I was able to get a couple of plots done today. Using the Rigol DSA 815TG, I first did an insertion loss test. Results were good (see plot).
Next I checked the output of the FWD port from a range 1 MHZ to 500 MHz. Recall the advertised range is 25 to 400 MHz with a coupling of -50 dB, and I want to be able to use it up to 450 MHz. As you can see, my desires have been met and I can easily use it at 450 MHz. It's only off by 1.12 dB @ 450 MHz. You can see the output starts to curve @ 400 MHz (the advertised upper F). Even so, if you know how to account for a variation at any point, you can use it with no problems at all. Anything like that will be accounted for in the software of my project.
I also reversed all the connections, using the output as the input, the REV then being the FWD port. No differences in the plots were noted from the original setup.
Conclusion: So far this unit seems to be a very well designed dual directional coupler. I am not through with testing though. Next up I want to use a more powerful signal from a Baofeng hand held radio that can output up to 5W (+37 dBm). I will first accurately measure the transmitter power with my hp 437B power meter. Then connecting it to the coupler, I'll measure both the FWD and REV power levels at the same time using two power measurement devices, record the levels, and calculate an SWR using a 50 ohm load giving SWR 1 (it won't be 1.000 since nothing is perfect). For the conditions of SWR 2 & 3, I will use a little test rig I made years ago that is a switch selectable RF load and presents both these SWR levels to a test transmitter.
The Daiwa cross needle SWR meter allows you to read an SWR of up to 5 (the next last graph line is infinity). However, any SWR of 2 to 3 or above is reason to check your transmitting system. In other words, reading an actual value of SWR 4 or 5 doesn't really help you all that much.