I have used the RFA4059 sampler and similar for years. As PRCGUY points out you will need to calibrate it for each use as they are fiddly in this way. I have best luck putting a 6dB pad at the sample port so that my RF signal generator sees a better than 12dB return loss rather than open circuit.
You can determine the cal factor by comparing the normal measured receiver 12dB SINAD sensitivity with the generator level in dBM required to attain 12 dB SINAD through the sampler when it is terminated into a GOOD QUALITY resistive 50 OHM load. You should do this whether using a T sampler or directional coupler,
Be sure to use quality double shielded cables from your duplexer as well as test cabling. Avoid UHF connectors, BNC connectors (and inter-series adapters) unless that is only termination for the duplexer or sampler.
I keep on hand a pre made RG142 cable with type N male to quality Teflon insulated UHF male for these cases. Likewise a RG142 cable with type N male to quality Teflon insulated BNC male for feeding the sampler.
https://www.rfparts.com/signal-samplers/rfa4059a.html