11
\$\begingroup\$

I would like to calibrate a VNA with the reference plane at the end of a male BNC feedline. Surprisingly, female SOL kits are hard to find. There are many BNC 50-ohm loads available but I'm having trouble finding a BNC open and a BNC short.

Obviously I can make one easily enough, but what about the "short" or "open" gives it high tolerance? For example, Fairview sells a 10GHz open and short for ~$1000 each, but my goodness, that is a lot for a high-tolerance short and open.

My target is only 440MHz, so I certainly don't need accuracy at 10GHz, but what should I consider when building my own short and open?

\$\endgroup\$
5
  • \$\begingroup\$ The open is easy - no connection. The short is a little trickier as you want the fattest, shortest connection between the inner pin and the outer. I’d suggest a copper washer of the correct dimensions - outer dia same as the connector body and the inner hole the same as the pin dia. Solder it all together and that is probably an adequate short circuit. \$\endgroup\$
    – Kartman
    Dec 4, 2022 at 23:13
  • \$\begingroup\$ There are some good videos. The key seems to be getting a common plane between the connectors so using identical connectors modified to give a flat plane at the end, and connectors with rigidly held center pins. One guy claims the open is the hardest to get really right because of fringing fields. \$\endgroup\$ Dec 4, 2022 at 23:31
  • \$\begingroup\$ @Kartman, what about covering the bottom of the bulkhead connector with copper adhesive tape so it shorts to the shield? Also, does the "open" need a shield around the bare pin, and if so, does the spacing to the shield matter? \$\endgroup\$
    – KJ7LNW
    Dec 4, 2022 at 23:31
  • 1
    \$\begingroup\$ Obviously BNC are less than ideal. Also, it is better to use 2x 100 ohm 1206 resistors rather than 1x 50 ohm or 4x 200 ohm. \$\endgroup\$ Dec 5, 2022 at 0:21
  • \$\begingroup\$ " I would like to calibrate a VNA with the reference plane at the end of a male BNC feedline. " what does that even mean? the tapered pin of a BNC male has the wrong impedance it's not going to resolve as a precise location. \$\endgroup\$ Dec 5, 2022 at 7:59

4 Answers 4

13
\$\begingroup\$

You're not paying the big bucks for the engineering, so much as the calibration certificates, and the tracability back to standards.

The reason you're having trouble finding BNC standards is that BNC is not a repeatable connector. It cannot be used for precision work. You might just as well use DIY standards, because you are not going to get high repeatability. N-type is far better, it's similar to BNC internally, but with a screw thread to hold it together. APC-7 is best. SMA is very common on VNAs.

With a short, the only thing you need to worry about is distance to the reflection plane. You can measure this physically, or calibrate it against a better standard. Dielectric will of course increase the distance.

A simple open radiates, making its |S11| < 1.000. You can fix this by making a shielded open. It also suffers from a capacitive fringing field, which you can't fix. There are tables to estimate how much this shifts the reflection plane from the physical end of the coax.

You would certainly get more repeatable results, and accurate results, and likely more usable results, if you did all your R&D in SMA, even if the final implementation was in BNC.

If you do want to make your own BNC standards, then probably the best route is to use connectors to 0.141" or 0.25" semi-rigid. As a hack, I've generally bought a ready made cable, and then cut it in half. Cut your short and open cables to the same length. Solder a short across the end of the cable for the short. This is your good length reference. Shield the open by soldering a copper foil thimble round the end. For shielding, it doesn't matter how far the shield is from the cable open end, however it will affect the estimate of end capacitance. Keep it three or more cable diameters away for negligible impact on the end capacitance, and then look up tables for the end correction. You could make a load, ideally using two 100 Ω resistors in parallel, but it would be easier and more reliable to buy a well-specified load (which wouldn't have much general use) or an attenuator (which will be useful elsewhere in the lab). An unterminated attenuator will have a return loss of twice its attenuation. A loaded attenuator will improve the performance of any rough load it's connected to by twice its attenuation. Generally you'll find the S11 of low value attenuators specified better than high value ones. I've sometimes used a 6dB + 20dB pair of pads, in that order, in a ghetto calibration set.

Before you put in a lot of work, try this experiment with your VNA (does it have N, SMA, or APC connectors?) Using a high reflection device (so short or open), break and make a connection several times, to observe the repeatability. Now put a BNC adaptor on it, and do the same with a BNC device. Compare. Can you live with the levels of repeatability you observe?

\$\endgroup\$
6
  • \$\begingroup\$ Can you elaborate why BNC is not repeatable? Is it because of the 180 degrees turn possibility? Do you think that SMA and the like is also not repeatable? \$\endgroup\$
    – lalala
    Dec 5, 2022 at 9:12
  • 2
    \$\begingroup\$ @lalala You obviously haven't used a BNC, or if you have, you haven't used any better quality connectors. It's not repeatable because of the sloppy spring mechanism that holds it connected. But, it depends what level you're working at, what you would call repeatable, or precision. If you only use it to connect to a 'scope input, then it's fine. \$\endgroup\$
    – Neil_UK
    Dec 5, 2022 at 9:16
  • 1
    \$\begingroup\$ I worked with various connectors, and I am not to shy to ask to learn new things. I also believe one doesnt have to use a higher frequency connector than the application demands, so I was curious about your criticism of bnc. When I did a small search now, it seems that keysight disagrees with your assessment. There is a connector kit: keysight.com/de/de/product/11854A/… and the manual states: "The characteristics of the components in this kit ensure high quality RF measurements of such devices." [for e.g. S-Parameter]. Should be fine for 440 MHz \$\endgroup\$
    – lalala
    Dec 5, 2022 at 9:35
  • \$\begingroup\$ @lalala You've just confirmed exactly what I've said. That kit you linked is for N-BNC adapters so you can use your N-type VNA with BNC devices. The BNC short is for getting a length reference. You'll note they don't provide a BNC open or load. You don't need a higher frequency connector than you need, you just need a repeatable one. As ever, it depends whether you just need power at an antenna, or are working in a calibration lab, what you mean by precision or repeatability. \$\endgroup\$
    – Neil_UK
    Dec 5, 2022 at 9:43
  • 1
    \$\begingroup\$ You are right that one needs repeatability. If BNC were really not repeatable, there would be no point in providing such a measurement kit though. The repeatability is not limited to the calibration kit. \$\endgroup\$
    – lalala
    Dec 5, 2022 at 10:05
7
\$\begingroup\$

