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On 8/4/22 10:16 PM, Chris Gardner via groups.io wrote:

Yes of course, the ohmic, or DC resistance itself in the context of the antenna impedance measurement?is a far less significant contributor than the reactance's; capacitive, and inductive. But, in terms of the device?calibration the DC components are all we use. So, I was wondering that how flawed (or not) any subsequent?analysis might be as a result of using a 50 ohm calibrator that was somewhat less or greater than 50 ohms.?I suspect not much at all.

As long as you tell the calibration algorithms the impedance of the 3 test loads, then it doesn't matter a whole lot. There are better and worse values, of course, but ultimately, it's a "solve three equations for three unknowns" kind of thing. By convention (and practical ease) the 3 standards are 0 ohms, infinite ohms, and 50 ohms.

If an open is truly an open, and not some sort of capacitor, then all is good. The problem is that, particularly as the frequency gets higher, an "open" starts to look like a capacitor. It's the capacitance between the end of the center conductor and the end of the shield - imagine a dot and a ring on a PWB. There's not a lot of capacitance, but there is some.

Where does it become a problem? That's hard to say without running some example cases. 1 pF at 1 GHz is -160j ohms. That's a long way from infinity (or more to the point, the reflection coefficient is 0.822-0.569j, not 1.0)

And this gets back to "how accurately do you want to measure?" That 1 pF is about 35 degrees. Say you want to get to 0.1 degree. That needs zload to be at least -50,000j or, at 1 GHz, 0.032 pF. That's pretty small.

My recollection is that two parallel 50 mil traces 50 mils apart would be about 0.5 pF/inch. And I'm too lazy to go hunt down the capacitance between a dot and a ring. But if we're looking at 0.141" semi rigid coax, the center conductor is 0.036" and the outer conductor is about 0.011", the dielectric is about 0.041". Let's just take that 0.041" * 0.5 pF/" and we get about 0.021 pF.

So, at 1 GHz, just cutting the end squarely off 0.141 semirigid is probably a decent "open". But at 10 GHz, nope.

I just happened to have a datasheet for 141 here, but one could do the same sort of calculation for a SMA connector. Or, even better, load a CAD model into a field solver (which is what mfrs do now).


When you get a fancy calibration standard set, typically, there's also a set of files that come with it that give the Z for each standard vs frequency. Some VNAs let you load that in (e.g. a 3.5" floppy on an 8753C, USB stick or over the wire on newer VNAs).

Be aware that you can really go down a rabbit hole on this. Once you start worrying about 0.1% kinds of uncertainty, you need to worry about all kinds of things, like mate/demate repeatability, temperatures, etc.

On Friday, August 5, 2022 at 12:00:35 AM EDT, Dragan Milivojevic <d.milivojevic@...> wrote:
You are thinking in DC, switch to AC + calibration plane, fringing
capacitance etc.
On Fri, 5 Aug 2022 at 05:08, Chris Gardner via groups.io <seaman98=
[email protected]> wrote:

? Pondering a simple question relating to SMA calibration devices as in
those that are generally supplied with a newly purchased Nano VNA as an
example. Open is Open, that is obvious. The connector simply shields
the the female connector from any outside interference during calibration.
The short is obvious as well, 0 (Zero Ohms) resistive load. But, more to
the point, my biggest concern is with the supposedly 50 Ohm resistive
calibration load. I have 3 such devices from various sources, and none are
precisely 50 Ohms. My most recent purchase has a calibration sticker
enclosed in the kit indicating the 50 ohm load is 48.9 ohms. I confirmed
that value with my DVM. Would that be considered adequate, or good enough.
How significant does that difference in the ohmic resistance become in the
final sweep product. Of course assuming I am calibrating the VNA at the end
of the feed line just before the actual antenna.
? ? On Thursday, August 4, 2022 at 09:36:02 PM EDT, Donald S Brant Jr <
dsbrantjr@...> wrote:

? I got my cal kit from Dr. David Kirkby kirkbymicrowave.co.uk.? It
includes not only extensive characterization data but also optionally
verification standards which will allow you to check the quality of your
calibrations.
73, Don N2VGU