[TowerTalk] CM Choke

[Wes Stewart]

On 10/10/2017 4:30 PM, Jim Brown wrote:
> On 10/10/2017 9:23 AM, Máximo EA1DDO_HK1H wrote:
>> Jim, I´ve seen someone else making a S11 measurement for CM choke?
>>
>> Is that correct?
>
> That's a reflection-based measurement, and has VERY poor accuracy when Z being 
> measured is much greater than or much less than the system impedance (50 ohms).
As you know, because he specifically mentions you, Kurt Poulsen, OZ7OU, has 
shown that with care and knowledge S11 measurements can, for the impedances in 
question, be just as accurate as S21 or RF-IV measurements.  See: 
http://www.hamcom.dk/VNWA/How%20to%20measure%20very%20high%20impedances.pdf

>
>> http://www.lb3hc.net/archives/1358
>
> This appears to be an S11 measurement.  The common measurement error is that 
> the measured resonant frequency is much lower than the actual resonant frequency.
>
>> I always use S21.
>
> This has the greatest potential for success, but it requires a load resistance 
> to form a voltage divider with the choke. This is the method shown in that set 
> of slides, and it correlated well with a set of round robin measurements done 
> by others using classical methods (Q-meter, etc.). The choke being measured 
> was wound on a material that seemed to be #61, thus a fairly high Q, and thus 
> quite sensitive to measurement error.
With a decent vector network analyzer, the ports are already well-known (50 Ohm) 
terminations.  If the round robin you mention was in the July 2007 time frame 
then I was a party and measured the device using both my N2PK VNA and a Boonton 
RX meter.  I have pictures of the setups, but alas can't find the data.  I got 
some feedback that you said of my measurements, that they were "dead nuts on." :-)
>
> 73, Jim K9YC

Wes  N7WS