I haven't ever done any VNA work with BNC connectors, so I can't speak to the specific difficulties related to that connector type, but, ...

An open intended for VNA calibration is not simply "no connection". A good one will be carefully calibrated (at the factory) for its parasitic capacitance. Its outer conductor should extend a bit beyond the end of the mating connector to maintain the field pattern of the propagating mode. In olden times they sometimes had a bit of dielectric rod attached to tune the capacitance and to ensure the phase delay of the reflection it produced was correct for the reference plane of the connector being calibrated.

The short is simpler, but it must be carefully made to place the terminating surface at the reference plane of the connector it attaches to, so that the reflection it produces has the correct phase delay.

In both cases, the vendor will provide a set of correction factors that can be given to the VNA, so that the VNA software can adjust the calibration for slight non-idealities of the standards. The exact definition of these correction factors might be different for different VNA vendors and models.

\$\endgroup\$
5
\$\begingroup\$

Be practical. Calibrating at 440 MHz isn't nearly as tricky as 10 GHz. And then, what are you trying to achieve? If the system under test has a BNC connector, well, you're already committed to its limitations. You can use the VNA to see the effect of your termination: how much does the measurement change between calibration using SMA at the end versus through a BNC adapter?

Where do you want to know the impedance? Calibrate with open, short, and load as close as practical to there, through the actual hardware you intend to use.

And then, the ability of a VNA to measure system properties usually vastly exceeds the significance of the variation in properties. A mismatch factor of 2.6 corresponds to a loss of only 1 dB.

\$\endgroup\$
4
\$\begingroup\$

More than 20 years ago I used an HP 8753B network analyser with the amazing APC7 connectors on the front. The regular N-type calibration kit was broken - for the N-F open, HP supplied a brass slug outer with a hole in it, and a little open circuit inner socket which you'd push in after screwing in the brass, so as not to rotate the connector on the inner pin.
enter image description here Image from Kirkby Microwave
The six (not four) tiny fingers were prone to breaking, and eventually it gave out.

In the back of the lab was a lovely 14x6x2 inch hardwood box with the HP 85054B kit in it - Opens, Shorts, Load and a sliding load which I think could be moved along to further improve the precision of the load.

The instrument didn't know about this special kit. Different kits had different distances from reference plane to short, and open, and different capacitance models. The standards were visibly different from the old ones, and it didn't give good results, even at 3 GHz.

You could type in the values for Offset length, Loss, and the coefficients of C0 C1 C2 C3 (polynominal in F), but we had no calibration coefficients. So I called Agilent in the USA; their support line was answered by a guy with a PhD in Electrical Engineering who promptly faxed or mailed me the appropriate coefficients for this 20-year-old kit.

Just look at these specifications (found again here):

  • 48 dB return loss on the load (0-2 GHz)
  • 42 dB return loss on the sliding load (2-18 GHz)
  • Open deviation from the model: < 1.5 degrees at 18 GHz
  • Short deviation < 1 degree at 18 GHz

That kit broke too. From then on, for many years, we used a simple kit I made with three flange-and-pin SMA connectors screwed to a bit of plastic. Open was just cut off flat. Short was soldered closed in the same plane. Load had two 100 Ohm 0805 resistors, one each side. All coefficients set to zero. This was quite good enough for (professional) 0-3 GHz antenna work.

So the answer is there's a time and place for $4k cal kits
enter image description here
and for $4 kits made from a few ebay SMA connectors.
enter image description here
I think this one is still is better than 25 dB return loss, 1.1 VSWR up to 3 GHz, good enough for making antennas.

Keep the open and short roughly in the same plane, remove any spare wire like the pin of the connector. Use two resistors for the load instead of one.

I recommend building your kit in SMA, and then doing a port extension to get to your desired reference plane. Add (or remove) adapters and connect your antenna (or a fresh connector of the same exact kind). Short circuit the connector with a scrap of copper tape or a knife, and adjust the port extension to get a nice short circuit on the Smith chart. Short is better than open because you can apply it in situ. See this answer for more detail and photos of that process.

\$\endgroup\$
3
  • \$\begingroup\$ Good luck getting that level of service from Keysight. The first thing they ask will be "Do you have a KeysightCare contract for that cal kit? " \$\endgroup\$
    – The Photon
    Dec 5, 2022 at 15:11
  • \$\begingroup\$ He was literally the first human that answered the phone, perhaps after some automated button-pushing. I wonder if I still have that email. \$\endgroup\$
    – tomnexus
    Dec 5, 2022 at 16:14
  • \$\begingroup\$ @thephoton Just coming back to say that today, 2023, I called the R&S toll free hotline, pressed 4 for technical support, and immediately spoke to someone who knew exactly what I needed, without opening a book or transferring me. So it's still a thing to offer good quality support like this. \$\endgroup\$
    – tomnexus
    Mar 31, 2023 at 17:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.