Flysight,
You can see the “typical” directivity of the ZFDC-10 rolls off to about 20 dB at around 1900 MHz, and from personal experience I can tell you it is horrible above 2 GHz. We do offer a better directional coupler that goes to 6 GHz.
That being said, there is additional impedance mismatch from the SA44B and TG44A. Adding a 3 to 6 dB pad (SMA fixed attenuator) to the SMA port of each can improve your return loss a great deal, and should get you very close to the performance of the directional coupler.
Since we do not have phase data, we cannot do an open-short-load calibration, but a good directional coupler and a couple of 3-6 dB pads should yield 25-30 dB of directivity.
