[TowerTalk] CM Choke

[Jim Brown]

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).

> 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.

73, Jim K9YC