Using open termination to intentionally reflect RF signal
It depends how accurate a calibration you need.

The open standard in the cal kit has some parasitic capacitance. When you use it to calibrate a network analyzer (VNA) you'll specify this (and a couple other parasitics) to the VNA to get the most accurate calibration.

It's usually possible to make a short standard that is closer to an ideal short than the open standard is to an ideal open. So if you want a more accurate calibration with a single standard that you assume is ideal, you'll probably want to use a short instead of an open. 

But even a short standard has some parasitic inductance and/or capacitance. Which you would want to characterize and use in the calibration math if you want a calibration comparable to what you get with a VNA.

And of course for the most accurate calibration, you'd probably want to combine open, short, and match standards like the VNA does. (Using the match standard, for example, would help you to separate the effects of reflections by the circulator, rather than at the end of the feed cable)

Of course there's also some complexity to the math for using a full set of well characterized standards for calibration, which would take more than a SE answer to describe.