it do not works as i wish. I find directional couplers are often used in S11 measurement circuit.  Could you tell me how the resistive return loss bridge works and how to calculate the values of those resistor?
Thank you very much!