This will be in an upcoming app note, but I couldn’t find a good explanation on the internet of the difference between a directional coupler and a dual directional coupler, so here it is:
A dual directional coupler is exactly the same circuit as two directional couplers placed back to back.
The prime reason most companies make, and most people use, dual directional couplers is that most commercially available directional couplers only have one coupled port accessible. The other port is generally terminated in a near 50 ohm load that is tuned to improve the directivity. Because Marki directional couplers are capable of superior directivity without tuning on the reflected port our customers are able to measure both the forward and reflected power using a single directional coupler instead of a dual directional coupler.
Marki makes dual directional couplers with all four coupled ports accessible. In this case the two inner ports can be terminated with 50 ohm loads, and the outer ports can be connected to potentially mismatched loads without affecting the directivity or output at the other coupled port.