[TowerTalk] measuring impedances with a VNA

[Edward Mccann]

In fact, common mode impedance through a common mode choke (read 1:1 current Balun) actually involves the inside of the coax shield, the outside of the coax shield, and the center conductor.

So, how my question was hoping to have some response as to how choking impedance might actually be measured. Jim Brown’s work is immense, and used laboratory-level equipment to determine his results.

Would be pleased to hear what connects to what, after calibration, on a one port VNA (say Sark-110 or AIM-417O) and the more advanced two port units, with the coax in and out of the 1:1 current Balun.

Would also be interested to hear what success has been had (and what accuracy) in such measurements involving s bifilar-wound 1:1 current Balun.

Not much written on how to do it, or how to interpret results, best I can find.

73,
Ed McCann
AG6CX

Sent from my iPhone

> On Jul 30, 2019, at 5:59 PM, jimlux <jimlux at earthlink.net> wrote:
> 
>> On 7/30/19 4:32 PM, Edward Mccann wrote:
>> How about advice on measuring impedance offered to common mode current in coax with a one-port and two port VNA?
>> Ed McCann
>> AG6CX
> 
> something like coax, with a choke. So you're measuring the impedance of the outside of the coax?  I would think that would be applicable to the stuff in the article.. It was basically describing how to measure Z of any two terminal device.
> 
> As always, fixtures are the tricky part.
> 
> 
> If you wanted to measure a 3,4, or more terminal device, then it probably is more complex.  Devices like RF transformers especially with a center tap on a winding often get measured on a 4 port VNA.
> 
> It is theoretically possible to make a N port measurement with a 2 port analyzer, but it could take a while because you need to make N(N-1)/2 measurements.
> 
> A relay box with some high quality relays and some calibration would make the process easier.
> (for N ports, you'd need 2 1:N switches (to connect the 2 VNA ports to the pairs of ports), and N SPDT switches (to connect loads to the unmeasured ports)
> 
> One might hope that the calibration on your relay box is stable, because doing all the SOLT calibrations would take forever.
> 
> There might be better ways to do Massive N port measurements (maybe some form of 6 port analyzer on steroids - everything is done with precision power measurements, no phase measurement needed)
> 
> 
>> Sent from my iPhone
>>> On Jul 30, 2019, at 3:56 PM, jimlux <jimlux at earthlink.net> wrote:
>>> 
>>> We've discussed the measuring of things like antenna traps, and we've had varying levels of success.
>>> 
>>> 
>>> https://www.mwrf.com/test-measurement/make-accurate-impedance-measurements-using-vna
>>> 
>>> Discusses three different configurations (one port, and a couple two port measurements), gives the math, and looks at the uncertainties.. each works best in a particular range of impedances.
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