I realize this is an old thread, but thought someone might benefit from seeing a concrete example. I built an open/short/load set of female SMA standards very similar to Claudio's and others. For the load I used 2x 100-ohm thin film resistors from Susumu:

https://www.digikey.com/product-detail/en/susumu/RG2012V-101-P-T1/RG2012V100PCT-ND/1248326METAS VNA Tools also offers a cal-kit model fitting tool, which I used in conjunction with Claudio's Octave script. The VNA Tools fitter is a bit more user friendly, offering a nice dialog where you can set the parameter initialization and lock parameters at will, while watching a graph of the fit accuracy. OTOH I caught VNA Tools making an error which unfortunately means I cannot entirely trust its output; under certain conditions when the parameters hit their limits, the rms fit statistic and the graphs are incorrect. So I mostly used VNA Tools to get a feel for how to constrain the parameters, then used the Octave script for final optimization.

I did not have access to a fancy calibrated VNA for the initial measurements, but a friend loaned me his Copper Mountain cal kit (good to 9 GHz). I entered the data-sheet coefs into my Agilent 4396B combo analyzer (300 kHz to 1.8 GHz) as a user kit, then measured my homemade standards. I fixed the test cable in place as best I could for stable measurements. Results:

Open fit -61 dB rms:

offset delay = 38.07 ps

offset loss = 11.96 Gohm/s

offset Z0 = 50 ohms (constrained)

C0 = -0.1448e-15 F

C1 = 1904e-27 F/Hz

C2 = -968e-36 F/Hz^2

Short fit -59 dB rms:

offset delay = 37.14 ps

offset loss = 2.67 Gohm/s

offset Z0 = 49.44 ohms

Load fit -77 dB rms:

offset delay = 80.4 ps

offset loss = 0

offset Z0 = 50.95 ohms

arbitrary termination = 50.02 ohms (of dubious utility, but, why not)

The load measurement is curious. My resistors were .02% tolerance, and at DC the impedance is as close to 50 ohms as I am able to measure, yet the calibrated return-loss measurement is "only" -42 dB at ~ 1 MHz. The 4396B supports "arbitrary impedance" load standards, so by calling my load 49.21 ohms I get an excellent fit to the measured data. Something doesn't add up.

**UPDATE:** The reason for this was fairly obvious: I measured the primary calibration standard and found its DC resistance ~ 50.8 ohms! This is still within spec (better than -38 dB return loss to 9 GHz), but much-improved modeling results were obtained by entering that load in the VNA as "arbitrary impedance" with the measured DC value, re-measuring the DIY standards, and re-fitting. The modeling results are now better all around. Data and plots updated to reflect this.

In the meantime, I had to rip apart my VNA and test set again to try and diagnose a problem with two-port measurements. Disappointingly, all indications are that the 4396B does not support test sets with mechanical transfer switches (see

https://www.eevblog.com/forum/repair/agilent-4396b-spectrumnetworkimpedance-analyzer-repair/msg2459730/#msg2459730) despite it being a contemporary of the 8753 series

Thus I am now looking at ways to upgrade my test set to electronic switching.

Thanks again to Claudio IN3OTD for his useful model-fitting code!