[TowerTalk] measuring impedances with a VNA

[jimlux]

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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