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
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