[TowerTalk] measuring impedances with a VNA

[Máximo EA1DDO_HK1H]

Hi all,

As Jim knows, I've spent some time investigating and learning how to measure chokes correctly. That was a year or two ago.
Transformer type baluns are more difficult to measure with a two port VNA (balun has 3 ports), but chokes are not that difficult, at least to get relative values, not absolute.

As you know, during S21 measure, VNA only takes amplitude and phase values. Rest of the information is maths calculated, including impedance.

Because not every VNA shows Zcm, I've made an upgrade to Dan's AC6LA ZPlots (with Dan's permission) to calculate and show Zcm, Phase, LogMag, Mag, Cp, Rp, Rs and Xs from any S21 Touchstone file.
I called it ZPlots Xtra, and you can grab a copy on below link:
Zplots_Xtra.xlsm<http://www.puertobalsillas.com/radio/Balun_Guanella_4_1/Zplots_Xtra.xlsm>
<http://foro.ea1ddo.es/>


It is my small bit to help everyone on this matter.

Thanks

73, Maximo - EA1DDO




________________________________
De: TowerTalk <towertalk-bounces at contesting.com> en nombre de Jim Brown <jim at audiosystemsgroup.com>
Enviado: miércoles, 31 de julio de 2019 5:47
Para: towertalk at contesting.com <towertalk at contesting.com>
Asunto: Re: [TowerTalk] measuring impedances with a VNA

On 7/30/2019 10:16 PM, Edward Mccann via TowerTalk wrote:
> Not much written on how to do it, or how to interpret results, best I can find.

http://k9yc.com/2018Cookbook.pdf

There's a photo of a well thought-out test fixture here. Scroll to the
very bottom of the page.

http://www.karinya.net/g3txq/chokes/

The author succumbed to cancer a year or two ago. While I trust his
measurement work, his recommendations for chokes should NOT be trusted,
simply because he failed to realize the significance of the relatively
broad manufacturing tolerances of the ferrite cores on which the chokes
are wound.

The work that produced my Cookbook included the measurement of more than
200 cores to understand where they fit in the tolerance window, from
which I selected cores at tolerance limits to wind chokes, and based the
cookbook on the worst case of each winding! That took a year. :)

To evaluate Steve's recommendations for #52 chokes, I purchased 40 of
the 2.4-in o.d. #52 cores, splitting the purchase between four major
electronics distributors over a period of about six weeks. I
characterized each core as I had my work for the cookbook, then wound
chokes on cores selected from the limits. None of the chokes matched
Steve's recommendations.

While I haven't done this for #43 cores, I'm pretty certain that the
result would be the same, because #43 and #52 cores are NiZn ferrite
mixes, which have much higher Q at HF (and thus much narrower resonance
curves) than #31, which is a MnZn mix especially designed for use at HF,
and which exhibits far broader resonance at HF.

This test circuit is optimum for measuring high values of impedance.
The choke is placed in series between input and output of a Vector
Network analyzer, forming a simple voltage divider between the choke and
the 50 ohm input Z of the VNA. The impedance of the choke is found by
solving the voltage divider equation.

The VNA I use is the VNWA3e, designed by DG8SAQ and sold by some hams in
the UK. DG8SAQ's control and measurement software includes a math
"solver" that reads data for S21 (Vout/Vin) and can be programmed to
solve that equation and plot the results for each sweep in real time.

73, Jim K9YC

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