Abstract: A six port network for use in determining a complex reflection coefficient associated with a microwave network. A directional coupler is connected between first and second ports of the six port network for transmitting microwave energy from a microwave source connected to the first port, to the microwave network connected to the second port. This coupler provides two signals respectively representative of the microwave energy supplied to the first port, and of the transmitted microwave energy reflected at the microwave network. The six port network further comprises a microwave circuit for producing from the two representative signals, four microwave power readings respectively associated with third, fourth, fifth and sixth ports of the six port network and proportional to .vertline.a.vertline..sup.2, .vertline.1+.alpha.e.sup.j.phi. .GAMMA..vertline..sup.2, .vertline.1+.alpha.e.sup.j(.phi.+120.degree.) .GAMMA..vertline..sup.2 and .vertline.1+.alpha.e.sup.j(.phi.-120.degree.) .GAMMA..vertline..sup.