[TowerTalk] measuring impedances with a VNA

[Jim Brown]

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