N2PK VNA Calibration Standards

Proper calibration is absolutely essential for high accuracy measurements.
Here are some notes on the calibration standards for the N2PK VNA. Initially, I've made my own Open-Short-Load-Thru kit. My VNA is equipped with BNC connectors, so I used "clamp type" male BNC connectors for large diameter coaxial cable (such as LMR400/RG-8) to house my calibration standards. I also wanted to have some reference calibration standard to compare against.

I was able to acquire (tnx to eBay) a precision commercial VNA calibration kit at very reasonable price. I did some comparison between my home-brewed calibration standards and the Maury Microwave VNA Kit (MMC) - good for VNA calibration DC to 18GHz.

I wanted all of my cal standards to use the same type of housing in order to easily maintain the same reference plane. Looking at the commercial standards - that's not case - the shells are different size but the reference plane inside must be the same.
As expected, the home-brewed Open and Short standards (which are fairly easy to fabricate), when compared to the expensive commercial standards seem to be pretty close. The Short standard must have low inductance and the Open - low capacitance. Looking at the MMC Short standard - it seems to be made out of one piece of metal for the center pin and the ground shell - the electrical short occurs at the far end of the center pin right at the back-face of the actual connector and the pin's length seems to be shorter than the normal N-connector center pin. The commercial MMC Open load has longer center pin attached to a dielectric disc in the back of the housing but maintains the same "reference plane".

The story is much different for the 50 Ohm Load standard. I made two different Load standards using different components (bulk-metal foil resistor and edge-trimmed precision thin film resistor - both rated for high frequency use and optimized for minimal self-reactance) and different construction techniques. Both Loads turned out to be not as good as the MMC Load (which I kind of expected) but the Load with the expensive bulk-metal foil resistor seems to be worse than the thin-film one. I suspect the difference is due to the construction and the much larger SMD component.
In addition, I tested the combination of a Male BNC-to-female SMA adapter + home-brewed SMA Load (housed in RA male SMA connector shell, /w edge-trimmed thin-film resistor) was much closer to the commercial MMC Load and actually quite acceptable.
I'll be trying yet different construction technique for the BNC Load standard. The main challenge with the BNC Loads is the installation of the SMD resistor. The center pin of the BNC is not captive, and it needs to be fixed somehow in order to remove any mechanical stress form the resistor. So far I have not used any glue or epoxy in my cal standard constructions, but I think it is about time to try it.

November Update: This is my latest (and hopefully final) version of the BNC calibration standards. I was fortunate to find on eBay these "Male BNC / PCB mount connectors" - something you don't see every day! Looks like they are perfect for home-brewed calibration standards/test fixture. Good news is that the center pin is fixed in place, it doesn't rotate, and it is not as long as the one in the "clamp-type coax connector"!

Here is the 50 Ohm LOAD standard using a Vishay FC series, high frequency precision (0.1%) 0603 SMD resistor - Digikey p/n FC0603-50BWCT. This size resistor is a perfect fit between the barrel and the center pin. The center pin of the connector is trimmed down, and the resistor is soldered in place.

The other two standards - OPEN and SHORT were very easy to make! One should be careful when soldering the SHORT standard not to melt the center insulator with excessive heat (I used a water bath during soldering). On the OPEN standard the center pin must be trimmed down to keep the reference plane the same across all 3 standards.

In order to create RF shielding, provide mechanical protection and facilitate easier handling, I prepared 3 short pieces (~1 cm length) - 2 made with brass tubing (Stock #137, K&S Engineering, 7/16" (11.1mm)) and one of same diameter plastic tube (used for the OPEN standard). I soldered each piece on the inside to the 4 ground pins of the SHORT and LOAD BNCs. Closed-cell foam was used create an endcap for each barrel. The LOAD standard was closed on the back with additional RF shield - tin-plated brass disc, soldered to the inside wall. The body of the OPEN standard was made with the plastic sleeve and glued to the 4 BNC pins - the reason for using plastic is to reduce stray capacitance, caused otherwise by a brass sleeve.

Final touch - heat-shrink tubing provides color-coding and nice finish.

This is my full set of (m) BNC calibration standards - in addition to the OSL and LLC standards, I made some extra loads (12.5, 25, 75, 100 and 200 ohms) to aid calibration of other instruments besides the VNA.

Update 2011: The excellent article by William Highton on standard definitions just confirms the fact that BNC connectors make poor calibration standards for ultra high frequencies (500 MHz and up). The home-brewed BNC standards are perfectly fine for the N2PK VNA with its max 60 MHz frequency, but I would not use such DIY kit with commercial GHz VNAs. The standards described above are intended for use with N2PK VNA. Using the same construction method but using different connector - SMA or N type will yield better accuracy with DG8SAQ VNWA or N2PK with N or SMA connectors.