Use cal kit standard #5 for sliding load.
If your calibration kit contains a sliding load, you should
be aware that some models of the HP 8753 and HP 8720 network
analyzer families will only calibrate properly if the sliding
load is standard #5. If you assign the sliding load to
any other standard number, the calibration will not work - the
error corrected data will be grossly wrong in the sliding
load's frequency range. Furthermore, on these VNAs standard
#5 can only be used as a sliding load - when you attempt
to measure #5 during a calibration, the VNA will ask you to
"set the slide" no matter how you defined the
standard. This problem is corrected in the latest models.